Announcements

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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
• Objects - attributes behaviors
• Classification - Inheritance
• Today
• Review Inheritance
• Abstract Classes

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Why Inheritance?

• Reuse of common functionality in related classes
• Reducing redundancy eases programmer effort
• Classification lets humans deal with complexity,
  inheritance is a form of classification
• General to specific, e.g., cat to siamese

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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
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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(...)

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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

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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

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Abstract Classes

• How can we partially declare a class?
• Example
• Shape class with subtypes Rectangle and Triangle
• Is there a default draw method?
• Is there a default toString?

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Syntax for Abstract Classes

• public abstract class Shapes
• protected static String name // Rectangle,
  Triangle, ...
• public abstract String toString() // must
  implement toString
• public String getName()
• return name
• public class Rectangle extends Shapes
• protected static String name Rectangle
• ....
• public String toString()
• return name

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BlueJ examples ...