Learning Java, 4th Edition

Pop-Up Menus

One of Swing’s nifty components is JPopupMenu, a context menu that appears at the mouse location when you press the appropriate mouse button or keystroke. (On a two-button mouse, clicking the right mouse button invokes a pop-up menu. On a single-button Mac, you Command-click.) Which button you press depends on the platform you’re using; fortunately, from the code’s point of view you don’t have to care—Swing figures it out for you.

The care and feeding of JPopupMenu is basically the same as any other menu. You use a different constructor—JPopupMenu()—to create it, but otherwise, you build a menu and add elements to it the same way. The big difference is that you don’t attach it to a JMenuBar. Instead, just pop up the menu whenever and wherever you need it. Prior to Java 5.0, this process is a little cumbersome; you have to register to receive the appropriate mouse events, check them to see if they are the pop-up trigger and then pop the menu manually. With Java 5.0, the process is simplified by having components manage their own pop-up menus.

First, we’ll show an example of explicit pop-up handling. The following example, PopupColorMenu, contains three buttons. You can use a JPopupMenu to set the color of each button or the background frame itself, depending on where you click the mouse.

    //file: PopUpColorMenu.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class PopUpColorMenu implements ActionListener
    {
      Component selectedComponent;

      public PopUpColorMenu() {
        JFrame frame = new JFrame("PopUpColorMenu v1.0");

        final JPopupMenu colorMenu = new JPopupMenu("Color");
        colorMenu.add(makeMenuItem("Red"));
        colorMenu.add(makeMenuItem("Green"));
        colorMenu.add(makeMenuItem("Blue"));

        MouseListener mouseListener = new MouseAdapter() {
          public void mousePressed(MouseEvent e) { checkPopup(e); }
          public void mouseClicked(MouseEvent e) { checkPopup(e); }
          public void mouseReleased(MouseEvent e) { checkPopup(e); }
          private void checkPopup(MouseEvent e) {
            if (e.isPopupTrigger()) {
              selectedComponent = e.getComponent();
              colorMenu.show(e.getComponent(), e.getX(), e.getY());
            }
          }
        };

        Container content = frame.getContentPane(); // unnecessary  in 5.0+
        content.setLayout(new FlowLayout());
        JButton button = new JButton("Uno");
        button.addMouseListener(mouseListener);
        content.add(button);
        button = new JButton("Due");
        button.addMouseListener(mouseListener);
        content.add(button);
        button = new JButton("Tre");
        button.addMouseListener(mouseListener);
        content.add(button);

        frame.getContentPane().addMouseListener(mouseListener);

        frame.setSize(200,50);
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setVisible(true);
      }

      public void actionPerformed(ActionEvent e) {
        String color = e.getActionCommand();
        if (color.equals("Red"))
          selectedComponent.setBackground(Color.red);
        else if (color.equals("Green"))
          selectedComponent.setBackground(Color.green);
        else if (color.equals("Blue"))
          selectedComponent.setBackground(Color.blue);
      }

      private JMenuItem makeMenuItem(String label) {
        JMenuItem item = new JMenuItem(label);
        item.addActionListener( this );
        return item;
      }

      public static void main(String[] args) {
         new PopUpColorMenu();
      }
    }

Figure 17-8 shows the example in action; the user is preparing to change the color of the bottom button.

Figure 17-8. The PopupColorMenu application

Because the pop-up menu is triggered by mouse events (in this example), we need to register a MouseListener for any of the components to which it applies. In this example, all three buttons and the content pane of the frame are eligible for the color pop-up menu. Therefore, we add a mouse event listener for all these components explicitly. The same instance of an anonymous inner MouseAdapter subclass is used in each case. In this class, we override the mousePressed(), mouseReleased(), and mouseClicked() methods to display the pop-up menu when we get an appropriate event. How do we know what an “appropriate event” is? Fortunately, we don’t need to worry about the specifics of our user’s platform; we just need to call the event’s isPopupTrigger() method. If this method returns true, we know the user has done whatever normally displays a pop-up menu on his system.

Once we know that the user wants to raise a pop-up menu, we display it by calling its show() method with the mouse event coordinates as arguments.

If we want to provide different menus for different types of components or the background, we create different mouse listeners for each different kind of component. The mouse listeners invoke different kinds of pop-up menus as appropriate.

The only thing left is to handle the action events from the pop-up menu items. We use a helper method called makeMenuItem() to register the PopUpColorMenu window as an action listener for every item we add. The example implements ActionListener and has the required actionPerformed() method. This method reads the action command from the event, which is equal to the selected menu item’s label by default. It then sets the background color of the selected component appropriately.

Component-Managed Pop Ups

Things get a bit easier in Java 5.0, using the new pop-up menu API for components. In Java 5.0, any JComponent can manage a JPopupMenu directly with the setComponentPopupMenu() method. JComponents can also be told to simply inherit their parent container’s pop-up menu via the setInheritsPopupMenu() method. This combination makes it very simple to implement a context menu that should appear in many components within a container.

Unfortunately, this doesn’t lend itself well to our previous example (PopupColorMenu) for two reasons. First, we need to know which component the mouse was in when the pop up was triggered and we don’t get that information using this API. The pop-up handling is actually delegated to the container, not inherited. Second, not all types of components are registered to receive mouse events by default.^[40] As a result, we’ll create a new example that is more appropriate for a “one context menu to rule them all” application. The following example, ContextMenu, shows a TextArea and TextField that both inherit the same JPopupMenu from their JPanel container. When you select a menu item, the action is displayed in the text area.

    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class ContextMenu implements ActionListener
    {
      JTextArea textArea = new JTextArea();

      public ContextMenu()
      {
        final JPopupMenu contextMenu = new JPopupMenu("Edit");
        contextMenu.add(makeMenuItem("Save"));
        contextMenu.add(makeMenuItem("Save As"));
        contextMenu.add(makeMenuItem("Close"));

        JFrame frame = new JFrame("ContextMenu v1.0");
        JPanel panel = new JPanel();
        panel.setLayout( new BorderLayout() );
        frame.getContentPane( ).add( panel );
        panel.setComponentPopupMenu( contextMenu );

        textArea.setInheritsPopupMenu( true );
        panel.add( BorderLayout.CENTER, textArea );

        JTextField textField = new JTextField();
        textField.setInheritsPopupMenu( true );
        panel.add( BorderLayout.SOUTH, textField );

        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize(400,200);
        frame.setVisible(true);
      }

      public void actionPerformed( ActionEvent e ) {
        textArea.append( e.getActionCommand() +"\n" );
      }

      private JMenuItem makeMenuItem(String label) {
        JMenuItem item = new JMenuItem(label);
        item.addActionListener( this );
        return item;
      }

      public static void main(String[] args) {
         new ContextMenu();
      }
    }

We’ve constructed our JPopupMenu as before, but this time we are not responsible for listening for mouse clicks or triggering the pop up explicitly. Instead, we use the setComponentPopupMenu() method to ask the JPanel to handle it for us. We use setInheritsPopupMenu() on both the JTextArea and JTextField so that they will both delegate pop-up trigger mouse clicks to the JPanel automatically.

---

^[40] Components such as JPanel and JLabel by default do not expect to handle mouse events. When you register a listener such as MouseListener, it registers itself internally to begin processing these events. Unfortunately, at the time of this writing, using setInheritsPopupMenu() does not trigger this functionality. As a workaround, you could register a dummy mouse listener with these components to prompt them to expect mouse events and properly trigger context menus if you want them.