Objects, Classes, and Interfaces

The Method Body

In the code sample that follows, the method bodies for the isEmpty and pop methods are shown in bold:

class Stack {
    static final int STACK_EMPTY = -1;
    Object[] stackelements;
    int topelement = STACK_EMPTY;
    . . .
    boolean isEmpty() {
        if (topelement == STACK_EMPTY)
            return true;
        else
            return false;
    }
    Object pop() {
        if (topelement == STACK_EMPTY)
            return null;
        else {
            return stackelements[topelement--];
        }
    }
}

Besides regular Java language elements, you can use this in the method body to refer to members in the current object. The current object is the object whose method is being called. You can also use super to refer to members in the superclass that the current object has hidden or overriden. Also, a method body may contain declarations for variables that are local to that method.

this

Typically, within an object's method body you can just refer directly to the object's member variables. However, sometimes you need to disambiguate the member variable name if one of the arguments to the method has the same name.

For example, the following constructor for the HSBColor class initializes some of an object's member variables according to the arguments passed into the constructor. Each argument to the constructor has the same name as the object's member variable whose initial value the argument contains.

class HSBColor {
    int hue, saturation, brightness;
    HSBColor (int hue, int saturation, int brightness) {
        this.hue = hue;
        this.saturation = saturation;
        this.brightness = brightness;
    }

You must use this in this constructor because you have to disambiguate the argument hue from the member variable hue (and so on with the other arguments). Writing hue = hue; makes no sense. Argument names take precedence and hide member variables with the same name. So to refer to the member variable you must do so through the current object--this--explicitly.

Some programmers prefer to always use this when referring to a member variable of the object whose method the reference appears. Doing so makes the intent of the code explicit and reduces errors based on name sharing.

You can also use this to call one of the current object's methods. Again this is only necessary if there is some ambiguity in the method name and is often used to make the intent of the code clearer.

super

If your method hides one of its superclass's member variables, your method can refer to the hidden variable through the use of super. Similarly, if your method overrides one of its superclass's methods, your method can invoke the overriden method throught the use of super.

Consider this class:

class ASillyClass {
    boolean aVariable;
    void aMethod() {
        aVariable = true;
    }
}

and its subclass which hides aVariable and overrides aMethod:

class ASillierClass extends ASillyClass {
    boolean aVariable;
    void aMethod() {
        aVariable = false;
        super.aMethod();
        System.out.println(aVariable);
        System.out.println(super.aVariable);
    }
}

First aMethod sets aVariable (the one declared in ASillierClass that hides the one declared in ASillyClass) to false. Next aMethod invoked its overriden method with this statement:

super.aMethod();

This sets the hidden version of the aVariable (the one declared in ASillyClass) to true. Then aMethod displays both versions of aVariable which have different values:

false
true

Local Variables

Within the body of the method you can declare more variables for use within that method. These variables are local variables and live only while control remains within the method. This method declares a local variable i that it uses to iterate over the elements of its array argument.

Object findObjectInArray(Object o, Object[] arrayOfObjects) {
    int i;      // local variable
    for (i = 0; i < arrayOfObjects.length; i++) {
        if (arrayOfObjects[i] == o)
            return o;
    }
    return null;
}

After this method returns, i no longer exists.

Objects, Classes, and Interfaces