Overriding

Look at Computer class and Laptop class instance variables

Write a toString method for Computer class

Will it work for the Laptop class?

We have to write another toString for the Laptop class, but we’d like it to call the toString of the super class

Overloading

It is very common to overload the constructor in a class

Polymorphism

Suppose we have in our client using the Computer and Laptop classes this:

Computer[ ] labCom = new Computer[10];

//what exactly happens in RAM wit this statement

for(i = 0; i<10; i++)

{ System.out.println(“Is the computer you want to record data for a laptop or desktop?”);

compType = in.next( );

// propmt for the for instance variables for all computers and save input

if (compType.equals(“Laptop”)

// prompt for the extra two items

Computer labComp[i] = new LapTop( six arguments);

else Compute labComp[i] = new Computer( four args);

The type of an object referenced is determined by the constructor;

The type of the reference variable may or may not be the same as the object referenced; if it is not the same, it is the type of a superclass

Abstract methods and classes

•Sometimes you want to require that all subclasses override a method

•Suppose we want to keep track of a bunch of shapes, but we won’t know much about the shapes until the user enters the data

•We do know that we will want to find the perimeter and area of all the shapes

•We design a class hierarchy to help us keep track of this data

•The superclass will be Shapes, and will require all its subclasses have methods to calculate area and perimeter

•We can not know how to implement the methods for the superclass so we declare the methods to be abstract, and write only the headers

•An abstract method is one that has not body i.e. not implemented

public abstract class Shape

{ public abstract double area ( ); // must return the area

public abstract double perimeter ( );

} if a class has even one abstract method, the class is abstract

•Now we write the subclasses, which must implement the abstract methods

•public class Circle

{ private double radius;

public Circle( ) { radius = 0.0;}

public Circle ( double r) { radius = r; }

public double area( ) { return PI * r * r; }

public double perimeter( ) { return PI*2*r; }

}

•Now the client of the classes can make an array of Shapes