Announcements & Review

1. 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/concepts/inheritance.html Last Time Objects - attributes & behaviors Classification - Inheritance Today Review Inheritance Abstract Classes Announcements & Review Lecture 27: Abstract Classes & Inheritance

2. 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 Lecture 27: Abstract Classes & Inheritance

3. 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 Lecture 27: Abstract Classes & Inheritance

4. Super class:Rectangle 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(...) {} Lecture 27: Abstract Classes & Inheritance

5. 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; } Lecture 27: Abstract Classes & Inheritance

6. 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 <variable instanceof TypeName> returns a boolean if (r instanceof Rectangle) {...} if (r instanceof ColoredRectangle) {...} • BlueJ Lecture 27: Abstract Classes & Inheritance

7. 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? Lecture 27: Abstract Classes & Inheritance

8. 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; } } Lecture 27: Abstract Classes & Inheritance

9. BlueJ examples ... Lecture 27: Abstract Classes & Inheritance