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J2ME in a Nutshell by Kim Topley

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The High-Level User Interface API

A MIDlet written using the high-level API typically consists of one or more screens built using the Form, List, or TextBox classes, together with a set of Commands that allow the user to tell the MIDlet what actions to perform and how to navigate from screen to screen. Let’s start our examination of the high-level API by creating a simple MIDlet with a single screen containing a TextBox.

A TextBox Example

TextBox is a component used to display and modify text. Since it is derived from Screen, TextBox occupies the entire screen of the device and therefore can accomodate relatively large amounts of text spread over several lines. Most of the API provided by TextBox is identical to that of a similar component called TextField, which is covered in detail in Section 4.2.9, later in this chapter. In this example, we use only the features that TextBox inherits from Screen (and which are not available to TextField, because it is not derived from Screen). The code for this example is shown in Example 4-1.

Example 4-1. Creating and Using a TextBox

package ora.ch4;

import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.IOException;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.TextBox;
import javax.microedition.lcdui.TextField;
import javax.microedition.lcdui.Ticker;
import javax.microedition.midlet.MIDlet;

public class TextBoxMIDlet extends MIDlet {

    // Maximum size of the text in the TextBox
    private static final int MAX_TEXT_SIZE = 64;
    
    // The TextBox
    protected TextBox textBox;
    
    // The MIDlet's Display object
    protected Display display;
    
    // Flag indicating first call of startApp
    protected boolean started;
    
    protected void startApp( ) {
        if (!started) {
            // First time through - initialize  
            // Get the text to be displayed
            String str = null;
            try {
                InputStream is = getClass( ).getResourceAsStream(
                    "resources/text.txt");
                InputStreamReader r = new InputStreamReader(is);
                char[] buffer = new char[32];
                StringBuffer sb = new StringBuffer( );
                int count;
                while ((count = r.read(buffer, 0, buffer.length)) > -1) {
                    sb.append(buffer, 0, count);
                }
                str = sb.toString( );
            } catch (IOException ex) {
                str = "Failed to load text";
            }
            
            // Create the TextBox
            textBox = new TextBox("TextBox Example", str, 
                                MAX_TEXT_SIZE, TextField.ANY);
            
            // Create a ticker and install it
            Ticker ticker = new Ticker("This is a ticker...");
            textBox.setTicker(ticker);
            
            // Install the TextBox as the current screen
            display = Display.getDisplay(this);  
            display.setCurrent(textBox);

            started = true;
        }  
    }

    protected void pauseApp( ) {
    }

    protected void destroyApp(boolean unconditional) {
    }
}

In this simple MIDlet, all of the code is in the startApp( ) method, which simply reads some text from a file, installs it in a TextBox, and arranges for the TextBox to appear on the screen. Since the startApp( ) method could be called more than once during the lifetime of a MIDlet, this initialization code is protected by a boolean flag that ensures that it is performed only on the first invocation of startApp( ).

Skipping for a moment the code that obtains the actual text, let’s look at how the user interface is created. The TextBox is created using its only constructor:

public TextBox(String title, String text, int maxSize, int constraints)

The title argument sets the title that appears above the TextBox; you can set it to null if no title is required. The second argument specifies the text that will initially be displayed in the TextBox, and the final two arguments allow you to exercise some control over what the TextBox is allowed to contain, as follows:

maxSize

Specifies the maximum number of characters that the TextBox can contain at any time. Once the TextBox contains the maximum number of characters, the user will not be allowed to enter any more. The same restriction also applies to the text supplied to the constructor and to all the other methods that allow you to change programmatically the content of the TextBox, which you’ll see later when we look at the TextField component. There is no way to avoid specifying an upper bound on the number of characters that the TextBox can hold; specifying 0, for example, creates a TextBox that cannot contain any text at all! Furthermore, the implementation is permitted to apply a smaller upper bound than the one you specify, so trying to avoid this constraint by setting a large maximum size is unlikely to work. You can find out the actual maximum size that applies to a TextBox by calling its getMaxSize( ) method.

constraints

Specifies the type of content that should be allowed in the TextBox. Using this argument, you can, for example, restrict the user to entering only numbers or more complex things such as phone numbers or URLs without having to write the code to validate the content yourself. Since this is another feature that TextBox shares with TextField, we’ll defer further discussion of it until later in the chapter. In this example, the constraint has the value TextField.ANY, which places no restriction on what the TextBox can contain.

TextBox inherits the ability to display a title from its superclass (Screen). Here is how Screen itself is defined:

public abstract class Screen extends Displayable {
    public Ticker getTicker( );
    public String getTitle( );
    public void setTicker(Ticker ticker);
    public void setTitle(String title);
}

You can change the title associated with the TextBox at any time by calling the setTitle( ) method, and you can also use the setTicker( ) method to add a Ticker to the screen. Ticker is a very simple class that displays a string that continuously scrolls across the screen area allocated to it, which is usually at the top. Here’s the definition of this class:

public class Ticker {
    public Ticker(String str);
    public String getString( );
    public void setString(String str);
}

You’ll notice that there is no way to explicitly start or stop the ticker or to control the direction or rate at which it scrolls its content; these aspects are all controlled by the MIDP implementation itself. This lack of direct control is a deliberate design feature of the high-level API, which emphasizes simplicity, partly to minimize the size of the API and partly to make it possible to port both the platform itself and the MIDlets that rely on it to devices with varying user interface capabilities. In this example, we add a Ticker to the TextBox so that you can see how it works and where it is placed:

Ticker ticker = new Ticker("This is a ticker...")
textBox.setTicker(ticker);

It is worth noting that a single Ticker can be associated with any number of screens at the same time. This is a very useful feature, not only because it potentially saves resources, but also because any changes made to the Ticker by calling its setString( ) method (e.g., updating stock prices) takes effect immediately for all the screens on which the Ticker appears.

To run this example, you can use the Run MIDP Application utility that comes with the Wireless Toolkit. Point it at the file ora\ch4\Chapter4.jad in the example source code for this book and select TextBoxMIDlet. The MIDlet’s user interface, as seen on the default color phone, is shown in Figure 4-3.

A TextBox with a ticker and screen title

Figure 4-3. A TextBox with a ticker and screen title

This phone arranges the three parts of the screen so that the Ticker is placed at the top with the title below it and the content of the TextBox itself at the bottom. Other devices might take a different approach. For example, if you run this code on a PalmOS-based handheld, the result looks like Figure 4-4, where the title and ticker are placed side by side. Notice also that because less space is allocated for the title on the PalmOS platform, the text is truncated..

Title and Ticker as shown on a PalmOS-based handheld

Figure 4-4. Title and Ticker as shown on a PalmOS-based handheld

Although the text used in this example is fairly short due to the small size of the phone’s screen and the space taken up by the title and ticker, it isn’t possible for the TextBox to show all of the text at once. When this happens, the TextBox allows the user to scroll its content using the up and down arrow keys on the keypad and draws a scroll arrow on the screen to indicate that there is more text to be seen. On other devices, such as handhelds with pointing devices, a scrollbar that could be dragged using the pointer might be provided. The presence and nature of these visual cues and the way in which they work is transparent to the MIDlet, which doesn’t need to include any code to deal with them or even be concerned about whether they are required.

Since TextBox provides editing facilities, you can use the keys on the emulated phone’s keyboard to change the text or add extra characters. If you try to add more than 6 characters, however, you will fail, because this TextBox has a capacity of only 64 characters, and the initial text is 58 characters long. Using the arrow keys, you can move the insertion point around within the TextBox and insert or delete characters anywhere you like, provided you don’t exceed the 64-character limit.

The emulated devices provided by the Wireless Toolkit attempt to mimic the input mechanisms of the real devices. In the case of a cell phone, the small number of keys available means that most of the keys are overloaded to perform several functions. Most of the keys give numbers when pressed, but if you press them repeatedly, they yield other characters. On the default color phone, for example, the 2 key can be used to input the number 2 or the letters A, B, or C, provided you press the key quickly enough. You can use the MODE key to shift into a separate mode to make the input of alphabetics quicker or to force each key to represent only the number on its face. You can also use the MODE key to select a screen that contains special symbols. The RIM wireless handheld, on the other hand, has a larger set of keys that include alphabetics, with numbers and special characters accessible via a mode shift. When you use the TextBox or TextField components, you don’t need to concern yourself with the details of the keypad or keyboard, because the mapping from key strokes to Unicode characters is handled for you in a manner appropriate to the device that your MIDlet is running on.

When you are using the cell phone emulator, you will probably find it tedious and quite time-consuming to use the phone’s keypad to enter text. In the real world, this would not be quite so difficult, because you are probably used to using the real keypad of your own phone, but it is inconvenient to use such a slow approach when developing MIDlets. To alleviate this problem, the emulators allow you use your PC’s keyboard to edit the content of the TextBox instead of having to resort to the mouse. The quickest way to enter this mode is to press the Return key on your keyboard. This replaces the MIDlet’s screen with a full-screen editor that accepts keystrokes from your keyboard, as shown in Figure 4-5. When you have finished editing, you can return to normal mode by pressing Return again. You can also abandon any changes you have made by pressing the Escape key. Another way to enter and leave full-screen editing mode is to use the mouse to “press” the key that corresponds to the SELECT action on the emulator’s keypad. In the case of the default color phone, this is the round white button just below the screen, as shown in Figure 4-5. The full-screen editing facility is, of course, not available on real devices, and you should perform some testing without using this facility before deciding that your MIDlets are error-free.

Using the emulator’s full-screen editor to enter text into a TextBox

Figure 4-5. Using the emulator’s full-screen editor to enter text into a TextBox

Displaying the TextBox

Once you’ve created the TextBox , the next step is to make it visible to the user, which requires two lines of code:

display = Display.getDisplay(this);
display.setCurrent(textBox);

The static getDisplay( ) method of Display gets the Display object for the MIDlet passed as its only argument. Since this call is made directly from the MIDlet’s startApp( ) method, it is appropriate to use this as the MIDlet reference. It is necessary to call getDisplay( ) only once in the lifetime of a MIDlet, because the returned reference is valid until the MIDlet is destroyed. Most MIDlets, therefore, simply store the reference in an instance variable, as shown in this example. To make the TextBox visible, the Display setCurrent( ) method is used with the TextBox reference supplied as the argument. The TextBox will appear on the user’s screen sometime shortly after the setCurrent( ) method returns.

Accessing Resources in the MIDlet JAR File

For this example, instead of hard-coding the text to be displayed in the TextBox, I put it into a text file that is included in the MIDlet JAR file. Separating text from code is a useful technique that can be used to allow tailoring of a MIDlet suite to meet locale- or customer-specific requirements, such as the need to translate text in the user interface into other languages. The only problem with this approach is getting access to the file while the MIDlet is executing.

To solve this problem, the CLDC version of the class java.lang.Class provides an implementation of the J2SE method getResourceAsStream( ) :

public InputStream getResourceAsStream(String name);

Given the name of a resource, this method returns an InputStream that can be used to read its content. To use this method, however, you need to have a Class object on which to invoke it and a properly formed resource name.

Warning

CLDC/MIDP does not provide an implementation of the other J2SE method that is commonly used to access resources in JAR files:

public URL getResource(String name)

Supporting this method would require the URL class, which is not part of either CLDC or MIDP. Another reason for not providing it is that it is of limited use even in J2SE, because some web browsers did not support it for applets but did implement getResourceAsStream( ). Therefore, probably much less existing code uses getResource( ) than getResourceAsStream( ).

