Learning Java

Object Creation

Objects in Java are allocated from a system heap space, much like malloc‘ed storage in C or C++. Unlike in C or C++, however, we needn’t manage that memory ourselves. Java takes care of memory allocation and deallocation for you. Java explicitly allocates storage for an object when you create it with the new operator. More importantly, objects are removed by garbage collection when they’re no longer referenced.

Constructors

Objects are allocated by specifying the new operator with an object constructor. A constructor is a special method with the same name as its class and no return type. It’s called when a new class instance is created, which gives the class an opportunity to set up the object for use. Constructors, like other methods, can accept arguments and can be overloaded (they are not, however, inherited like other methods; we’ll discuss inheritance in Chapter 6).

class Date {  
    long time;  
  
    Date( ) {  
        time = currentTime( );  
    }  
  
    Date( String date ) {  
        time = parseDate( date );  
    }  
    ...  
}

In this example, the class Date has two constructors. The first takes no arguments; it’s known as the default constructor. Default constructors play a special role: if we don’t define any constructors for a class, an empty default constructor is supplied for us. The default constructor is what gets called whenever you create an object by calling its constructor with no arguments. Here we have implemented the default constructor so that it sets the instance variable time by calling a hypothetical method, currentTime( ), which resembles the functionality of the real java.util.Date class. The second constructor takes a String argument. Presumably, this String contains a string representation of the time that can be parsed to set the time variable. Given the constructors in the previous example, we create a Date object in the following ways:

Date now = new Date( );  
Date christmas = new Date("Dec 25, 1999");

In each case, Java chooses the appropriate constructor at compile time based on the rules for overloaded method selection.

If we later remove all references to an allocated object, it’ll be garbage-collected, as we’ll discuss shortly:

christmas = null;          // fair game for the garbage collector

Setting this reference to null means it’s no longer pointing to the “Dec 25, 1999” object. (So would setting christmas to any other value.) Unless that object is referenced by another variable, it’s now inaccessible and can be garbage-collected.

A few more notes: constructors can’t be abstract, synchronized, or final. Constructors can, however, be declared with the visibility modifiers public, private, or protected, to control their accessibility. We’ll talk in detail about visibility modifiers later in the chapter.

Working with Overloaded Constructors

A constructor can refer to another constructor in the same class or the immediate superclass using special forms of the this and super references. We’ll discuss the first case here, and return to that of the superclass constructor after we have talked more about subclassing and inheritance. A constructor can invoke another, overloaded constructor in its class using the reference this( ) with appropriate arguments to select the desired constructor. If a constructor calls another constructor, it must do so as its first statement:

class Car {  
    String model;  
    int doors;  
  
    Car( String m, int d ) {  
        model = m;  
        doors = d;  
        // other, complicated setup  
        ...  
    }  
  
    Car( String m ) {  
        this( m, 4 );  
    }  
    ...  
}

In this example, the class Car has two constructors. The first, more explicit one, accepts arguments specifying the car’s model and its number of doors. The second constructor takes just the model as an argument and, in turn, calls the first constructor with a default value of four doors. The advantage of this approach is that you can have a single constructor do all the complicated setup work; other auxiliary constructors simply feed the appropriate arguments to that constructor.

The call to this( )must appear as the first statement in our second constructor. The syntax is restricted in this way because there’s a need to identify a clear chain of command in the calling of constructors. At one end of the chain, Java invokes the constructor of the superclass (if we don’t do it explicitly) to ensure that inherited members are initialized properly before we proceed.

There’s also a point in the chain, just after the constructor of the superclass is invoked, where the initializers of the current class’s instance variables are evaluated. Before that point, we can’t even reference the instance variables of our class. We’ll explain this situation again in complete detail after we have talked about inheritance.

For now, all you need to know is that you can invoke a second constructor only as the first statement of another constructor. For example, the following code is illegal and causes a compile-time error:

Car( String m ) {  
    int doors = determineDoors( );  
    this( m, doors );   // Error: constructor call
                        // must be first statement
}

The simple model name constructor can’t do any additional setup before calling the more explicit constructor. It can’t even refer to an instance member for a constant value:

class Car {   
    ...  
    final int default_doors = 4;  
    ...  
  
    Car( String m ) {  
        this( m, default_doors ); // Error: referencing  
                                  // uninitialized variable
    }  
    ...  
}

The instance variable defaultDoors is not initialized until a later point in the chain of constructor calls, so the compiler doesn’t let us access it yet. Fortunately, we can solve this particular problem by using a static variable instead of an instance variable:

class Car {  
    ...  
    static final int DEFAULT_DOORS = 4;  
    ...  
  
    Car( String m ) {  
        this( m, DEFAULT_DOORS );  // Okay now  
    }  
    ...  
}

The static members of a class are initialized when the class is first loaded, so it’s safe to access them in a constructor.

Static and Nonstatic Code Blocks

It’s possible to declare a code block (some statements within curly braces) directly within the scope of a class. This code block doesn’t belong to any method; instead, it’s executed once, at the time the object is constructed, or, in the case of a code block marked static, at the time the class is loaded.

Nonstatic code blocks can be thought of as extensions of instance variable initialization. They’re called at the time the instance variable’s initializers are evaluated (after superclass construction), in the order that they appear in the Java source:

class MyClass { 
    Properties myProps = new Properties( ); 
    // set up myProps 
    { 
        myProps.put("foo", "bar"); 
        myProps.put("boo", "gee"); 
    } 
    int a = 5; 
...

You can use static code blocks to initialize static class members. So the static members of a class can have complex initialization just like objects:

class ColorWheel {  
    static Hashtable colors = new Hashtable( );  
  
    // set up colors 
    static {  
        colors.put("Red", Color.red );  
        colors.put("Green", Color.green );  
        colors.put("Blue", Color.blue );  
        ...  
    }  
    ...  
}

The class ColorWheel provides a variable colors that maps the names of colors to Color objects in a Hashtable. The first time the class ColorWheel is referenced and loaded, the static components of ColorWheel are evaluated, in the order they appear in the source. In this case, the static code block simply adds elements to the colors Hashtable.