Nested Classes and Event Handling - Learning Java Through Games - page 215

Method cal lback is when a method is passed as a parameter. When an event occurs,
the method will be called back. Java does not directly support method callbacks.
The reason is that methods themselves cannot be passed as arguments to methods.
Alternatively, one needs to create a class that supports the required methods and
pass an object of that class as a parameter. Java uses interfaces to guarantee that the
object supports the required methods.

10.2 Nested Classes
It seems burdensome to create a brand new class every time we want to implement a
method callback. To simplify the process, Java allows us to create a class within another
class. Such classes are referred to as nested classes .

10.2.1 Static Nested Classes
Let us now explore how nested classes apply to our typing program. First, consider the
following rewrite of our typing game.
public class TypingGame

{
public static void main(String [] args) {

Timer t = new Timer(1000, new TimerListener ()) ;
t. start() ;
JFrame frame = new JFrame () ;
frame. setVisible( true );

}

public static class TimerListener implements ActionListener {

ArrayList < Character > charList = new ArrayList <> () ;

public void actionPerformed(ActionEvent e) {
charList .add(( char )( 'a' +( int ) ((Math . random() 26)))) ;

∗

System. out . println ( charList ) ;

}

}

}

Now the TimerListener class is defined inside the TypingGame class. The class is defined
as static because it is not connected to a particular instance of the outer TypingGame class.
In other words, objects that belong to the TimerListener class will not be associated with
an object of the TypingGame class. Note that defining the TimerListener class without
using the static keyword will result in an error. The reason is that this will imply that
the class is associated with an instance of the TypingGame class. However, the TypingGame
class is never instantiated.
The nested class is defined as public , which means that everyone can create an instance
of it. For example, the following code is valid outside the TypingGame class.
TypingGame . TimerListener l i s t e n e r = new TypingGame . TimerListener ( ) ;