There are two different ways to specify the resource: with a relative name or an absolute name. To see how the resource name is constructed, you need to keep in mind how the JAR file is logically arranged. The simplest way to understand the layout is simply to imagine the JAR file expanded out into a filesystem hierarchy. This is usually very easy to do, because most JAR files are constructed from a filesystem anyway. In this example, the MIDlet class file is in a package called ora\ch4 and, therefore, in terms of a filesystem layout, the class file would be called ora\ch4\TextBoxMIDlet.class. The text file is called text.txt and was placed in a package called ora\ch4\resources. Therefore, the filesystem pathnames for these two files would be:

ora\ch4\TextBoxMIDlet.class
ora\ch4\resources\text.txt

For the purposes of this example, we want to access the latter of these files while executing the code of the former. The simplest way to do this is to use an absolute resource name for the text file, which can be created by taking the logical pathname of the file, replacing all the “\” characters with “/”, and prefixing the result with a “/” to form an absolute pathname:

/ora/ch4/resources/text.txt

When you use an absolute resource name, you can invoke the getResourceAsStream( ) method of any class in the same JAR file to get an InputStream for the resource. In this example, the simplest approach to take is to use the Class object of the MIDlet itself. Hence, one way to locate the text file is to write the following:

InputStream is = getClass( ).getResourceAsStream(
    "/ora/ch4/resources/text.txt");

Alternatively, you can use a relative resource name. Normally, you use a resource name that is relative to the class whose code is using it, so in this case you need a resource name relative to ora\ch4\TextBoxMIDlet.class. If you view the JAR as a filesystem, it is easy to see that the appropriate relative resource name would be resources/text.txt. Note that relative resource names do not begin with a “/” character. Because this name is relative to TextBoxMIDlet.class, you need to use the Class object of that class (or, in fact, any other class in the same package, since all such classes are in the same directory in a filesystem representation of the JAR file structure). Hence, to use a relative pathname, you would code the following:

InputStream is = getClass( ).getResourceAsStream("resources/text.txt");

Relative resource names are a little more flexible than absolute names because they are unaffected by package name changes, provided that you keep the relative locations of the class file and the target file unchanged. Hence, if the MIDlet were moved from the package ora.ch4 into a different package called ora.ch8, the relative resource name would continue to work, provided that the text file is moved to ora/ch8/resources. No code changes would need to be made, other than to change the package line at the top of the source file and recompile. If you use absolute resource names, changing the package hierarchy requires that you search for and change all affected instances of getResourceAsStream( ).

Once you have an InputStream for the resource, you can use the usual mechanisms to load its content. Here, we simply wrap the InputStream with an InputStreamReader to convert the content of the file into Unicode characters and read it into a StringBuffer a piece at a time.

The MIDP specification allows you to use getResourceAsStream( ) to access anything in the JAR file apart from the class files. This includes the JAR’s manifest file, which can be obtained as follows:

InputStream is = getClass( ).getResourceAsStream("/META-INF/MANIFEST.MF");

Commands

The TextBoxMIDlet example allows you to view and edit text, but there is no way to tell the MIDlet to save your changes in persistent storage, and it is not possible to terminate the MIDlet in an orderly manner. To provide this functionality, you need to use Commands. Commands are a feature of the Displayable class, so you can add them to any user interface, even those created using the low-level API.

Creating Commands

The Command class has a single constructor:

public Command(String label, int type, int priority);

The label argument supplies the text that will be used to represent the Command in the user interface, and the type and priority arguments are hints that the MIDP implementation can use when deciding where the Command will be placed. The type and priority arguments are required because of the diversity of the devices on which MIDP is intended to be used. Following construction, you cannot change the label, type and priority attributes of a Command.

If you were writing a J2SE application using AWT or Swing, you would add a command action to the user interface by creating a button or a menu item and connecting to it a listener that would perform the action associated with the command upon activation by the user. The limited capabilities of most MIDP devices make it impossible to rely on the general availability of anything that resembles a menu, nor do you have the screen space to display more than a couple of buttons. Cell phones, for example, typically have only two soft keys to which application actions can be assigned. PalmOS applications are more fortunate: they have access to a traditional pull-down menu system and a larger number of buttons that can be drawn on the screen.

Clearly, a portable MIDlet cannot be coded in such a way as to assign command actions explicitly to individual menus or buttons, because these may not be available on any given device. On the other hand, forcing all MIDlets to work to the lowest common denominator (i.e., two soft keys) would be overly restrictive, especially for PDAs. For this reason, the responsibility for mapping Commands to GUI resources rests with the MIDP implementation, which is specific to each platform and, therefore, aware of what is available. MIDlets can use the type and priority constructor arguments to supply hints to the MIDP implementation regarding the semantic meanings of Commands and their relative importance, so that those likely to be most frequently used can be made most easily accessible to the user.

The type argument is used to convey the meaning of a Command in terms of a small set of commonly required application operations. The possible values for this argument and their interpretations are given in Table 4-1.

Table 4-1. The Command type Parameter

type Paramter Value

Meaning

OK

Implies agreement by the user for some operation to be performed. Commands of this type would normally be placed to be easily accessible to the user.

BACK

Replaces the currently displayed screen with the one that preceded it.

CANCEL

Abandons an operation before it has been initiated. This command, along with the OK command, is typically made available while setting up the parameters for the operation. It might also be available on an Alert screen used to explicitly prompt the user for confirmation of an operation that might not easily be reversible.

STOP

Stops an operation that is already in progress.

EXIT

Requests that the MIDlet stop all outstanding operations and terminate in an orderly manner.

HELP

Requests general or context-sensitive help.

SCREEN

Relates to the function of the current screen, but does not fit into one of the specific categories listed previously. Most application-specific actions are of this type.

ITEM

Indicates a command that is associated with a particular user interface component.

Adding Commands to the user interface

Once you have created a Command object, the next step is to arrange for it to appear in the user interface. This is achieved by calling the addCommand( ) method of Displayable:

public void addCommand(Command cmd);

MIDP platforms are allowed to follow their own rules when determining how to represent Commands in the user interface. In general, however, the choice is made first based on the Command type and then on the priority, where lower priority values tend to result in a more favorable placement. The order in which Commands are added to a Displayable is not usually of any significance in the determination of placement, and the label text is not used at all, because the semantic meaning of the command is supposed to be conveyed via the type attribute.

On a cell phone, for example, the type might be used to favor well-known operations (such as OK, CANCEL, BACK, etc.) that the user would normally expect to be able to access via a soft key. Where the number of these Commands exceeds the number of soft keys available, the phone might use the priority to determine which Commands should be installed on the soft keys, with lower values increasing the likelihood of assignment to a soft key. The remaining Commands would then be placed on a menu that would itself be accessible via a soft key. When the number of Commands does not exceed the number of soft keys, they can all be allocated a soft key. When a platform has both soft keys and pull-down menus, it may choose to place Commands on menus as well as, or instead of, on soft keys, with the choice again being made usually based on the type and priority attributes.

Some commands, such as EXIT, might need to appear on more than one application screen. When this is the case, it is not necessary to create a dedicated instance for each screen, because a single Command can be added to any number of screens:

Command exitCommand = new Command("Exit", Command.EXIT, 0);
form1.addCommand(exitCommand);
form2.addCommand(exitCommand);

Responding to user activation of Commands

In order to be notified when the user activates a Command, you have to register a CommandListener with the Displayable to which the Command was added. You do this by invoking its setCommandListener( ) method:

public void setCommandListener(CommandListener l);

CommandListener is an interface with a single method:

public void commandAction(Command c, Displayable d)

The commandAction( ) method is called when any Command on the Displayable is activated. The first argument is the more useful, because it allows you to determine which operation the user wants to perform. The Displayable argument is useful if you add the same Command to more than one screen, and the resulting action is dependent on the current screen. It can also be useful if the action needs a reference to the screen in order to perform its assigned function.

Note that the setCommandListener() method allows only a single CommandListener to be registered at a time. Calling this method again replaces any existing listener with the new one, and calling it with a null argument removes the previous listener. This is very different from J2SE event handling, which normally allows you to add as many listeners as you like and requires you to register with the component itself rather than an enclosing container. Although it is very flexible, the J2SE model tends to result in the creation of lots of small event handler classes, which is very expensive in terms of memory and class-loading time; it is therefore not suitable for small-memory devices. MIDlets can get away with only one listener per screen and, if the MIDlet itself implements the CommandListener interface, this won’t even entail creating a new class. If a MIDlet has several screens, it can choose to create a single listener class for each, or it can save even that overhead by subclassing the screen class to implement CommandListener, as follows:

public class MyTextBox extends TextBox implements CommandListener {
    public MyTextBox(String title, String text, int maxSize, 
        int constraints) {
        super(title, text, maxSize, constraints);
        setCommandListener(this);
                             // Add Commands (not shown)
 }
    // Handle command actions
                         public void commandAction(Command c, Displayable d) {
                             // Code not shown
                         }
}

A Command example

We can easily illustrate the use of Commands by extending the TextBoxMIDlet example to include four operations:

  • An Exit command that terminates the MIDlet.

  • An OK command that prints a message to standard output. (In a real MIDlet, this would obviously do something a little more useful!)

  • A Clear command that removes all of the text from the TextBox.

  • A Reverse command that reverses the text in the TextBox.

The implementation of this modified example is shown in Example 4-2.

Example 4-2. Adding Commands to the TextBoxMIDlet Example

package ora.ch4;

import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Displayable;

import javax.microedition.lcdui.*;

public class TextBox2MIDlet extends TextBoxMIDlet implements CommandListener {

    // Exit command
    private static final Command EXIT_COMMAND = 
                        new Command("Exit", Command.EXIT, 0);
    
    // OK command
    private static final Command OK_COMMAND =
                        new Command("OK", Command.OK, 0);
    
    // Clear text box content
    private static final Command CLEAR_COMMAND =
                        new Command("Clear", Command.SCREEN, 1);
    
    // Reverse the content of the text box
    private static final Command REVERSE_COMMAND =
                        new Command("Reverse", Command.SCREEN, 1);

    protected void startApp( ) {
        boolean firstTime = !started;
        super.startApp( );
        
        // If this is the first execution of startApp, install commands
        if (firstTime) {
            textBox.addCommand(OK_COMMAND);  
            textBox.addCommand(EXIT_COMMAND);
            textBox.addCommand(CLEAR_COMMAND);  
            textBox.addCommand(REVERSE_COMMAND);  
            textBox.setCommandListener(this);
        }
    }
    
    // Command implementations.
    public void commandAction(Command c, Displayable d) {
        if (c == EXIT_COMMAND) {
            destroyApp(true);
            notifyDestroyed( );
        } else if (c == OK_COMMAND) {
            System.out.println("OK pressed");
        } else if (c == CLEAR_COMMAND) {
            textBox.setString(null);
        } else if (c == REVERSE_COMMAND) {
            String str = textBox.getString( );
            if (str != null) {
                StringBuffer sb = new StringBuffer(str);
                textBox.setString(sb.reverse().toString( ));
            }  
        }
    }  
}

Notice that this example is implemented by deriving it directly from the TextBoxMIDlet class from the previous example. Of course, you wouldn’t normally have to do this in the real world, but here it serves to show how easy it is to add command handling to an existing class, and you don’t need to replicate code that you saw earlier!

The four Commands are defined as static class members, for example:

private static final Command EXIT_COMMAND = new Command("Exit", 
    Command.EXIT, 0);

Since Commands are simply constant-valued objects, you can usually define them in this way and then reuse them wherever you need to, which would include adding the same instance to more than one screen, if necessary. You can see from Example 4-2 that the EXIT and OK commands use the standard types Command.EXIT and Command.OK, respectively, which allows the device on which the MIDlet will be run to represent them in whatever way it would normally present EXIT and OK actions. By constrast, the other two commands are of type Command.SCREEN, because they are application-defined actions that have no generic meaning. Notice that the OK and EXIT actions have priority 0, whereas the other two have priority 1. This hints to the device that if it has no built-in preferences, we would rather have the OK and EXIT actions more quickly accessible to the user than Clear and Reverse. However, there is no guarantee that the device will take this hint.

