Inner Classes - Java

Inner Classes

In Chapter 7, the basics of inner classes were explained. Here, you will see why they are important. Recall that an inner class is a class defined within another class, or even within an expression. This section illustrates how inner classes can be used to simplify the code when using event adapter classes.

To understand the benefit provided by inner classes, consider the applet shown in the following listing. It does not use an inner class. Its goal is to display the string "Mouse Pressed" in the status bar of the applet viewer or browser when the mouse is pressed. There are two top-level classes in this program. MousePressedDemo extends Applet, and MyMouseAdapter extends MouseAdapter. The init( ) method of MousePressedDemo instantiates

MyMouseAdapter and provides this object as an argument to the addMouseListener( ) method.

Notice that a reference to the applet is supplied as an argument to the MyMouseAdapter constructor. This reference is stored in an instance variable for later use by the mousePressed( ) method. When the mouse is pressed, it invokes the showStatus( ) method of the applet through the stored applet reference. In other words, showStatus( ) is invoked relative to the applet reference stored by MyMouseAdapter.

// This applet does NOT use an inner class.

import java.applet.*;

import java.awt.event.*; /*

<applet code="MousePressedDemo" width=200 height=100> </applet>

*/

public class MousePressedDemo extends Applet {

public void init() {

addMouseListener(new MyMouseAdapter(this));

}

}

class MyMouseAdapter extends MouseAdapter { MousePressedDemo mousePressedDemo;

public MyMouseAdapter(MousePressedDemo mousePressedDemo) { this.mousePressedDemo = mousePressedDemo;

}

public void mousePressed(MouseEvent me) { mousePressedDemo.showStatus("Mouse Pressed.");

}

}

The following listing shows how the preceding program can be improved by using an inner class. Here, InnerClassDemo is a top-level class that extends Applet. MyMouseAdapter is an inner class that extends MouseAdapter. Because MyMouseAdapter is defined within the scope of InnerClassDemo, it has access to all of the variables and methods within the scope of that class. Therefore, the mousePressed( ) method can call the showStatus( ) method directly. It no longer needs to do this via a stored reference to the applet. Thus, it is no longer necessary to pass MyMouseAdapter( ) a reference to the invoking object.

// Inner class demo.

import java.applet.*;

import java.awt.event.*; /*

<applet code="InnerClassDemo" width=200 height=100> </applet>

*/

public class InnerClassDemo extends Applet { public void init() {

addMouseListener(new MyMouseAdapter());

}

class MyMouseAdapter extends MouseAdapter { public void mousePressed(MouseEvent me) {

showStatus("Mouse Pressed");

}

}

}

Anonymous Inner Classes

An anonymous inner class is one that is not assigned a name. This section illustrates how an anonymous inner class can facilitate the writing of event handlers. Consider the applet shown in the following listing. As before, its goal is to display the string "Mouse Pressed" in the status bar of the applet viewer or browser when the mouse is pressed.

// Anonymous inner class demo.

import java.applet.*;

import java.awt.event.*; /*

<applet code="AnonymousInnerClassDemo" width=200 height=100> </applet>

*/

public class AnonymousInnerClassDemo extends Applet { public void init() {

addMouseListener(new MouseAdapter() { public void mousePressed(MouseEvent me) {

showStatus("Mouse Pressed");

}

});

}

}

There is one top-level class in this program: AnonymousInnerClassDemo. The init( ) method calls the addMouseListener( ) method. Its argument is an expression that defines and instantiates an anonymous inner class. Let’s analyze this expression carefully.

The syntax new MouseAdapter(){...} indicates to the compiler that the code between the braces defines an anonymous inner class. Furthermore, that class extends MouseAdapter. This new class is not named, but it is automatically instantiated when this expression is executed.

Because this anonymous inner class is defined within the scope of AnonymousInnerClassDemo, it has access to all of the variables and methods within the scope of that class. Therefore, it can call the showStatus( ) method directly.

As just illustrated, both named and anonymous inner classes solve some annoying problems in a simple yet effective way. They also allow you to create more efficient code.