Inner Classes in Java

1. Inner Classes in Java CMSC 432 Shon Vick

2. Conceptual Structure package p1    class A1   class A2      ...   class An...package n withoutinner classes package p1    class A1    class A2      class A2_1 classA2_2   ... inner classes

3. Example public x + z; } class OC { // outer class private int x = 2; public class IC { // inner class int z = 4; public int getSum() { return } }

4. Why? • Logical grouping of functionality • Data hiding • Yet another encapsulation mechanism • Linkage to outer class • We’ll see about this later

5. Simple Motivating Example public class MyList { private Object [ ] a; private int size; } Say we want to iterate over the elements of this guy …

6. Problems Need to maintain reference to MyList Need to access private data in MyList 1st Design public class MyIterator { private MyList list; private int pos; MyIterator(MyList list) { this.list = list; pos = 0; } public boolean hasNext() { return (pos < list.size); } public Object next() { return list.a[pos++]; } }

7. 2nd Try – Using Inner Classes public class MyList { private Object [ ] a; private int size; public class MyIterator { private int pos; MyIterator() { pos = 0; } public boolean hasNext() { return (pos < size); } public Object next() { return a[pos++]; } } }

8. Inner Classes Example public class OC { // outer class private int x = 2; public class IC { // inner class int z = 4; public int getSum() { return x + z; } } }

9. Mechanics and Syntax • Class referencing syntax • OuterClass.InnerClass • Example OC oc = new OC( ); OC.IC ic;name of inner class ic = new OC.IC()doesn’t work! ic = oc.new IC();instantiates inner class -ic now will "know about" oc, but not vice versa ic.getSum()yields 6 can access private x in oc!

10. Accessing Outer Scope public class OC { outer class int x = 2; public class IC { inner class int x = 6; public void getX() { inner class method int x = 8; System.out.println( x ); prints 8 System.out.println( this.x ); prints 6 System.out.println( OC.this.x ); prints 2 } } }

11. Instantiating Inner Class • Common Technique • Outer class method returns instance of inner class • Used by Java Collections Library for Iterators • Code Example • public class MyList { • public class IC implements Iterator { … } • public Iterator iterator() { • return new IC(); // creates instance of IC • } • } • MyList m = new MyList(); • Iterator it = m.iterator();

12. Listeners Event Source • Each event is represented by an object that gives information about the event and identifies the event source. • Event sources are typically components. • Each event source can have multiple listeners.Conversely, a single listener can register with multiple event sources.

13. Making the Applet the Listener import java.applet.*; import java.awt.*; import java.awt.event.*; public class Scribble extends Applet implements MouseListener, MouseMotionListener { private int last_x, last_y; public void init() { this.addMouseListener(this); this.addMouseMotionListener(this); } …

14. Mouse Events public void mousePressed(MouseEvent e) { last_x = e.getX(); last_y = e.getY(); } public void mouseDragged(MouseEvent e) { Graphics g = this.getGraphics(); int x = e.getX(), y = e.getY(); g.drawLine(last_x, last_y, x, y); last_x = x; last_y = y; }

15. Need to implement the whole Interface // The other, unused methods of the MouseListener interface. public void mouseReleased(MouseEvent e) {;} public void mouseClicked(MouseEvent e) {;} public void mouseEntered(MouseEvent e) {;} public void mouseExited(MouseEvent e) {;} // The other method of the MouseMotionListener interface. public void mouseMoved(MouseEvent e) {;} }

16. Using Adapters and Inner Classes to Handle Events • The empty method bodies of the MouseListener interface from the previous example make the code harder to read and maintain • Adapter classes are a means around this problem • An adaptor allows you to override just the methods for the events of interest

17. Adapters and Anonymous Classes public void init() { // Define, instantiate, and register a MouseListener object. this.addMouseListener(new MouseAdapter() { public void mousePressed(MouseEvent e) { last_x = e.getX(); last_y = e.getY(); More Info on Innerclasses } } );

18. References • http://www.cs.umd.edu/class/spring2005/cmsc132/lecs/lec27.pdf • http://java.sun.com/docs/books/tutorial/java/javaOO/nested.html