Making these operations available from the user interface is a simple matter of adding the Command instances to the TextBox and registering the MIDlet class itself as the CommandListener:

textBox.addCommand(OK_COMMAND);
textBox.addCommand(EXIT_COMMAND);
textBox.addCommand(CLEAR_COMMAND);
textBox.addCommand(REVERSE_COMMAND);
textBox.setCommandListener(this);

The last step is to implement the CommandListener interface by providing a commandAction method, which is responsible for carrying out the operations associated with the Commands. The commandAction method shown in Example 4-2 is typical of most event handling in MIDlets. Because there is only a single command handler for each screen, its first task is to determine which operation the user wants to perform. To do this, it examines the first method argument to see which Command has been activated. The neatest way to do this is with a switch statement, but this is not possible because Command is not an integral value. Instead, MIDlet event handlers tend to consist of if statements that compare the first method argument with each of the possible Commands. Once the correct operation is found, the code that performs the required function is trivial.

You can try this example by selecting TextBox2MIDlet from the MIDlet suite for this chapter. On the default color phone, the result is shown in Figure 4-6.

Commands on a typical cell phone

Figure 4-6. Commands on a typical cell phone

Command placement

The default color phone, like most cell phones, has two soft keys to which Commands can be assigned, but the TextBox used in this example has four Commands. As a result, the Exit command has been mapped to the left soft key, and the right key provides access to a menu of the remaining three Commands, as shown in Figure 4-7. The fact that the Exit command has been given its own key in preference to the OK command is a feature of this particular MIDP implementation. The result might not be the same on other devices, and the menu might also not look the same as it does in Figure 4-7. The MIDlet developer, of course, has no real control over these decisions and can only provide hints in the form of the type and priority arguments to the Command constructor.

Command assigned to a separate menu

Figure 4-7. Command assigned to a separate menu

Command placement on a PalmOS device

The same MIDlet looks slightly different when run on a PalmOS platform, where the larger screen space means that more Commands can be assigned to buttons that are always visible to the user. Figure 4-8 shows two views of this MIDlet running on a PalmOS-based handheld. In this case, three of the four Commands have been assigned to buttons below the TextBox. Commands are assigned to buttons based on their types, as listed here in descending order of preference:

  • Command.BACK

  • Command.OK

  • Command.CANCEL

  • Command.STOP

  • Command.SCREEN

  • Command.CANCEL

Commands on a PalmOS device

Figure 4-8. Commands on a PalmOS device

If the number of commands exceeds the number of buttons that can be created in the button area, the command priority is also taken into account when assigning commands to buttons. Note, however, that commands of type Command.EXIT and Command.HELP are never mapped to buttons.

PalmOS also has pull-down menus, and, as these two views show, the application-specific Commands have been assigned to the Actions menu, while the OK and Exit commands appear on a menu labeled Go. In this implementation, the Actions menu is used to hold application-specific commands of type Command.SCREEN or Command.ITEM. If both types of Command are installed in the same screen, they all appear on the same menu, with Commands of the same type grouped together, and the two groups separated by a horizontal line, as shown in Figure 4-9. Commands of type Command.BACK, Command.OK, Command.CANCEL, Command.STOP, and Command.EXIT are placed on the Go menu, and Command.HELP appears in the Option menu.

Grouping of commands on pull-down menus

Figure 4-9. Grouping of commands on pull-down menus

Forms and Items

Form is a subclass of Screen that can be used to construct a user interface from simpler elements such as text fields, strings, and labels. Like TextBox, Form covers the entire screen and inherits from its superclasses the ability to have a title, display a Ticker, and be associated with Commands. The elements that you can add to a Form are all derived from the abstract class Item:

public abstract class Item {
    public String getLabel( );
    public void setLabel(String label);
}

On its own, Item provides only the ability to store and retrieve a text label, but because each component that can be added to a Form is derived from Item, it follows that all of them can have an associated label. The implementation displays this somewhere near the component in such a way as to make the association between the label and the component clear. The components that MIDP provides are described briefly in Table 4-2; each of them will be discussed in greater detail in later sections of this chapter.

Table 4-2. Items That Can Be Added to a Form

Item

Description

StringItem

An item that allows a text string to be placed in the user interface

TextField

A single-line input field much like the full-screen TextBox

DateField

A version of TextField that is specialized for the input of dates; it includes a visual helper that simplifies the process of choosing a date

Gauge

A component that can be used to show the progress of an ongoing operation or allow selection of a value from a contiguous range of values

ChoiceGroup

A component that provides a set of choices that may or may not be mutually exclusive and therefore may operate either as a collection of checkboxes or radio buttons

ImageItem

A holder that allows graphic images to be placed in the user interface

The Form class has two constructors:

public Form(String title);
public Form(String title, Item[] items);

The first constructor creates an empty Form with a given title, which may be null in the unlikely event that no title is required; the second constructor can be used to install an initial set of Items on the Form. The Item s that are associated with the Form are held in an internal list, the order of which determines how they are placed on the form. Form has three methods that allow items to be added to the end of this internal list, which causes them to appear on the Form itself:

public void append(Item item);
public void append(Image image);
public void append(String string);

The second and third methods provide a quick and convenient way to include an image or string on the Form: just create and append an ImageItem containing a supplied Image or a StringItem containing the given string.

Unlike an AWT container, Form does not have the concept of a separate layout manager that you can select to control how items are arranged on the screen. Instead, Form has a few simple rules that determine how items are arranged:

  • Items that involve user input (that is, TextField, DateField, Gauge, and ChoiceGroup) are laid out vertically, with the first item in the Form’s internal list at the top of the screen, the second one directly below it, and so on.

  • Adjacent StringItems and ImageItems that have a null or empty label are laid out horizontally. If there is insufficient space to fit a complete StringItem in the horizontal space remaining in a row, the text is wrapped to the next line, and the implementation breaks at whitespace where possible. If there is insufficient space to fit an entire ImageItem, the image is simply clipped.

  • StringItems and ImageItems with a nonempty label cause a line break before the label is rendered.

  • Newlines in StringItems cause a line break. A similar effect can be obtained using layout directives of the ImageItem class, as described in Section 4.2.11, later in this chapter.

  • The width of the Form is always the same as that of the screen. The Form may, however, be taller than the screen. If so, the implementation provides a means for the user to scroll the Form vertically. Horizontal scrolling is not provided.

  • Where it is necessary to scroll vertically, the implementation attempts to ensure that scrolling never obscures the label associated with a visible item, if the item has one.

To clarify how these rules work in practice, let’s look at a simple example that places strings and TextFields on a Form.. The code that builds the Form is shown in Example 4-3. You can run it by selecting FormExampleMIDlet from the MIDlet suite in Chapter4.jad.

Example 4-3. A Demonstration of Form Layout Rules

Form form = new Form("Item Layout");

form.append("Hello");
form.append("World");

form.append("\nLet's start\na new line\n");
form.append("This is quite a long string that may not fit on one line");

form.append(new TextField("Name", "J. Doe", 32, TextField.ANY));
form.append("Address");
form.append(new TextField(null, null, 32, TextField.ANY));

The first four append( ) calls add text strings to the Form, the results of which can be seen in the leftmost two screenshots in Figure 4-10. These screenshots show the MIDlet running on the relatively small screen of the default color phone emulator from the Wireless Toolkit. The top line of the screen holds the two separate items “Hello” and “World”, which have been laid out horizontally because they are string items. Note that, even though they were added separately, no space has been left between them.

The next item to be added begins and ends with newline characters; you can see that it is placed vertically below the first two items because of the leading newline, and the trailing newline also causes a line break. Notice that in this string, and in the next, rather longer, one, the text is automatically wrapped, and line breaks are placed between words.

Form layout on a cell phone

Figure 4-10. Form layout on a cell phone

Since the Form is too large to fit on the screen, the implementation draws an arrow at the bottom to indicate that the screen can be scrolled vertically, as has been done in the middle and right views.

Following the text strings, a TextField is added:

form.append(new TextField("Name", "J. Doe", 32, TextField.ANY))

The constructor supplies both the Item label (“Name”) and the initial content of the field itself (“J. Doe”). As you can see, the label has been placed below the previous text string, even though the string did not end with a newline, but above the input field itself. If you scroll the screen up and down, you’ll find that it is impossible to arrange for the label to be visible without the text field, and vice versa.

The last two items are the text string “Address” and another TextField. Because this device’s screen is so narrow, it would be difficult to see the difference between the effect of the code used here:

form.append("Address");
form.append(new TextField(null, null, 32, TextField.ANY));

and the apparently similar:

form.append(new TextField("Address", null, 32, TextField.ANY));

which includes the string “Address” as the item’s label. To see the difference, you need to run this example using the PalmOS emulator. Because this emulator has a much larger screen, it can lay out the items differently, as shown in Figure 4-11.

Form layout on a PDA

Figure 4-11. Form layout on a PDA

Most of the items are shifted over to the right side of the screen, leaving mostly blank space to the left. This is because the MIDP for PalmOS implementation allocates the left side of the screen to the label part of each Item and places the active part of the Item to the right. Hence, all the strings (which are actually StringItems with no label) appear on the right side of the screen. The only Item with a real label is the first TextField, and its label has been placed on the left of the input field itself, rendered in a bold font, and been appended with a colon. Compare this to the next TextField: the “Address” string was added as a separate string and not installed as the Item label, and it therefore appears above the input field itself. Although the difference between using a label and using a separate text string was hard to detect with the cell phone emulator, here it becomes very obvious and underlines the fact that the Item label should be used instead of installing a separate a text string to describe the following input field. Another important reason to take advantage of the Item label is the automatic font highlighting provided for the label. You cannot achieve this in any other way, because the high-level API does not allow you to select fonts or colors.

Form has a small number of other methods, in addition to the three variants of append( ), that allow the list of Items it contains to be manipulated:

public void delete(int index);
public Item get(int index);
public void insert(int index, Item item);
public void set(int index, Item item);
public int size( );

Most of these methods use an index argument to specify the list position to be operated on, where the first item has index 0. The delete( ) method removes the Item at the given index; like all the other methods that change the Item list, it causes the screen layout to be updated immediately to reflect the change. The get( ) method returns the Item at the given index without modifying the list at all. The insert( ) method places a new Item at the given index within the list, moving the Item at that index and greater indices down by one position. The set( ) method, by contrast, replaces the Item at the index supplied as its first argument and does not affect any other Item in the Form. Finally, the size( ) method returns the number of Items on the Form.

Warning

A single Command or Ticker instance can be shared between multiple screens simply by adding it to each screen in turn. However, an Item is allowed to be on only one Form at any given time. If you try to add the same Item to another Form without first removing it from the original, an IllegalStateException is thrown.

Item State Changes

Since Form is subclassed indirectly from Displayable, it is possible to add a Command to a Form to allow the user to request that values entered into it be processed. The logic for this processing is implemented in the commandAction method of a CommandListener attached to the Form, as illustrated in Example 4-2. Sometimes, however, it is necessary to take action as soon as the value in an input field is changed. Changes in the state of Items that accept user input are notified to an ItemStateListener registered with the Form. ItemStateListener is an interface with a single method, which is called when any Item on the Form has a state change to report:

public void itemStateChanged(Item item);

An ItemStateListener is registered using the following Form method:

public void setItemStateListener(ItemStateListener l);

As was the case with CommandListeners, only one ItemStateListener can be associated with a Form at any time and calling setItemStateListener( ) removes any listener that was previously installed. Calling this method with the argument null removes any existing listener.

The conditions under which the ItemStateListener is notified of a state change are specific to each individual type of Item; these conditions are described in the sections that follow. It is important to note, however, that only user actions result in the listener’s itemStateChanged method being called. Changing the state of an Item programmatically does not cause notification to the listener.

High-Level API User Interface Components

In the rest of this section, we take a closer look at each of the Items you can use with the Form class, together with the TextBox and List components. TextBox and List are derived from Screen, so they are not suitable for use with Forms, but they have Form-based counterparts that are sufficiently similar that they are best described together.

