Java

Inner Classes

Interfaces

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

• An interface describes methods but does not supply bodies for them.

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