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4 OOPS and its

application in Java

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Topics: Classes and Objects

Defining a class Defining instance variables and

methods

Creating objects out of a class

Method calls via object references

Abstraction Interfaces and Abstract classes

Abstract and non-abstract methods

Inheritance

Extends and implements keywords

in Java Super class and Sub class

this keyword, super keyword in Javafor inheritance

Concrete classes in Java

Polymorphism

Compile time polymorphism -- Overloading of methods

Run time polymorphism --Overriding of methods

Method Overriding rules andmethod overloading rules

Introduction to Object classand it's methods

Encapsulation

Protection of data

Java Bean, POJO

Getters/Setters

Memory management in Java

Heap

Stack

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Class

Definition:

A class is a blueprint or prototype

Defines the variables and methodscommon to all objects of a certain kind.

The Benefits of Classes

Reusability -- Software programmers use the

same class, and the same code to create many

objects.

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Object

An object is a chunk of memory:

holdsfield values

holds an associated object type

All objects of the same type share code

they all have same object type, but canhave different field values.

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Object & Class Comparison

Class ObjectClass is a type/template

for similar objects Object is an instance ofthe class, with eachinstance behaving

identically

Class is purely a staticconcept, represented by

program textObject is dynamic/run-time entity, occupying

space in memory

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Object Interaction Software World

As in real world, Software Objects also interact bypassing messages.

In Software World, messages are dispatched to

Methods : which defines the behavior of the object

for a message Code to be executed by an object (to invoke a

behavior of an object), when a message is sent to it

is known as a Method.

Message passing is achieved by invoking themethodsof the objects.

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The class hierarchy

Classes are arranged in a hierarchy

The root, or topmost, class is Object

Every class but Object has at least one

super class.

A class may have subclasses

Each class inheritsall the fields and

methods of its (possibly numerous)super classes

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Encapsulation

Hides the implementation details of a class

Forces an interface to access the data.

Makes the code more maintainable.

It encapsulates all members within class

Data can hide by using access specifiers.

MyDate

+Today :Date

getToday() : Date

setToday(Date d);

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Benefits of Encapsulation

The fields of a class can made read-only or

write-only.

A class can have total control over what data

should store in fields.

The user of a class do not know how data stores

in class.

A class can change Data Type of filed and userof the class need not to change their code.

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Inheritance We can understand this better by considering the is

aidea

A subclass object is a super class object

However, some extra instance variables andmethods may have been added and some othermethods may have been changed

Note that is a is a one way operation

Subclass is a super class (specific "is a" general)

With modifications / additions Super class is NOT a subclass (general not "is a"

specific

Missing some properties

Ex: Bird is a Animal

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Inheritance and is a

Animal

is a

is ais a

Bird

Human Fish

Bird, Human and Fish are all Animals However, an Animal is not necessarily a

Bird, Human or Fish

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Constructors

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Usageof this

Inside a constructor, you can usethisto invokeanother constructor in the same class. This is

called explicit constructor invocation. It MUST

be the first statement in the constructor body if

exists.

thiscan also be used as a reference of the current

object. It CANNOTbe used in a static method

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Usage ofsuper

Inside a constructor, you can usesuperto

invoke constructor of the parent class. This is

called explicit constructor invocation. It MUST

be the first statement in the constructor body ifexists.

supercan also be used as a reference of the

super class object. It CANNOTbe used in astatic method.

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Example: usage of thisas a reference of thecurrent object

class Body {

private long idNum;

private String name;

private Body orbits;

private static long nextID = 0;

private static LinkedList bodyList =

new LinkedList();. . .

Body(String name, Body orbits) {

this.name = name;this.orbits = orbits;

}. . .private void inQueue() {

bodyList.add(this);}

. . .

}

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Data Abstraction

It is a process of hiding data by encapsulating in a

class and wraps through access specifiers.

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Abstract class

Abstract classes created using the abstractkeyword:

public abstract class MotorVehicle { }

In an abstract class, several abstract methods are

declared.

An abstract method is not implemented in the class, only

declared. The body of the method is then implemented in

subclass.

An abstract methodis decorated with an extra abstract

keyword. Abstract classes can not be instantiated! So the

following is illegal:

MotorVehicle m = new MotorVehicle;

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Abstract methods

Abstract methods are declared but do not

contain an implementation.

For example, the Motor Vehicle class may

have an abstract method gas( ):

public abstractclass Motor Vehicle {

private double speed, maxSpeed;

void accToMax( ) { speed = maxSpeed;} public abstractvoid gas( );

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Abstract Class Syntax

abstractclass Class Name

{ ...

abstractType MethodName1();

Type Method2(){

// method body

}

}

When a class contains one or more abstract methods, itshould be declared as abstract class.

