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:
Last Time
Objects - attributes & behaviors
Classification - Inheritance
Today
Review Inheritance
Abstract Classes
Lecture 27: Abstract Classes & Inheritance

2. Lecture 27: Abstract Classes & Inheritance
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. Lecture 27: Abstract Classes & Inheritance
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 Inheritance Example: 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
} */
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. Lecture 27: Abstract Classes & Inheritance
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. Lecture 27: Abstract Classes & Inheritance
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() {
Lecture 27: Abstract Classes & Inheritance

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