Java Inner Classes

Interfaces An interface describes methods but does not supply bodies for them. interface KeyListener { public void keyPressed(KeyEvent e); public void keyReleased(KeyEvent e); public void keyTyped(KeyEvent e); }

Four kinds of inner classes Member classes simple Anonymous classes syntax is ugly but anonymous classes are useful Static member classes (not too useful) Local classes (not too useful) Every class compiles to a separate .class file

Member classes A member class is an “ordinary” inner class class Outer { int n; class Inner { int ten = 10; void setNToTen ( ) { n = 10; } } void setN ( ) { new Inner ( ).setNToTen ( ); } }

Member classes II Member classes are useful for handling events Button b = new Button (“Click Me”); b.addActionListener (new Clicker ( )); … class Clicker implements ActionListener { … } Can access the variables of the outer class

Anonymous inner classes Convenient for short code b.addActionListener (anonymous inner class); The anonymous inner class can be either: new Superclass (args) { body } new Interface (args) { body } Notice that no class name is given--only the name of the superclass or interface

Example anonymous inner class b.addActionListener (new ActionListener ( ) { public void actionPerformed (ActionEvent e) { System.out.println (“Ouch!”); } } ) ;

Static member classes static class Inner { … } A static member class can access only static variables of the outer class A static member class isn't "really" an inner class Inner classes did not exist in Java 1.0

Local classes A local class is a class defined inside a method. A local class cannot access variables declared in the method (!) There are many other restrictions on local classes.

Interfaces, again interface KeyListener { public void keyPressed(KeyEvent e); public void keyReleased(KeyEvent e); public void keyTyped(KeyEvent e); }

Adapters class KeyAdapter implements KeyListener { public void keyPressed(KeyEvent e) { }; public void keyReleased(KeyEvent e) { }; public void keyTyped(KeyEvent e) { }; }

The End