The examples used in this section are all part of a single MIDlet called ItemMIDlet. You can run it with the Wireless Toolkit by opening the project called Chapter4 and pressing the Run button, then selecting ItemMIDlet. This displays a screen (actually a List) that has an entry that runs the example for each of the following sections. To run the example code for these sections, simply highlight the appropriate entry in the list and press the SELECT button on the emulated phone’s keypad, as shown in Figure 4-5.[13]

StringItems

StringItem , the simplest of the MIDP user interface components, provides the ability to place a string or pair of strings on a Form. Initial values for both strings may be supplied to the constructor:

public StringItem(String label, String text)

The label part is the label that is inherited by all Items from their base class; its value can be retrieved or changed using the Item getLabel( ) and setLabel( ) methods. StringItem provides similar methods for its own text attribute:

public String getText( )
public void setText(String text)

Either or both of the label and text string may be null.

A technique often used when adding text to a Form is simply to use the variant of the append method that accepts a String argument:

form.append("Name");

This code, in fact, amounts to the use of a StringItem with a null label and so could also be written like this:

form.append(new StringItem(null, "Name"));

It might seem strange to provide a component that displays two text strings, when the same effect could apparently be achieved by creating a component that supports only one string and the ability to place two of them next to each other. In fact, this would not lead to the same result, because the label and text string parts of a StringItem are not equivalent. The difference between the label and the text is the same for StringItem as it is for the label and content of any Item, namely:

  • The layout management code of the MIDP platform should attempt to display the label close to the text and ensure that they are either both visible or both not visible when scrolling takes place.[14]

  • The platform may choose to render the label differently from the content to make clear the distinction between them.

As described in Section 4.2.5, the layout policy for StringItems required by the MIDP specification results in a horizontal arrangement, unless a line break is forced by the use of newline characters within the label or text, or if there is insufficient space to fit the entire StringItem in the current line. Additionally, the Sun reference implementations force a line break before a StringItem that has a non-null label.

A typical example in which it would be advantageous to use both the label and text attributes of a StringItem is a labeled item in which the content can be updated by the MIDlet but must not by the user. Such a StringItem might be used to show the state of a connection to a web server:

StringItem status = new StringItem("Status ", "Not connected");
status.setText("Connecting");  // Change the state

In Example 4-3, you’ve already seen several examples of the use of StringItem created indirectly by appending a String to a Form. ItemMIDlet includes a screen that has a few more StringItem examples. The code that creates this Form is shown in Example 4-4.

Example 4-4. Using StringItem

Form form = new Form("StringItem");
form.append(new StringItem("State ", "OK"));
form.append(new StringItem(null, "No label\n"));
form.append(new StringItem(null, "Line\nbreak"));
form.append(new StringItem("Label", "Text."));
form.append(new StringItem("Label2 ", "Text2."));

The results of running this example on both the default color phone and on the PalmOS device are shown in Figure 4-12. The first StringItem uses both the label and text attributes. Notice that the color phone doesn’t distinguish between the label and the text in any way, whereas the PalmOS MIDP implementation uses a bold font to represent the label, adds a colon, and places all the labels in a dedicated area on the left side of the screen. The second StringItem contains only the text and is placed immediately after the text of the first StringItem, with no line break. Because the text ends with a newline character, however, it is followed by a line break.

StringItems on the default phone and PalmOS emulators

Figure 4-12. StringItems on the default phone and PalmOS emulators

The third example shows the effect of embedding a newline in the text, which results in a line break on the screen. Although it isn’t illustrated here, you can also include a newline in the label part, and the effect is the same. The final two examples illustrate an important difference in the handling of labels between the PalmOS platform and the cell phone version. In the first case, the label and text are set up as follows:

form.append(new StringItem("Label", "Text."));

As you can see, the color phone does not interpose any whitespace between the label and text, whereas the PalmOS version displays them with a clear gap, owing to its special handling for labels. In most cases, you want to clearly separate the label from the text; you can do this by adding a space at the end of the label:

form.append(new StringItem("Label2 ", "Text2."));

This produces the desired effect on the color phone and also works on the PalmOS platform, which strips out trailing whitespace before appending the colon that marks the end of the label, as you can see on the right side of Figure 4-12.

TextFields and TextBoxes

TextField and TextBox are two very similar components that have almost the same programming interface. The differences between them are as follows:

  • TextBox is derived from Screen and therefore occupies the entire display. TextField is an Item that occupies space on a Form. Usually, a TextField appears as a single-line input field, but some implementations spread its content over extra lines if a single line is not sufficient.

  • TextBox does not have a way to report changes in its content to a listener, but modifications to a TextField are reported to the ItemStateListener associated with the Form on which the TextField is displayed.

Since the specifics of TextBox have already been covered, the rest of this section focuses on the common features of these two components and illustrates them with TextFields.

Construction

TextField has only one constructor:

public TextField(String label, String text, int maxSize, 
    int constraints);

The label and text arguments specify, respectively, the Item label to be placed near the component and the string to be placed initially in the TextField; either or both of these arguments may be null. The constraints argument can be used to limit the type of data that can be entered into the TextField. See Section 4.2.9.3, later in this chapter, for details.

The maxSize argument determines the maximum number of characters that the TextField can hold. The MIDP implementation is allowed to place an upper limit on the allowed values of maxSize and may therefore impose a lower limit than the one specified in the constructor. The actual limit applied to a particular TextField can be obtained by calling the getMaxSize( ) method. The maximum size is applied whenever the field content is changed, that is:

  • When the initial value is set at construction time

  • When a new value is supplied by calling the setString( ) method

  • When some or all of the field content is modified using the insert or setChars methods

  • As the user amends the content of the TextField by adding characters anywhere in the string

In the first three cases, the result of attempting to install a value whose length exceeds the capacity of the TextField is an IllegalArgumentException. If the user tries to type more characters than the field can hold, the extra characters are ignored, and the device may supply audible feedback.

The capacity of the TextField can be changed by calling the setMaxSize( ) method. If the number of characters in the TextField exceeds the new capacity, it is truncated to the maximum size.

Field content changes and listener notification

If the Form that contains the TextField has an ItemStateListener installed, it will be notified of changes made by the user to its content. You can get the value held in the TextField by calling its getString( ) or getChars( ) methods, which return a String or an array of characters, respectively:

public String getString( ) 
public int getChars(char[] chars)

To use the getChars( ) method, you have to allocate the character array to be filled. The return value of this method is the number of characters of the array that were used. If the array is too short to hold the content of the TextField, an ArrayIndexOutOfBoundsException is thrown. You can avoid this by using the size( ) method to get the number of characters that are currently in the TextField:

char[] chars = new char[textField.size( )];
int copied = textField.getChars(chars);

The following code extract shows how a listener might use getString( ) to retrieve the last value that the user entered as a String:

public void itemStateChanged(Item item) {  
    if (item instanceof TextField) {
        System.out.println("Text field content: <" +
                        ((TextField)item).getString( ) + ">");
    }
}

The point at which the ItemStateListener is called following a change in the content of the TextField is implementation-dependent. The MIDP specification requires only that this should happen no later than when the user moves the input focus away from the TextField or activates a command on the Form. The reference implementation provides notification when the user completes an editing operation in the TextField; the MIDP for PalmOS version does it after any character has been inserted or deleted.

The TextField (and TextBox) API contains several methods that allow programmatic changes to its content.[15] All of these methods throw an IllegalArgumentException and leave the TextField content unchanged if the result of performing the requested operation would make the content inconsistent with the constraint, if any, applied to the TextField. This means, for example, that an exception would be thrown if an attempt were made to insert non-numeric characters into a TextField to which the TextField.NUMERIC constraint has been applied. Constraints are described in Section 4.2.9.3.

The following are the methods that enable programmatic changes to TextField and TextBoxes:

public void delete(int offset, int length)

Removes length characters from the TextField, starting with the character at position offset.

public void insert(char[ ] chars, int offset, int length, int position)

Inserts the characters from chars[offset] through chars[offset + length - 1] into the TextField, starting at the given position. The characters that originally occupied offsets position and higher are moved to the right to make room for the new characters. An IllegalArgumentException is thrown if this operation would make the content of the TextField exceed its maximum size.

public void insert(String src, int position)

Inserts the characters that make up the given String into the TextField, starting at the given position. The characters that originally occupied offsets position and higher are moved to the right to make room for the new characters. An IllegalArgumentException is thrown if this operation would make the content of the TextField exceed its maximum size.

public void setChars(char[ ] chars, int offset, int length)

Replaces the content of the TextField with chars[offset] through chars[offset + length - 1] of the given character array. An IllegalArgumentException is thrown if this operation would make the content of the TextField exceed its maximum size.

public void setString(String src)

Replaces the content of the TextField with the characters from the given String. An IllegalArgumentException is thrown if this operation would make the content of the TextField exceed its maximum size.

Note that programmatic changes are not notified to ItemStateListeners.

In general, application code that modifies the content of a TextField uses either the setString( ) or setChars( ) methods to replace its entire content. Less frequently, it is necessary to insert content starting at the location of the TextField’s insertion point, which is indicated on the screen by a cursor, otherwise known as a caret. You can get the offset of the cursor within the TextField using the following method:

public int getCaretPosition( );

The following code could be used to insert three characters starting at the cursor position:

textField.insert("ABC", textField.getCaretPosition( ));

Constraints

The constraints argument of the constructor or the setConstraints method can be used to limit the characters that the user can type into a TextField. The effect of each constraint may be device-dependent. Table 4-3 describes what these constraints do in the MIDP reference implementation.

Table 4-3. TextField Input Constraints

Constraint Value

Effect

TextField.ANY

Allows any characters to be typed into the input field.

TextField.EMAILADDR

Limits the user’s input to a legal email address. The format of a valid email address may vary from device to device, so vendors are expected to implement this in a manner appropriate to the network to which their device will be connected. In the reference implementation, the constraint has no effect.

TextField.NUMERIC

Limits input to integer values. The first character may be a minus sign, and the other characters must be digits 0 through 9. On a cell phone, the implementation typically forces the keypad into a mode where it assumes that each key press represents the number on the face of the key when this constraint is applied.

TextField.PHONENUMBER

Specifies that the field should contain a phone number. The format of a valid phone number may vary from device to device and network to network. The reference implementation provides a default implementation of this constraint that is described later in this section.

TextField.URL

Although this constraint signifies that the input field should only be allowed to hold a valid URL, it has no effect in the reference implementation.

TextField.PASSWORD

This constraint may be specified in conjunction with TextField.ANY or TextField.NUMERIC to convert the TextField into a field intended to hold a password, for example:

TextField.PASSWORD | TextField.ANY

The implementation usually displays the content of a password field differently from that of a plain TextField. Typically, the characters are displayed as asterisks for security reasons.

When input is constrained, the user cannot type any characters that would result in the field content becoming inconsistent with the constraint. Calling a method to change the field content results in an IllegalArgumentException if the result would not match the constraint.

You can change the constraint associated with a TextField or TextBox at any time by calling the setConstraints( ) method:

public void setConstraints(int constraints);

When this method is called, the current content of the control is checked to ensure that it is consistent with the new constraints; if not, the field is cleared.

The effect of some of the constraint values can be seen by launching the ItemMIDlet and selecting the TextField example. This example contains four TextFields with different constraints, as shown in Figure 4-13.

TextFields with various input constraints

Figure 4-13. TextFields with various input constraints

The first field, shown at the top on the left side of Figure 4-13, has constraint TextField.ANY, which permits any characters to be entered. If you start typing into this field, either by clicking with the mouse on the emulator’s onscreen keypad or using your PC keyboard, the display switches to a full-screen TextBox that you can use to type and edit the value that you want, as shown in Figure 4-14. To enter the displayed value into the TextField, press the Save soft key, or press Back to abandon editing and leave the field content unchanged.

