Inner Classes in Java

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Inner Classes in Java

• CMSC 432
• Shon Vick

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Conceptual Structure
package p1    class A1   class
A2      ...   class An...package n without
inner classes
package p1    class A1    class A2     
class A2_1 class A2_2        
...         inner classes
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Example
public x z class OC // outer
class private int x 2 public class
IC // inner class int z 4
public int getSum() return
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Why?

• Logical grouping of functionality
• Data hiding
• Yet another encapsulation mechanism
• Linkage to outer class
• Well see about this later

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Simple Motivating Example
public class MyList private Object a
private int size
Say we want to iterate over the elements of this
guy
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1st Design
public class MyIterator private MyList
list private int pos MyIterator(MyList
list) this.list list pos 0
public boolean hasNext() return (pos lt
list.size) public Object next()
return list.apos

• Problems
• Need to maintain reference to MyList
• Need to access private data in MyList

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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 lt size) public Object next()
return apos
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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
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Mechanics and Syntax

• Class referencing syntax
• OuterClass.InnerClass
• Example
• OC oc new OC( )
• OC.IC ic name of inner class
• ic new OC.IC() doesnt 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!

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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
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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()

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Listeners

• 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.

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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)
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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
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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)
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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

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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 )
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References

• http//www.cs.umd.edu/class/spring2005/cmsc132/lec
  s/lec27.pdf
• http//java.sun.com/docs/books/tutorial/java/javaO
  O/nested.html