The abstract methods of an abstract class must bedefined in its subclass.

We cannot declare abstract constructors or abstractstatic methods.

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Abstract Classes Properties

A class with one or more abstract methods isautomatically abstract and it cannot beinstantiated.

A class declared abstract, even with no abstract

methods can not be instantiated. A subclass of an abstract class can be instantiated

if it overrides all abstract methods byimplementation them.

A subclass that does not implement all of thesuper class abstract methods is itself abstract; andit cannot be instantiated

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Example of Abstract Classes

Example:abstractclass Stack{

abstract void push(Object o);

abstract Object pop();

}

public class ArrayStack extends Stack{.... // declare elems[] and top;void push(Object o) { elems[top++] = o; }Object pop() { return elems[--top]; }

}

class LinkedStack extends Stack{.... // declare ...void push(Object o) { .... }Object pop() { .... }

}

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Creating and Using Interfaces

Interface

Keyword interface

Has set of public abstractmethods

Can contain public final staticdata

Using interfaces

Class specifies it uses interface with keyword

implements

Multiple interfaces use comma-separated list

Class must define all abstractmethods in interface Must use same number of arguments, same return

type

Using interface like signing a contract

"I will define all methods specified in the interface

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Using interfaces (continued)

Interfaces used in place of abstractclasses Used when no default implementation

Typicallypublicdata types

Interface defined in its own .javafile

Interface name same as file name

1 // Fig. 27.5: Shape.java

2 // Definition of interface Shape

3

4publicinterfaceShape {

5 publicabstractdoublearea();

6 publicabstractdoublevolume();

7 publicabstractString getName();

8 }8 }

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Polymorphism

Many-forms

Two types:

Compile time: resolved at compile time

(method overloading)

Runtime time: resolved at runtime.

(method overridding)

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Method Overloading.

If a method has same name but a differentargument list in a same class is overload

method of the existing method.class overload{

public void show(){

System.out.println(Default show

method);

}

public void show(int i)

{

System.out.println(Parameterised show

method);

}

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Rules for method overloading..

The argument list of calling statement must

different enough to allow unambiguous

determination of the proper method call.

The return type of a method can be different,but it is not sufficient for the return type to

make the difference.

The argument can be differ in their count, type

or their order in method declaration.

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Rules continued

Method can be overload in same class as wellas in its child class.

It is also called as early binding or more

specifically we can say it is Compile time

polymorphism.

Compiler will decide which method should

get called based upon the parameters passed

during method call.

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Method Overriding

If a method in subclass has same name,

return type and argument list match with the

method in super class then the new method is

said to override the old one.

Class Animal

{

public void eat() {

System.out.println(Animal

Eat method);

}

}

Class Horse extends

Animal

{

public void eat() {

System.out.println(Hor

se Eat method);

}

}

Overridden

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Rules for method Overriding

The argument list should exactly match with

overridden method.

The return type must be same of super class.

The access level cant be more restrictive thanlevel use in super class.

Overridden method cant throw any new or wider

exception than the overridden method. We cant override the method marked final in

super class.

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Rules for method Overriding

Static methods can not be override.

The overriding method can throw

unchecked(runtime) exception regardless

that the overridden method has declared

exception.

Constructor cant override.

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Use of Final Declaring variables final

Indicates they cannot be modified after declaration

Must be initialized when declared

Declaring methods final

Cannot be overridden in a subclass staticandprivatemethods are implicitly final

Program can inline finalmethods

Actually inserts method code at method call locations

Improves program performance

Declaring classes final

Cannot be a subclass (cannot inherit from it)

All methods in class are implicitly final

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Assignment

WAP in java for the following inheritance:

Student

Test Sports

Result

WAP to demonstrate the use of constructors in inheritance

Default/no parameter constructor

1param

2 param

WAP to demonstrate the use of super keyword in inheritance.

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Assignment

WAP in java to demonstrate the use if this keyword

WAP to demonstrate the use of abstract classes and

abstract method in inheritance.

WAP for creating a interface STACK, which will contain 2

abstract methods: push() & pop(). Implement them in a

demo class.

WAP to demonstrate

one interface inheriting other interface.

One abstract class implementing interface.

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Assignment

WAP in java to demonstrate the use of final keyword

WAP for method overloading to find the area of:

square

Circle Rectangle

Cylinder

Define a Mango object which is derived from the Treeclass .In addition to the attributes of Tree, the Mango

class has yield attribute. Override the displayTree() and

annualUpdate() and suitable Constructor.

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Thank You

Documents

4OOPS (2 Days)