Full-screen TextBox for entering or editing a value

Figure 4-14. Full-screen TextBox for entering or editing a value

The second TextField has the TextField.PHONENUMBER constraint. In the reference implementation, this constraint limits the characters that can be typed to the digits 0 through 9 and the characters +, *, and #. This constraint also causes the content of the TextField to be displayed so that it looks like a telephone number by separating the digits into groups separated by space characters. The appropriate grouping depends entirely on the part of the world in which the cell phone or PDA is being used, since different conventions apply in different countries. The reference implementation uses the following rules:

  • If the first digit is zero, the number is assumed to be for international dialing and is represented in the form “0xx xxx xxxx . . . “.

  • If the first digit is 1, the number is formatted as “1 xxx xxx xxxx . . . “.

  • In all other cases, the number is displayed as “xxx xxx xxx . . . “.

Note that the spaces used to separate the number groups are purely visual and do not appear in the TextField content. For example, if the TextField displayed “044 171 1234567”, the result of calling the getString( ) method would be “0441711234567”. Similarly, an attempt to store a value containing spaces would result in an IllegalArgumentException. If you run this example using the Wireless Toolkit, you can observe the results of typing different values into this field or any of the other fields by looking at the Wireless Toolkit console, to which a message is written whenever any of the fields calls the ItemStateListener registered with this screen.

The third field has the constraint TextField.NUMERIC applied to it. As you can verify for yourself, this field will allow you to type only positive and negative integer values and zero.

The final field is set up with the constraint TextField.PASSWORD|TextField.NUMERIC, which limits the user to numeric values but also displays each character that is typed as an asterisk, as shown on the right side of Figure 4-13. On PalmOS, a field that includes the constraint TextField.PASSWORD is handled slightly differently. When the field is empty, its content is shown as “-Prompt-”, as shown on the left side of Figure 4-15. When an attempt is made to enter a value, a separate window opens up to allow you to type the required password. As you can see from the screen shot in Figure 4-15, this window displays the actual password value instead of disguising it. Once a password has been entered, the TextField displays “-Assigned-”, as shown at the right side of the figure.

Password fields on the PalmOS platform

Figure 4-15. Password fields on the PalmOS platform

DateFields

DateField is a component that allows you to display and edit the value of an object of type Date. The DateField class has two constructors:

public DateField(String label, int mode) 
public DateField(String label, int mode, TimeZone timeZone)

The date and time value held in a Date object is always relative to midnight UTC on January 1, 1970. When displaying the time, a correction needs to be made for the time zone in which the user is working. On the east coast of the United States, for example, a Date value that corresponds to 9:00 P.M. on January 31, 2002 (UTC), would need to be displayed as 4:00 P.M., January 31, 2002, and in Tokyo, it would need to be shown as 6:00 A.M., February 1, 2002. You can use the timeZone argument to supply a TimeZone object that can be used to determine how to display the date and time for a specific location in the world. If this argument is not supplied (or is null), the device’s default TimeZone is used, which should properly display local time. Therefore, it should be necessary to supply a TimeZone value only when the date and time for a different time zone are to be displayed.

Warning

The DateField component works with any valid TimeZone object and therefore should be able to properly display the date and time anywhere in the world. However, the CLDC specification requires only that the time zone for GMT be supported. Practical considerations dictate that a device also support the time zone in which it normally operates, but there is no guarantee that other time zones will be available.

The mode argument determines what the DateField will display and takes one of the following values:

DateField.TIME

The DateField should display only the time.

DateField.DATE

The DateField should display only the date.

DateField.DATE_TIME

The DateField should display both the date and time.

An example of a DateField in each of these three modes can be seen by running the ItemMIDlet and selecting the DateField screen. The result is shown in Figure 4-16. The left side of this figure shows DateFields configured with mode DateField.TIME at the top and DateField.DATE at the bottom, while the bottom DateField on the right side has mode DateField.DATE_TIME.

DateFields on the default color phone

Figure 4-16. DateFields on the default color phone

DateField allows the user to edit the date and/or time that it displays. In the reference implementation, if you start pressing keys or press the SELECT button on the emulator keypad while a DateField has the input focus, a full-screen editor appears. There are separate editors for dates and times, as shown in Figure 4-17.

DateField date and time editing helper components on the default color phone emulator

Figure 4-17. DateField date and time editing helper components on the default color phone emulator

Note that DateField is derived from Item and not from TextField, so it is not possible to gain access to the characters displayed on the screen as would be the case with TextField.

Like all Items, when the user changes the date and/or time displayed by a DateField, the change is reported to the ItemStateListener, if any, registered with the Form that the DateField is displayed on. The value of the Date object associated with the DateField can be obtained or changed using the following methods:

public void setDate(Date date);
public Date getDate( );

When the setDate method is called, the DateField does not store a reference to the Date that is passed to it. Instead, it copies the value so that changes made within the DateField component are not reflected in the Date object supplied. Similarly, the value returned by getDate( ) is a newly created object that reflects the date and/or time in the DateField at the time of the method call.

The setDate method may be called with argument null. In this case, the DateField is considered to be in an uninitialized state and does not display a valid value. The DateField is also in this state following construction and until setDate( ) is called with a valid Date. The getDate method returns null when the DateField is in this state, and, in the reference implementation, the time part displays the string <time> while the date part displays <date>.

DateField has a very simple programming interface, but there are some traps waiting for the unwary. The nature of these traps depends on the mode in which the DateField is operating.

DateField in DATE_TIME mode

This is the simplest case to handle. The only possible problem here arises from the fact that the DateField does not preserve the seconds and milliseconds value of the Date object that is passed to it. As a consequence of this, for example, if the setDate( ) method is called with a Date object for 10:04:03 P.M. on January 31, 2002, and no changes are made by the user, the value returned by the getDate( ) method corresponds to 10:04 P.M. on the same date.

DateField in DATE mode

In this mode, the DateField works only with the year, month, and date parts of the time and does not preserve the time elements. Therefore, the value returned by getDate( ) in this mode reports zero values for the time.

DateField in TIME mode

TIME mode causes the greatest inconvenience. According to the specification, in this mode, the Date passed to the setDate( ) method must have the date parts initialized to the “epoch” date, January 1, 1970, and the Date returned by getDate( ) contains this same date. The problem with this is that code like the following does not necessarily work as you might want it to:

Date now = new Date( );  // Current date and time
dateField.setDate(now);  // We want to display only the time

Ideally, the setDate method would ignore the date and display only the time. Unfortunately, the specification excludes this possibility. For predictable results, you have to pass in a Date value with the date parts set to those for the epoch. In the reference inplementation, if you fail to do this, the DateField considers its content to be invalid and puts itself into the uninitialized state, as if setDate(null) had been called. The following code extract can be used to create a Date object that contains the current time and the year, month, and day values for the epoch, without assuming what the epoch date is:

// Get Calendar for the epoch date and time
Calendar baseCal = Calendar.getInstance( );
Date baseDate = new Date(0);
baseCal.setTime(baseDate);

// Get Calendar for now and use the epoch
// values to reset the date to the epoch.
Calendar cal = Calendar.getInstance( );
Date now = new Date( );  
cal.setTime(now);

// Set the year, month and day in month from the epoch
cal.set(Calendar.YEAR, baseCal.get(Calendar.YEAR));
cal.set(Calendar.MONTH, baseCal.get(Calendar.MONTH));
cal.set(Calendar.DATE, baseCal.get(Calendar.DATE));

Changing the DateField mode

Under most circumstances, the DateField mode would not be changed following construction. If required, however, the mode can be changed using the setInputMode( ) method:

public void setInputMode(int mode);

where the mode argument is DateField.DATE, DateField.TIME, or DateField.DATE_TIME. Changing the mode affects the visual appearance of the component and may also affect the Date value that it contains, as follows:

Changing to DateField.DATE mode

The time part is reset to 00:00 A.M. on the date contained in the DateField.

Changing to DateField.TIME mode

The date part is reset to the epoch date, January 1, 1970.

ImageItems

ImageItem lets you place an image on a Form with some limited control over how it is placed relative to other Items. The ImageItem class has a single constructor:

public ImageItem(String label, Image image, int layout, String altText)

Adding an ImageItem to a Form causes the optional label and the image to be placed subject to the constraints specified by the layout argument. The device is free to ignore the layout argument and apply its own layout rules. It may also use the text supplied by the altText argument in place of the image when, in the words of the MIDP specification, “the image exceeds the capability of the device to display it.”

The image is supplied in the form of an Image object, which will be described in detail when we discuss the low-level API in Chapter 5. There are several ways to create an Image, including loading data over a network connection, using graphics primitives to compose the Image from lines, points, curves and solid shapes, and loading encoded data from a file. For the purposes of illustration, we will use the last of these methods in this chapter because it is easy to demonstrate and creates an immutable image, which is a requirement for ImageItem. [16]

To load an image from a file, use the following static method of the Image class:

public static Image createImage(String name) throws IOException

name is a resource name that corresponds to the location of the file in the MIDlet suite’s JAR file. The name parameter is used as the argument to the getResourceAsStream( ) method that was described in Section 4.2.3, earlier in this chapter. Although getResourceAsStream( ) can be given either an absolute or relative resource name, the name parameter should always be absolute in this case, because a relative name would not be interpreted as being relative to your MIDlet’s class (and, in fact, the class relative to which a relative resource name would be interpreted is implementation-dependent). The indicated file must contain an image encoded in Portable Network Graphics (PNG) format, since this is the only graphics file format that MIDP devices are required to support. Most of the commonly used utilities that allow you to design graphics or manipulate images provide the option to save in PNG format.

The layout parameter is a bitmask made up from legal combinations of the following values:

ImageItem.LAYOUT_DEFAULT

The image should be placed according to the platform’s default layout policy.

ImageItem.LAYOUT_LEFT

The image should be left-justified in the space available to it.

ImageItem.LAYOUT_RIGHT

The image should be right-justified in the space available to it.

ImageItem.LAYOUT_CENTER

The image should be centered in the space available to it.

ImageItem.LAYOUT_NEWLINE_BEFORE

A line break should occur before the image is drawn.

ImageItem.LAYOUT_NEWLINE_AFTER

A line break should occur after the image is drawn.

When LAYOUT_DEFAULT is used, the device places the image according to implementation-dependent rules. In the reference implementation, this value causes the ImageItem to be handled in the same way as StringItem -- that is, it is placed on the same horizontal line as the Item that precedes it, providing that both of the following conditions are met:

  • The ImageItem does not contain a nonempty label, because this always forces a line break.

  • The space remaining in the current line is not less than the width of the image.

If these conditions are not met, a line break occurs before the optional label and image are drawn. The remaining layout constraints may be mixed together subject to the following rules:

  • LAYOUT_LEFT, LAYOUT_RIGHT, and LAYOUT_CENTER are mutually exclusive. They determine how the image is placed within the remaining space on the current line.

  • LAYOUT_NEWLINE_BEFORE and LAYOUT_NEWLINE_AFTER can be used separately or together; they may also be used in conjunction with either LAYOUT_DEFAULT or one of LAYOUT_LEFT, LAYOUT_RIGHT, or LAYOUT_CENTER. Because LAYOUT_DEFAULT has value 0, a layout value of LAYOUT_NEWLINE_BEFORE is equivalent to LAYOUT_NEWLINE_BEFORE | LAYOUT_DEFAULT.

As a shorthand, you can add an image to a Form using the following Form method:

public void append(Image image);

This is equivalent to creating and appending an ImageItem with layout LAYOUT_DEFAULT and no label, that is:

form.append(new ImageItem(null, image, ImageItem.LAYOUT_DEFAULT, null));

You can see some examples of ImageItems by selecting the ImageItem entry from the list presented by ItemMIDlet. The result of running this example on the default color phone is shown in Figure 4-18 and on the PalmOS platform in Figure 4-19.

