Announcements

1
Announcements Review

• Lab 8 Due Thursday
• Image and color effects with 2D arrays
• Read
• Chapter 9 Cahoon Davidson
• Chapter 9 Regis Stepp
• Additional Reading
• http//java.sun.com/docs/books/tutorial/java/conce
  pts/inheritance.html

• Last Time
• Review Inheritance
• Dynamic dispatch
• Today
• Dynamic dispatch
• Abstract Classes

2
Inheritance in Java ...

• All classes inherit from Object
• Rectangle inherits from Object
• ColoredRectangle inherits from
• Rectangle

Object provides these methods toString()
equals() // more on these clone()
// later... instanceof()
Object
Transformation
Graph
Rectangle
ColoredRectangle
3
Super classRectangle

• public class Rectangle
• protected int width // the instance
  variables
• protected int height
• public Rectangle() width 0 height 0
• public Rectangle(int w, int h) width w
  height h
• public void draw(...)

4
Syntax of InheritanceExample ColoredRectangle

• public class ColoredRectangle extends Rectangle
• private Color myColor // additional
  instance variables
• public ColoredRectangle()
• super() // optional
• myColor Color.white
• / Is the same as
• public ColoredRectangle() implicitly first
  calls super, because
• 
  the signatures match
• myColor Color.white
• /
• public ColoredRectangle(int w, int h, Color
  newColor)
• super(w,h) // required why do we need
  this one?
• myColor newColor

5
Dynamic Dispatch

• Declare variables of supertype and they can be
  the supertype or the subtype
• Rectangle r new ColoredRectangle()
• Declare variables of a subtype and they can only
  be of type subtype or lower.
• Test type with instanceof
• ltvariable instanceof TypeNamegt returns a boolean
• if (r instanceof Rectangle) ...
• if (r instanceof ColoredRectangle) ...
• BlueJ

6
Abstract Classes

• Want to partially declare a class
• Force an implementation of a method but do not
  offer a default implementation it
• require a draw method, but have not default draw

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Syntax for Abstract Classes

• public abstract class Shapes
• protected static String name // e.g.,
  Rectangle, Triangle, ...
• 
  // Subclass must implement draw
• public abstract void draw(DrawingBox box)
• public String getName() // Subclass
  need not implement getName
• return name
• public class Rectangle extends Shapes
• protected static String name Rectangle
• ....
• public void draw(DrawingBox box)
• box.drawRect(...)

8
BlueJ examples ...
9
Interfaces

• Want to partially specify a behavior that can
  apply to lots of classes
• For example, adding Color
• Enforces a consistant application of a feature
  across lots of classes

10
Interfaces

• Interface Colorable
• public void setColor(Color c)
• public Color getColor()
• public class ColorRectangle extends Shapes
  implements Colorable
• Color myColor
• public void setColor(Color c)
• myColor c
• public Color getColor()
• return myColor