Nested Classes and Event Handling - Learning Java Through Games - page 217

pointer to outer

object

charList = [2,3,5]

PopulateChars object TimerListener object

FIGURE 10.1: Example of inner class.

In our example, a TimerListener object is created within the constructor of the
PopulateChars class. The outer object for the new object of type TimerListener is the
object that created the TimerListener object. An inner object can also be created from
outside the outer class. However, the syntax is more involved because the outer object needs
to be explicitly identified. Here is an example.

public class TypingGame {
public static void main(String [] args) throws Exception

{

PopulateChars populateChars = new PopulateChars () ;
Timer timer = new Timer(200 ,populateChars . new TimerListener ()) ;
timer. start() ;
JFrame frame = new JFrame () ;
frame. setVisible( true );

}

}

{

public class PopulateChars

< >

ArrayList charList ;

public PopulateChars ()

Character

{
charList = new ArrayList < Character > () ;

}

public class TimerListener implements ActionListener {

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

∗

System. out . println ( charList ) ;

}

}

}

Examine the code populateChars.new TimerListener() . This means create a new
object of type TimerListener . However, instead of specifying the outer PopulateChars
class, we need to specify the outer object: populateChars . The reason is that every inner
object needs to have a reference to an outer object. This is similar to class inheritance
because every object that belongs to a subclass needs to have a reference to an object that
belongs to the superclass.

Objects of nested static classes have a reference to the outer class. Conversely,
objects of inner classes have a reference to an object of the outer class.