ImageItems as shown by the default color phone emulator

Figure 4-18. ImageItems as shown by the default color phone emulator

ImageItems displayed by MIDP for PalmOS

Figure 4-19. ImageItems displayed by MIDP for PalmOS

The top four lines all contain ImageItems that have both an image and a label. These components were created as follows:

Image red = Image.createImage("/ora/ch4/resources/red.png");
Image blue = Image.createImage("/ora/ch4/resources/blue.png");

// ImageItems with labels
form.append(new ImageItem("Center", red, ImageItem.LAYOUT_CENTER, null));
form.append(new ImageItem("Left", red, ImageItem.LAYOUT_LEFT, null));
form.append(new ImageItem("Right", red, ImageItem.LAYOUT_RIGHT, null));
form.append(new ImageItem("Default", red, ImageItem.LAYOUT_DEFAULT,
    null));

The layout arguments used here do not include LAYOUT_NEWLINE_BEFORE, so the images directly follow their labels. However, each ImageItem is placed on a line of its own even though LAYOUT_NEWLINE_AFTER is not specified, because each has a label, which forces a line break.

If you compare Figure 4-18 and Figure 4-19, you’ll notice that the image placements on the default color phone do not correspond to those requested by the layout argument: they all appear to be left-aligned, whereas the PalmOS implementation places them properly. This is not inconsistent with the MIDP specification, which allows a device to treat the layout parameter as a hint. It serves to illustrate that you cannot rely on having images placed exactly where you want them.

The last five ImageItems differ from the first four in two respects:

  • They do not have labels.

  • Three of them have layout values that include both LAYOUT_NEWLINE_BEFORE and LAYOUT_NEWLINE_AFTER.

The code used to add these components is as follows:

form.append(new ImageItem(null, blue, ageItem.LAYOUT_NEWLINE_BEFORE | 
    ImageItem.LAYOUT_CENTER |ImageItem.LAYOUT_NEWLINE_AFTER, null));
form.append(new ImageItem(null, blue, mageItem.LAYOUT_NEWLINE_BEFORE |
    ImageItem.LAYOUT_DEFAULT | ImageItem.LAYOUT_NEWLINE_AFTER, null));
form.append(new ImageItem(null, blue, ImageItem.LAYOUT_NEWLINE_BEFORE |
    ImageItem.LAYOUT_RIGHT | ImageItem.LAYOUT_NEWLINE_AFTER, null));
form.append(new ImageItem(null, blue, ImageItem.LAYOUT_DEFAULT, null)); 
form.append(new ImageItem(null, blue, ImageItem.LAYOUT_DEFAULT, null));

Because these ImageItems do not include labels, they would normally be laid out on a single line with no line breaks. The LAYOUT_NEWLINE_BEFORE and LAYOUT_NEWLINE_AFTER values cause each image to be preceded and followed by a line break. Note that only a single line break is used between each pair of images, even though it might appear that two newlines have been requested (i.e., one after each image and one before the image that follows it). The last two ImageItems are created with the layout argument set to LAYOUT_DEFAULT only. As a result, no line breaks are added, and, as you can see, they appear on the same line. The line break before the first ImageItem is due to the LAYOUT_NEWLINE_AFTER part of the layout attribute of the ImageItem on the line above.

Notice that the default color phone has obeyed the positioning constraints when placing these ImageItems, as you can see from the right side of Figure 4-18. At the time of writing, the MIDP reference implementation honors the LAYOUT_RIGHT and LAYOUT_CENTER constraints only if the layout attribute also includes both LAYOUT_NEWLINE_BEFORE and LAYOUT_NEWLINE_AFTER.

Gauges

A Gauge provides a way to represent a single selected value from a contiguous range of integers starting from 0 and ranging up to an application-supplied maximum. The Gauge class has a single constructor:

public Gauge(String label, boolean interactive, int maxValue, 
    int initialValue);

The maxValue and initialValue arguments specify, respectively, the largest value of the range covered by the gauge and the value that will be displayed initially. The minimum value is always implicitly zero, and the current value must always be positive and not greater than the maximum.

The interactive argument determines whether the user can adjust the value in the gauge. To use a gauge as a slider, you should set this argument to true. Adjustments made by the user are reported to the ItemStateListener attached to the Form on which the gauge is displayed. If interactive is false, the value of the gauge can be adjusted only under application control. In this mode, the gauge acts more like a progress bar.

The current value of a gauge can be obtained or changed using the following methods:

public int getValue( );
public void setValue(int value);

The value passed to the setValue( ) method must be nonnegative and less than or equal to the maximum value. The maximum value can itself be manipulated using similar methods:

public int getMaxValue( );
public void setMaxValue(int value);

The value passed to setMaxValue( ) must be greater than 0. If the new maximum value is less than the current value, the current value is reduced to the new maximum. Note that, as with all programmatic changes, this change in the current value is not reported to ItemStateListeners.

There is also a method that allows you to determine whether a gauge is interactive:

public boolean isInteractive( );

However, you cannot change this attribute: a gauge is either always interactive or always not interactive.

If you run the ItemMIDlet and select the Gauge example, you’ll see a screen displaying three gauges, all of which have a maximum value of 100, as shown in Figure 4-20. The code used to create this Form is as follows:

Form form = new Form("Gauge");
form.append(new Gauge(null, true, 100, 50));
form.append(new Gauge(null, true, 100, 25));
form.append(new Gauge(null, false, 100, 50));
Gauges as shown by the default color phone

Figure 4-20. Gauges as shown by the default color phone

The top two gauges are interactive, and the bottom one is not. Notice first that the Gauge that has the focus is distinguished from the the others in that its bars are fully drawn, while those of the other two are not. Also, the two interactive gauges have bars that increase in size from left to right, but the noninteractive one has bars of constant height.

These gauges represent their complete range using 10 bars, so that each bar corresponds to a range of 10 values. For a larger value range, each bar would correspond to a wider range of values. On the default color phone, the number of filled bars gives a guide to the current value of the gauge, but the user can see only an approximation of the real value, because each bar represents more than one possible value (a range of 10 possible values, in this case). On other devices, the gauge might use a different total number of bars to represent the same total value range, or it might not use bars at all. On the PalmOS platform, for example, both interactive and noninteractive gauges are represented quite differently from those on the default color phone, as shown in Figure 4-21.

Gauges on the PalmOS platform

Figure 4-21. Gauges on the PalmOS platform

On the default color phone, you can use the up and down arrow keys to move the input focus from gauge to gauge. When an interactive gauge has the focus, you can use the left and right arrow keys to adjust the current value up or down; horizontal arrows are drawn on the screen as a visual cue, as you can see at the bottom of the left screenshot in Figure 4-20. When the gauge is at its maximum value, the right-pointing arrow is not shown, and the right arrow key has no effect; the left arrow and key show similar behavior when the gauge is at its minimum value. No visual cues are shown when the input focus is assigned to a noninteractive gauge, as is the case in the right screen shot in Figure 4-20, because the user cannot change the value of this gauge.

If you change the value of either of the top two gauges with the arrow keys, you’ll notice that a message is written to the Wireless Toolkit console window to reflect every value change. This value is obtained by calling the Gauge.getValue( ) method from the Form’s itemStateChanged( ) method:

public void itemStateChanged(Item item) {  
    if (item instanceof Gauge) {
        int value = ((Gauge)item).getValue( );
        System.out.println("Gauge value set to " + value);
    } else {
        // Other code not shown here
    }
}

You have to click the right or left arrow key 10 times to affect the visual representation of the gauge, but the ItemStateListener is notified of each individual change.

An interactive gauge generally is used to allow the user to select one of a range of values, and the MIDlet usually interacts with it only when the user changes the value or when it is necessary to set a new value programmatically. By contrast, when the gauge is used as a progress bar, the MIDlet updates it regularly to reflect the state of an operation that it is performing.

ChoiceGroups and Lists

ChoiceGroup and List are two similar components that present the user with a set of choices and allow one or more them to be selected. The relationship between them is similar to that between TextField and TextBox: ChoiceGroup is an Item to be used as part of a Form, while List is derived from Screen and is therefore a freestanding component that occupies the entire screen. Most of the programming interface is common and is described by an interface called Choice. For simplicity, we’ll cover the common features by examining ChoiceGroup and then look at how List differs from it.

Creating a ChoiceGroup

There are two types of ChoiceGroup , distinguished by the number of items within the group that can be selected at the same time. The choice between these two types is made when the ChoiceGroup is created with one of its two constructors:

public ChoiceGroup(String label, int choiceType);
public ChoiceGroup(String label, int choiceType, String[] strings,
    Image[] images);

The choiceGroup parameter takes one of the following values, defined in the interface Choice, which ChoiceGroup implements:

Choice.EXCLUSIVE

Creates an exclusive ChoiceGroup in which only one item can be selected and which, therefore, acts like a collection of radio buttons

Choice.MULTIPLE

Creates a multiple-selection ChoiceGroup, which is like a set of check boxes, in which any number of items can be selected.

You can see examples of both types of ChoiceGroup by running the ItemMIDlet and choosing the ChoiceGroup entry. The result of running this on the default color phone is shown in Figure 4-22, with an EXCLUSIVE ChoiceGroup on the left and a MULTIPLE ChoiceGroup on the right.

ChoiceGroups on the default color phone

Figure 4-22. ChoiceGroups on the default color phone

There are two ways to initialize a ChoiceGroup: the selections can be added using the second of the constructors shown above, or they can be added following construction. The left ChoiceGroup in Figure 4-22 was initialized at construction time:

Image red = Image.createImage("/ora/ch4/resources/red.png");
Image green = Image.createImage("/ora/ch4/resources/green.png");
Image blue = Image.createImage("/ora/ch4/resources/blue.png");

// Exclusive choice group
String[] strings = new String[] { "Red", "Green", "Blue" };
Image[] images = new Image[] { red, green, blue };
ChoiceGroup exGroup = new ChoiceGroup("Choose one", 
    ChoiceGroup.EXCLUSIVE, strings, images);

Each element of a ChoiceGroup consists of a string and an optional Image that the device may display near the string, although it is not obliged to display the Image at all. When using the constructor to initialize a ChoiceGroup, the following rules must be followed:

  • The strings argument must not be null and no element of the strings array can be null. This restriction implies that image-only entries are not supported.

  • The images argument may be null if images are not required.

  • If the images argument is not null, it must have the same number of elements as the strings array. The image at index N of the images array corresponds to the string at element N of the strings array. Images must be immutable. Any element in the images array may be null if an image is not required for that entry of the ChoiceGroup.

The device is responsible for rendering the ChoiceGroup in such a way as to visually distinguish an EXCLUSIVE ChoiceGroup from a MULTIPLE one. As shown in Figure 4-22, the default color phone achieves this by following the common convention of using a circle to represent a radio button in the EXCLUSIVE group and a square for a check box in the MULTIPLE group. This is not the only way to achieve this differentiation, however, as you can see in Figure 4-23, later in this chapter, which shows the same ChoiceGroups as those in Figure 4-22 as they appear on the PalmOS platform. Note that the EXCLUSIVE ChoiceGroup is represented in the form of a popup menu, which shows only the selected item when the menu is not visible. This figure also illustrates that a platform is not obliged to use Images even if they are supplied.

An alternative way to initialize a ChoiceGroup is to add entries after construction. This is how the multiple-choice group shown on the right side of Figure 4-22 was created:

ChoiceGroup multiGroup = new ChoiceGroup("Choose any", 
    ChoiceGroup.MULTIPLE);
multiGroup.append("Use SSL", null);
multiGroup.append("Reconnect on failure", null);
multiGroup.append("Enable tracing", null);

The append( ) method supplies both the string and the optional image, in that order:

public int append(String string, Image image);

This method requires that the string argument is not null. The image argument may be null if no image is required; if it is not null, the Image that it refers to must be immutable. The value returned by this method is the index of the entry created within the ChoiceGroup, so the first call in the code example would return 0, the second would return 1, and so on. The append( ) method is one of several methods from the Choice interface that can be used to change the content of the ChoiceGroup at any time. The other methods are described in Section 4.2.13.4, a little later in the chapter.

