Java and OOP

OOP in Java : © W. Milner 2005 : Slide 1

Java and OOP

Part 3 – Extending classes


Inheritance

Suppose we want a version of an existing class, which is slightly different from it.

We want to avoid starting again from scratch We can define the new class to be a sub-class

of the first class.


Terms used

The original class is called the base class, the ancestor class or the super class

The process of designing the sub-class from the base class is called 'sub-classing' the base class


Sub-classing

The subclass inherits all members and methods of the first

where needed we can write new versions of inherited methods – which replace the old method

we can add extra members and methods You can’t ‘loose’ a member or method No limit to levels of inheritance


Example

Common type of sub-classing is specialization Suppose we want a type of product which is

perishable When we deliver new stock, we throw away

old stock – not add to it First review the Product class:


Product class definitionpublic class Product{

public Product(){ lastBarcodeUsed++; barcode=lastBarcodeUsed; stockLevel=100;}

public Product(int initStock){ lastBarcodeUsed++; barcode=lastBarcodeUsed; stockLevel=initStock;}

public static int count(){ return lastBarcodeUsed;}

public void display(){ System.out.println("Barcode = "+barcode); System.out.println("Stocklevel = "+stockLevel); System.out.println("=========================");}


Rest of Product

public void deliver(int howMany){ if (howMany<0) { System.out.println("Invalid delivery"); return; } else stockLevel+=howMany;}

public int getStockLevel(){ return stockLevel;}

private static int lastBarcodeUsed=0;private int barcode;protected int stockLevel;}


Implementation of Perishable

public class Perishable extends Product{

public void deliver(int howMany) { stockLevel=howMany; }

}


Protected

Problem – the deliver method in Perishable references the private field stockLevel – not allowed

Solution – use access control modifier protected Excerpt from modified Product definition –

..private static int lastBarcodeUsed=0;private int barcode;protected int stockLevel;}


Using the subclasspublic class First{public static void main(String[] args){Product prod = new Product();prod.deliver(100);Perishable perish1 = new Perishable();Perishable perish2 = new Perishable();perish1.deliver(50);perish2.deliver(60);prod.display();perish1.display();perish2.display();}}

All 3 use default constructor =stocklevel 100Barcode = 1Stocklevel = 200====================Barcode = 2Stocklevel = 50====================Barcode = 3Stocklevel = 60====================


Constructors of sub-classes

Constructors are not methods They are not inherited If you don't define one – the no-arg constructor

of the base class is called for you – see last example


super()

super(); can be used as the first statement in a constructor

It means the corresponding superclass constructor is called

Further statements can take further action For example..suppose Perishable products

have an extra store location code..


Using super()

public Perishable(int initStock, int initLocationCode){ super(initStock); locationCode = initLocationCode;}

public Product(int initStockLevel){ barcode=lastBarcodeUsed++; stockLevel=initStockLevel;}


More on super()

super() cannot be anywhere except the first line of a constructor

If you don’t use super(), the system executes it anyway

IOW a subclass constructor first executes the no-arg constructor of the super class


Exercise

Define an Employee class, with fields payrollNumber and rateOfPay

Define a Manager class as a sub-class of Employee. They are paid monthly – define their pay() method to display their pay

Define a Clerical class as a sub-class of Employee. They are hourly paid. Add an hoursWorked field, and a pay() method.


Object

All classes descend from the class Object public class MyClass.. Is in effect: public class MyClass extends Object.. While if you say public class MyClass extends MySuperClass Then MySuperClass, or its ancestor, descends from Object Object objects have few useful methods Except toString(), which converts the object to a

descriptive string Which is what System.out.println calls For example..


Object example

Object someObject= new Object();System.out.println(someObject);

Output:java.lang.Object@187c6c7


Changing and using toString

In Perishable definition....public String toString(){ return "Perishable ID="+barcode+" Loc="+locationCode+" stock="+stockLevel;}..

Output:Perishable ID=0 Loc=30 stock=20Perishable ID=1 Loc=45 stock=20

in use..Perishable p1 = new Perishable(20,30);Perishable p2 = new Perishable(20,45);System.out.println(p1);System.out.println(p2);

calls toString() of Perishable objects


final methods

A method declared as final in a superclass cannot be altered in a subclass

For example..


Defining a method as final

In Product..public final void display(){ System.out.println("Barcode = "+barcode); System.out.println("Stocklevel = "+stockLevel); System.out.println("=========================");}

In Perishable..public void display(){ .. ..}

In use:Perishable p1 = new Perishable(20,30);Output on next slide


Trying to override final - compile time error

C:\Walter\JavaProgs\Perishable.java:19: display() in Perishable cannot override display() in Product; overridden method is finalpublic void display() ^1 error


Final classes – and why

A class declared as final cannot be subclassed Methods and classes are usually declared as

final for security Otherwise – a subclass of a standard

superclass might be defined, with.. Unpleasant overridden methods But at run-time a subclass object would look

like the superclass Eg the String class is final for this reason


Abstract classes

Superclasses which are 'general' can be declared abstract

Used when subclasses will all implement the same method – in different ways, but

The superclass is too general to actually implement the method itself

For example..


Example abstract class

Suppose we had a superclass called Shape And subclasses called Triangle, Rectangle,

Square and so on. Each would need a draw method But we could not program the draw method of

a Shape instance So the draw method of Shape is declared

abstract As is the class as a whole This means Shape cannot be instantiated


Example abstract class

public abstract class Shape{

public Shape(int initHeight, int initWidth){

width=initWidth;height=initHeight;

}

public abstract void draw();

protected int width;protected int height;}


Subclass of abstract class

public class Rectangle extends Shape{

public Rectangle(int h, int w){ super(h,w);}

public void draw() { for (int i=0; i<height; i++) { for (int j=0; j<width; j++)

System.out.print("*"); System.out.println(); } }}


Using the subclass

Rectangle r = new Rectangle(4,5);r.draw();


Exercise

Copy the Shape class Define a Triangle class by subclassing Shape –

define the draw method How would you deal with a Square class?

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