Nested Classes and Event Handling - Learning Java Through Games - page 216

Nested static classes that are public can be accessed from everywhere by specifying
the name of the outer class followed by a dot and then the name of the nested classes.
Within the outer class, nested static classes can be accessed directly by specifying only
thenameoftheclass.

Note that nested classes can be defined without an access privilege modifier. Then
they will be accessible only within the package. Alternatively, they can be defined using
the private keyword. Such classes will only be visible within the outer class. Although
uncommon, nested classes can also be defined as protected . Then they will be accessible
within the package and within the subclasses of the outer class.
The new code for the typing game seems a little more compact. We did not have to
create a new outer class. Probably, we do not want anyone outside the TypingGame class to
be aware of our nested class and we can define it as private .
The classes Double and Float inside the class Rectangle2D are examples of nested
static classes.

10.2.2 Inner Classes
Nested classes in Java that are not static are called inner . In order to show an example
of an inner class, let us refactor our program and create a separate class that handles
populating the ArrayList with random characters.
public class TypingGame {
public static void main(String [] args) throws Exception {

new PopulateChars () ;
JFrame frame = new JFrame () ;
frame. setVisible( true );

}
} public class PopulateChars {
ArrayList < Character > charList ;
public PopulateChars () {
charList = new ArrayList < Character > () ;
Timer timer = new Timer(200, new TimerListener ()) ;
timer. start() ;

}

private class TimerListener implements ActionListener {

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

∗

System. out . println ( charList ) ;

}

}
} Now the TimerListener class is an inner class for the PopulateChars class. A nice
feature of inner classes is that they have access to the variables of the outer class. For ex-
ample, the actionPerformed method has access to the variable charList in the outer class.
However, note that there can be multiple instances of the PopulateChars class. Therefore,
every object of the TimerListener class must be associated with exactly one object that
belongs to the PopulateChars class. If this is not the case, then the reference to the vari-
able charList can be ambiguous. Figure 10.1 shows how the object that belongs to the
TimerListener class must be related to an object of the outer class.