Handling selection

When a ChoiceGroup has the input focus, the user can navigate from item to item within it with the up and down arrow keys (or their equivalents) on the phone keypad. These internal navigation operations are not visible to application code. To change the selected state of an entry, the user must press the device’s SELECT button. The location of the SELECT button on the default color phone is shown in Figure 4-5. The effect that this has depends on the ChoiceGroup type:

MULTIPLE ChoiceGroup

Pressing the SELECT button when an element that is not currently selected has the focus results in that element being selected, but it does not affect the state of any other item. Pressing the SELECT button for an item that is already selected has the effect of deselecting it.

EXCLUSIVE ChoiceGroup:

Because only one item in an exclusive group may be selected at any time, selecting an element clears the previous selection. Attempting to select an element that is already selected has no effect. (It does not deselect the entry; this would result in no element being selected, which is not allowed.)

Changes in the selection state of an element within a ChoiceGroup are reported to the ItemStateListener of the Form on which the ChoiceGroup is displayed. In the case of a multiple-selection group, notification occurs whenever an element is selected or deselected. For an exclusive group, selecting one element implicitly deselects another element, but only one notification takes place.

Handling state changes using an ItemStateListener is appropriate for applications where an immediate response is required, perhaps to update some other part of the user interface to reflect the user’s selection. On the other hand, the ChoiceGroup might be part of a larger input form whose contents will be processed as a single unit when all fields have been filled in. In this case, you add a Command (typically Command.OK) to the Form and implement the logic of the Form in its commandAction( ) method. Whichever approach you take, you need to be able to find out which elements of the ChoiceGroup are selected. ChoiceGroup has three methods that can be used to get the current selection state:

public boolean isSelected(int index);
public int getSelectedIndex( );
public int getSelectedFlags(boolean[] flags);

The isSelected( ) method returns true if the element with the given index is selected, false if it is not. This method is most often used with multiple-selection ChoiceGroups where each check box represents a different program action that is likely to be independent of the others. Typical code for this case might look like this:

public static final int USE_SSL = 0;
public static final int RECONNECT_ON_FAILURE = 1;
public static final int TRACING_ENABLED = 2;
....
do {
    if (multiGroup.isSelected(USE_SSL)) {
        // Connect using SSL
    } else {
        // Connect using vanilla sockets
    }
    if (failed && multiGroup.isSelected(TRACING_ENABLED)) {
        // Log failure
    }
} while (failed && multiGroup.isSelected(RECONNECT_ON_FAILURE));

In the case of an EXCLUSIVE ChoiceGroup, since only one element can be selected, the getSelectedIndex( ) method can be used to determine its index:

public static final int RED = 0;
public static final int GREEN = 1;
public static final int BLUE = 2;
....
int index = exGroup.getSelectedIndex( );
if (index == RED) {
    // Act on red selection
}

This method always returns -1 if it is called for a multiple-choice ChoiceGroup because there could be more than one selected element. It also returns -1 if the ChoiceGroup has no elements at all (which is unlikely in practice).

If you need to get the selection state of every element in the ChoiceGroup, the getSelectedFlags( ) method should be used. This method requires an array of booleans that has at least as many elements as there are items in the ChoiceGroup; it sets each entry in the array to true or false depending on whether the corresponding entry is selected. The return value is the number of items that are selected. Before invoking this method, you need to allocate a boolean array of the appropriate size. If the number of elements in the ChoiceGroup is not constant, you can use the size( ) method to find out how many there are:

boolean[] flags = new boolean[multiGroup.size( )];
int count = multiGroup.getSelectedFlags(flags);
do {
    if (flags[USE_SSL]) {
            // Connect using SSL
        } else {
            // Connect using vanilla sockets
        }
    if (failed && flags[TRACING_ENABLED]) {
        // Log failure
    }
} while (failed && flags[RECONNECT_ON_FAILURE]);

This technique works for both types of ChoiceGroup.

Finally, to get the value of an element within the ChoiceGroup, use the getString( ) method:

public String getString(int index);

The following code extract returns either “Red”, “Green”, or “Blue”:

String color = exGroup.getString(exGroup.getSelectedIndex( ));

The code that handles selection changes for the ChoiceGroups used in the ItemMIDlet is shown in Example 4-5.

Example 4-5. Handling Selection Changes in a ChoiceGroup or List Component

// Handles the selection for a Choice 
private void handleChoiceSelection(Choice choice) {
    int count = choice.size( );
    boolean[] states = new boolean[count];
    int selCount = choice.getSelectedFlags(states);
    if (selCount > 0) {
        System.out.println("Selected items:");
        for (int i = 0; i < count; i++) {
            if (states[i]) {
                System.out.println("\t" + choice.getString(i));
            }
        }
    } else {
        System.out.println("No selected items.");
    }
    int selectedIndex = choice.getSelectedIndex( );
    System.out.println("Selected index is " + selectedIndex);  
}

This method, which is called from the ItemStateListener attached to the Form in which the ChoiceGroups are contained, is given an argument of type Choice instead of ChoiceGroup. When it is invoked, however, the calling code passes a reference to the ChoiceGroup. This is acceptable, because ChoiceGroup implements the Choice interface. The benefit of requiring an argument of type Choice instead of ChoiceGroup is that this same code can also be used to handle selection changes for the List component, which also implements Choice.

The aim of this code is simply to demonstrate a couple of ways of handling selection changes. The first part of this code uses the getSelectedFlags( ) method to get an array of booleans that shows which elements are selected. The code then loops over the returned array, gets the strings corresponding to selected entries, and prints them. The second part of the method uses the getSelectedIndex( ) method to access directly the index of the selected item, which, as noted above, returns a meaningful result only for an EXCLUSIVE ChoiceGroup. Selecting the Green item in the ChoiceGroup example and pressing the SELECT button results in the following output in the Wireless Toolkit console:

Selected items:
        Green
Selected index is 1

Because this is an EXCLUSIVE ChoiceGroup, only one item can ever be selected, so the getSelectedIndex( ) method is able to return its index. Selecting Use SSL in the multiple-choice ChoiceGroup gives this result:

Selected items:
        Use SSL
Selected index is -1

Here, getSelectedIndex( ) has returned -1 because the type is MULTIPLE. Selecting another entry in the same ChoiceGroup results in the following:

Selected items:
        Use SSL
        Enable tracing
Selected index is -1

As you can see, the getSelectedFlags( ) method returns all the selected items.

Setting and changing the selection

The selection state of the elements within a ChoiceGroup can be changed programmatically using the following methods:

public void setSelectedIndex(int index, boolean selected);
public void setSelectedFlags(boolean[] flags);

The effect of the setSelectedIndex( ) method depends on the ChoiceGroup type. In the multiple-choice case, this method selects or deselects the element at the given index, depending on the value of the selected argument. In an exclusive ChoiceGroup, however, this method has an effect only if the selected argument has value true. In this case, it selects the element at index and deselects the element that was previously selected. If selected is false, the call is ignored. This happens because an exclusive ChoiceGroup must always have one selected element, so it is not possible simply to deselect the element that is currently selected without selecting another element at the same time.

You can set the selected state of all the elements in a ChoiceGroup by calling the setSelectedFlags( ) method, passing it an array of booleans containing true for those elements that are to be selected and false for those that are not. The boolean array must contain an entry for each element in the ChoiceGroup:

public boolean[] initialStates = new boolean[3];
initialStates[RECONNECT_ON_FAILURE] = true;  // Select just this element
multiGroup.setSelectedFlags(initialStates);

In the multiple-choice case, any number of entries in the array can be true. Since exclusive ChoiceGroups can have only one element selected, in this case the boolean array must have exactly one entry with value true. If this is not the case, the following selection rules apply:

  • If the array has no entries set to true, the first entry in the ChoiceGroup is selected.

  • If the array has more than one entry set to true, the element in the ChoiceGroup corresponding to the first true entry is selected.

Note that changing the selection using these methods does not result in notification to the Form’s ItemStateListener.

Changing the content of a ChoiceGroup

The content of a ChoiceGroup can be changed at any time using the following methods:

public int append(String string, Image image);
public void insert(int index, String string, Image image);
public void set(int index, String string, Image image);
public void delete(int index);

The append( ) method, which has already been discussed, adds a new element to the end of the ChoiceGroup. The insert( ) method is similar, except that it places the new entry at the given index, moving the element at that index and all higher indexes down to make room for the new one. This method can also be used to add an element at the end of the ChoiceGroup by supplying the size of the ChoiceGroup as the insertion index:

multiGroup.insert(multiGroup.size( ), "New Entry", null);

Insertion indexes greater than size( ) are invalid and cause an IndexOutOfBoundsException to be thrown.

The set( ) method replaces the content of an existing element with new values. Both the string and image parts of the element are changed: it is not possible to change only one of these attributes by supplying null for the other. For all of these methods, the image argument may be null if no image is required, but the string argument must not be null. If an image is supplied, it must be immutable.

Finally, an element can be removed from the ChoiceGroup using the delete( ) method:

public void delete(int index);

Changing the content of a ChoiceGroup may have an effect on its selection state. The rules that apply are as follows:

  • Adding an item using the append( ) method has no effect on the selection. The only exception to this is an EXCLUSIVE ChoiceGroup that was previously empty. In this case, the newly added element is selected.

  • Inserting an element using the insert( ) method preserves the selected state of each existing item in the list, but, of course, the indexes of the selected items may change. As an example of this, if elements 2 and 3 are selected and a new element is inserted at index 2, the selected item indexes change to 3 and 4. As a special case, as with the append( ) method, if an EXCLUSIVE ChoiceGroup was previously empty, the new element is selected.

  • Replacing an item using the set( ) method gives the new item the same selected state as the item that it replaced.

Deleting an item has no effect on the selection state of other items, except when the selected element in an EXCLUSIVE ChoiceGroup is deleted. In this case, if the deleted item is not at the end of the list, the item that replaces it is selected (that is, the selected index remains the same). If the selected item is the last item, then the element that becomes the last item is selected instead.

The List component

List is a full-screen version of ChoiceGroup that shares most of its programming interface. The common functionality is grouped into an interface called Choice, which has the following methods:

public int append(String string, Image image);
public void delete(int index);
public Image getImage(int index);
public int getSelectedFlags(boolean[] flags);
public int getSelectedIndex( );
public String getString(int index);
public void insert(int index, String string, Image image);
public boolean isSelected(int index);
public void set(int index, String string, Image image);
public void setSelectedFlags(boolean[] flags);
public void setSelectedIndex(int index, boolean selected);
public int size( );

List has two constructors that mirror those of ChoiceGroup and work in exactly the same way:

public List(String title, int type);
public List(String title, int type, String[] strings, Image[] images);

The only difference is that the first parameter is used to set the title of the List’s screen, whereas it is used as the label for a ChoiceGroup.

As well as supporting the EXCLUSIVE and MULTIPLE modes of operation, List has a third mode, selected by setting the type to Choice.IMPLICIT, which cannot be used with ChoiceGroup. IMPLICIT mode creates a List that behaves somewhat like a standard list (e.g., the Swing JList component), with the restriction that only one element can be selected at a time. This mode is often used to create a menu, and, in fact, this is how the list of MIDlets in a MIDlet suite is presented when you launch the Java VM (see Figure 3-8). If you select the List item from the menu presented by ItemMIDlet, you’ll see an IMPLICIT List, as shown on the left side of Figure 4-23. The code used to create this list is very similar to the corresponding ChoiceGroup code:

List list = new List("List", List.IMPLICIT);
Image red = Image.createImage("/ora/ch4/resources/red.png");
Image green = Image.createImage("/ora/ch4/resources/green.png");
Image blue = Image.createImage("/ora/ch4/resources/blue.png");

list.append("Red", red);
list.append("Green", green);
list.append("Blue", blue);
List component on the default color phone

Figure 4-23. List component on the default color phone

The same List as displayed on the PalmOS platform is shown in Figure 4-24. As you can see, the cell phone emulator displays both the string and the image associated with each element in the list, whereas the PalmOS implementation ignores the image, as it did in the case of the ChoiceGroup.

