Lecture 26: Inheritance

Announcements & Review

Lab 8 Due Thursday

Image and color effects with 2D arrays

Last TimeImages as 2D arrays• color representation

– red, green, blue values– integer values

• expanding & shrinking– mapping from a point in

one 2D space to another

Today• review & new object

oriented concepts

Lecture 26: Inheritance

Object OrientedObject Centered View

• Objects with attributes and behaviors– attributes: instance variables– behaviors: methods

• Designing a Class– use private for encapsulation

• scopes the instance variables to the class methods

• no one else can directly modify them

– public methods for behaviors• local variables “disappear” after the method

returns

– use private helper methods when necessary

Lecture 26: Inheritance

Example Local vs Instance Variable

public class Rectangle { private int width; // the instance variables private int height; public Rectangle() { width = 0; height = 0; } public Rectangle(int w, int h) { width = w; height = h; }

public int getArea() { int area = width * height; // area is local variable // Aside: int width = ... BAD PRACTICE, never name a

local // variable the same name as an instance variable if (area > 0) { return area; } else {

System.out.println (“Bad Rectangle”); } }

Lecture 26: Inheritance

Why Inheritance?

• Reuse in common functionality in related classes

• Reduce redundancy, ease programmer effort• Classification lets humans deal with

complexity, inheritance is a form of classification

• General to specific, e.g., cat to siameseRead:

– Chapter 9 Cahoon & Davidson– Chapter 9 Regis & Stepp

Additional Reading:http://java.sun.com/docs/books/tutorial/java/concepts/

inheritance.html

Lecture 26: Inheritance

Example Inheritance Hierarchy and Instance

Variables

Motor Vehicle

Car Truck Motorcycle

Harley Honda Kawasaki

miles per gallonmax speed...tires passengers

seat storagecup holder...

colorsstandard featuresextras

Lecture 26: Inheritance

Inheritance in Java ...

Object

Transformation Graph Rectangle

ColoredRectangle

Object provides these methods: toString() equals() // more on these clone() // later... instanceof()

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

Lecture 26: Inheritance

Syntax of InheritanceExample:

ColoredRectanglepublic 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; }

Also, we have to change Rectangle’s instance variables to protected