InheritanceChapter 10

Programs built from objects/instances of classes

An O.O. approach – build on earlier work. Use classes in library and ones you have

written before

Using a class similar to one already written in text “Sphere” page 193

• Sphere has radius and position (actually draws an Oval.)

• protected will be used for inheritance

• Class “Bubble” page 194 will “extend” Sphere. Bubble inherits from Sphere.

• Bubble is a sub class of Sphere

• “protected” less private than private– More private than public

private int x, y;

• // x and y can not be used by another class

• protected int x , y; // can be used by another subclass

Scope rules: access

• public – from anywhere

• protected – from the class and sub classes

• private – only from the current class– local variables only inside a particular method

Overriding/Overloading

• Overriding: same name and parameters

• Overloading: method with same name as class but different parameters

• Subclass may add additional methods, variables and override methods of it’s parent

Class Diagrams: show which classes inherit from others

Object

Bubble Ball

A sub class

Java uses single line inheritance

Any public or protected method may be used by any sub class if the child “extrended” the parent class

“super” – used by subclass to call it’s immediate superclass

super.display(paper); // uses super class method “display” and not the method in the child class (sub class) “display”

constructors

• Use “new”• A class with no constructors – Java assumes a

single one with zero parameters• A class with one or more constructors that

require zero parameter constructor must be explicitly written. (Java does not assume it)

• Constuctors not inherited – must be explicitly called by sub class.

“Balloon” class page 199

• Holds a constructor “Balloon” which is passed through parameters

• “DifferentBalloon” page 200 – holds a constructor passed in two parameters– super( ) is called from the immediate super

class.

• “ModifiedBalloon” – holds a constructor, passed three parameters– Super is called with three parameters

final:

• Methods: declared final can not be overridden

• Variables: declared final can not be changed

• Classes: declared final can not be sub classed

Abstract classes

• A program that maintains graphical shapes – they are similar (position, color, size)

• Super class “Shape”

• Page 202– public abstract class Shape { …– public class Circle extends Shape { …

• paper.drawOval ( x, y, size, size);

Shape is incomplete

• The keyword ‘abstract’ used in Shape prevents attempts to create an instance of the class Shape

• display method in Shape is just a header the subclass must implement it as ‘display’ in Circle does.

• Any class that includes methods that are abstract must also be abstract

• Abstract classes higher up the class hierarchy become more general or abstract