List component on the PalmOS platform

Figure 4-24. List component on the PalmOS platform

The IMPLICIT and EXCLUSIVE modes are very similar, in that both require exactly one element of the list to be selected at any time. In fact, all previous comments regarding EXCLUSIVE mode made in connection with ChoiceGroup also apply to the IMPLICIT mode of List. The difference between these modes can be seen by comparing the IMPLICIT List on the left side of Figure 4-23 with the EXCLUSIVE ChoiceGroup on the right. As you can see, the ChoiceGroup has a separate radio button that indicates which element is selected, whereas the List does not. This means that the highlighted element in the ChoiceGroup (Green) need not be the same as the selected element (Red). In the case of an IMPLICIT list, however, the highlighted element is implicitly considered to be the selected element, which is why this is referred to as IMPLICIT mode.

Since List is not an Item, changes in its selection state cannot be notified to application code via an ItemStateListener. In fact, there is no way to detect when the selected element of a List changes, in any of its modes, until the user activates a Command installed on the List that would prompt application code to examine its selection state.[17] In the case of an IMPLICIT List, however, if the user presses the SELECT key on the cell phone keypad or presses the arrow to the left of each item on the PalmOS platform (refer to Figure 4-24), the List’s CommandListener, if there is one, is notified of a selection change. Usually, when the commandAction( ) method of the CommandListener is called, it is passed a reference to the application-supplied Command that was activated and the Displayable to which the Command was attached. In this case, however, there is no application Command associated with the selection action, so the List provides a private Command called List.SELECT_COMMAND, which indicates that the commandAction( ) method has been called as a result of an IMPLICIT List selection. The Displayable argument passed to commandAction( ) refers to the List itself.

To associate a CommandListener with a List, you use the setCommandListener( ) method inherited from Displayable:

List list = new List("List", Choice.IMPLICIT);
list.setCommandListener(new CommandListener( ) {
    public void commandAction(Command c, Displayable d) {
        // Handle notification from the List
    }
});

The ItemMIDlet commandAction( ) method includes a case that detects the List selection change:

public void commandAction(Command c, Displayable d) {
    if (c == List.SELECT_COMMAND) {
        // Selection made in the IMPLICIT LIST
        handleChoiceSelection((Choice)d);
    } else {
         // Other cases not shown
    }
}

Because List implements the Choice interface, you can use the same methods to handle the selection as those shown in connection with ChoiceGroup. In fact, the previous code uses the same handleChoiceSelection( ) method as was used to handle the ChoiceGroup selection in Example 4-5. Notice that a reference to the List is obtained by casting the Displayable to an object of type Choice. This is correct because the notification comes from the List (the Displayable), which implements Choice.

Warning

Note carefully that this discussion applies only to Lists created with type IMPLICIT. The CommandListener will not be notified of any selection change for Lists of type MULTIPLE or EXCLUSIVE. For these types, it is necessary to attach to the List a Command that notifies application code that the List selection should be checked.

The behavior of the Choice methods concerned with selection handling in the IMPLICIT mode is the same as that for EXCLUSIVE mode, as described in the earlier sections Section 4.2.13.2 and Section 4.2.13.3.

Alerts

Alert is a subclass of Screen that behaves much like a dialog, albeit with very limited functionality. When an Alert is displayed by calling the Display setCurrent( ) method, it covers some or all of the device screen and receives all key and pointer events generated by user action while it is visible. An Alert may be modal or nonmodal. In this context, an Alert is modal if it remains displayed until the user explicitly dismisses it. A nonmodal dialog, by contrast, is displayed for a limited maximum time period before being closed automatically.

Alert has several attributes that determine its appearance and behavior:

Title

This attribute is inherited from Screen. An Alert is not required to have a title.

String

This attribute contains the message that the Alert displays to the user. Line breaks may be created within the message by including newline characters.

Image

An optional image may be provided to be displayed along with the message. The way in which the image is displayed, and whether it is displayed at all, is device-dependent.

Timeout

Specifies how long the Alert is displayed. A default timeout is applied if no explicit timeout value is set. The distinguished value Alert.FOREVER is used to indicate that the Alert should be displayed until the user dismisses it. There is no requirement for the device to provide a means for the user to remove an Alert with a finite timeout value before the timeout expires. This feature should be used with care to ensure that the user does not have to wait an unduly long period for a simple confirmation message to time out and dismiss itself.

Type

This attribute, which is of type AlertType , conveys the intent of the Alert to the platform. The platform may use this attribute to tailor the alert’s visual appearance to help the user distinguish between errors, warnings, and informational messages. The platform may also generate an appropriate sound to draw the user’s attention to the alert. An Alert is not required to have an AlertType, and the platform is not required to act upon it even if it does. The available types are:

AlertType.ALARM
AlertType.CONFIRMATION
AlertType.ERROR
AlertType.INFO
AlertType.WARNNG

Note that the CONFIRMATION type is intended to confirm to the user that an action previously requested has been completed, not to solicit something like a Yes, No, or Cancel response before an action is performed. In fact, it is not possible to construct an Alert that accepts any input. If you want to get confirmation from a user before performing an action, you must construct and display a Form containing the appropriate Commands to allow the user to approve or cancel the proposed action.

You can see all the available Alert types and how the timeout works by selecting the Alert example from the ItemMIDlet. This example lets you configure the attributes of an Alert and display the result. When the example starts, you see a Form containing two ChoiceGroup s. Figure 4-25 shows how this looks on the default color phone, where you need to scroll to see all of the Form.

Configuring an Alert on the default color phone

Figure 4-25. Configuring an Alert on the default color phone

The first ChoiceGroup, shown on the left side of Figure 4-25, lets you select the timeout for the Alert, which can be either Alert.FOREVER or a value specified in seconds. If you select the second item in the ChoiceGroup, a Gauge appears so that you can adjust the timeout value, as shown on the right side of the figure. The second ChoiceGroup allows you to choose the AlertType. On the PalmOS platform, this Form has a more compact representation, shown in Figure 4-26, but the functionaility is the same.

Configuring an Alert on the PalmOS platform

Figure 4-26. Configuring an Alert on the PalmOS platform

Once you’ve configured the Alert, you can use the OK command to display it. On the PalmOS platform, this is available as a button on the Form, but on other devices you might need to access a soft-key menu to locate it. Pressing the OK button displays the Alert, and, on some devices, a sound plays (the specific sound may depend on the AlertType). If you experiment with different types, you’ll notice that, with the exception of the text message (which is constructed by ItemMIDlet to remind you of the parameters that you selected), there is no difference in appearance on the default color phone. The alerts all look like the one shown on the upper left side of Figure 4-27. The exception is when you select a timeout of Alert.FOREVER: you get a command button labeled Done that allows you to dismiss the Alert at any time.

On the PalmOS platform, however, the Alerts use different icons to indicate the AlertType. Furthermore, as you can see from Figure 4-27, when a finite timeout is selected, the time remaining until the Alert is dismissed counts down in a small circle in its bottom left. When the timeout value is set to Alert.FOREVER, there is a Done button in this area instead.

Various Alerts on the default color phone and the PalmOS platform

Figure 4-27. Various Alerts on the default color phone and the PalmOS platform

Warning

Only Alerts with the timeout set to Alert.FOREVER can be dismissed by the user; the others remain displayed until their timeout expires. You can’t get around this by trying to add your own Done button, because Alert overrides the addCommand and setCommandListener methods of Displayable and throws an IllegalStateException if you try to add a Command or install a CommandListener.

The code used to create and display an Alert is simple:

Alert alert = new Alert("Alert Title", "This is an Alarm", alarmImage,
    AlertType.ALARM);
Display.getDisplay(this).setCurrent(alert);

As you can see, you use the Display setCurrent( ) method to display an Alert, just as you would any other type of Displayable. The Alert partially or completely covers the screen that was active when the setCurrent( ) method is called. When the Alert is dismissed, the original screen is redisplayed. In some cases, though, it might be appropriate to show a different screen once the Alert has closed. You can arrange for this to happen by using a different form of setCurrent( ):

public void setCurrent(Alert alert, Displayable displayable);

This method first displays the given Alert; when it closes, the Displayable given as the second argument appears instead of the screen that was originally displayed. As a special case, passing null for the Displayable reverts to the original screen -- that is, it behaves just like the single-argument variant of setCurrent( ).

Alert has several methods that you can use to customize it after creation or to get some of its attributes. The AlertType can be obtained or changed using the following methods:

public void setAlertType(AlertType alertType);
public AlertType getAlertType( );

You can get or change the text string and image using similar methods:

public void setString(String string);
public String getString( );
public void setImage(Image image);
public Image getImage( );

When an Alert is created, a default timeout is applied, which you can get using the getDefaultTimeout( ) method:

public int getDefaultTimeout( );

Note that this is not a static method, so you have to create an Alert before you can use it. There is no method to change the default timeout. The returned value is in milliseconds.

Finally, you can get or change the actual timeout for a specific Alert using the following methods, where the time is again measured in milliseconds:

public int getTimeout( );
public void setTimeout(int timeOut);

If you call setTimeout( ) with the argument Alert.FOREVER, then the Alert will be modal.

Warning

An Alert with timeout set to Alert.FOREVER is described in the MIDP specification as a modal dialog, but it is not modal in the same sense that a J2SE Dialog or JDialog is. In particular, when you display a J2SE modal dialog by calling its show( ) or setVisible( ) method, control is not returned until the dialog is dismissed. The same is not true of Alert. That is, in the following code the setCurrent( ) method returns control immediately; it does not wait for the Alert to be dismissed:

Alert alert = new Alert("Modal", "Modal Alert", null, AlertType.ALARM);
alert.setTimeout(Alert.FOREVER); // Make the Alert "modal"
Display.getDisplay(this).setCurrent(alert)
// Returns IMMEDIATELY

In fact, the Alert may not actually have been displayed when it returns, as discussed in Section 4.1.1, earlier in this chapter.

Playing Sounds

MIDP does not currently have an API for playing arbitrary sounds, but it is possible to create a small set of sounds on devices that support it by using the AlertType method on its own. The AlertType method has a public method that plays its associated sound:

public boolean playSound(Display display);

where display is the Display object associated with the MIDlet. The following code extract requests that the device play the sound associated with an ALARM:

AlertType.ALARM.playSound(Display.getDisplay(this));

The device is not obliged to play any sounds or to generate a different sound for each AlertType. If the playSound( ) method actually plays a sound, it returns true. You can experiment with the various sounds by selecting the Sounds example from ItemMIDlet, choosing each sound in turn from the List component that the MIDlet displays, and using the SELECT key (or its equivalent) to play it. The return value of the playSound( ) method is written to the Wireless Toolkit console. If you are running this example on a PalmOS device, and you don’t hear any sounds, be sure to enable System Sounds using the Prefs applet from the main screen.



[13] A small number of examples in this section produce output on the MIDlet’s standard output stream. When using the Wireless Toolkit, this stream usually directs its output to the Wireless Toolkit console. However, if you use the PalmOS device emulator, this information is written to a separate file instead. To examine the file content, you must stop the emulator. For further details, see Chapter 9.

[14] Unfortunately, at the time of writing, the MIDP implementation used in the Wireless Toolkit does not do this.

[15] As noted earlier, TextBox does not have any way of notifying application code that its content has changed because it is not an Item and therefore cannot be associated with an ItemStateListener. Application code normally retrieves the content of a TextBox (using getString( ) or getChars( )) only when prompted to do so by the activation of a Command attached to the TextBox.

[16] An immutable image is one that cannot be changed in situ. Some methods of building an Image produce an immutable Image, while others result in one that is mutable. As you’ll see in Chapter 5, an immutable Image can always be obtained from a mutable one, so any Image you create can be used in conjunction with an ImageItem, either directly or after being made immutable.

[17] Strictly speaking, this is not true, because you could periodically examine the selection state from a background thread or on expiration of a Timer, but such tactics are not likely to be useful in a real application!

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