Day 1 - OOPS Concepts - Java SE

MAVEN SCIENTISTS

2013

H T T P : / / B L O G . M A V E N S C I E N T I S T S . C O M

2 http://blog.mavenscientists.com OOPS Concepts - Java SE

Contents

1. Object 2. Class 3. Inheritance 4. Interface 5. Package


Object

The real world objects have two characteristics:

#Real World Objects

State Behaviour

Example:

Dog – State Behaviour Name Barking

Colour Running

Breed Wagging Tail

Hungry

Similarly, the software Objects have both the state and behaviour.

State – State of an Object is stored in the variables, notably called fields

in Java Language

Behaviour – Behaviour of an Object is expressed through the functions,

notably called methods in Java Language.

#Benefits of Objects

1. Modularity

2. Information – Hiding

3. Code Reuse

4. Pluggability and Debugging Ease


Class

In real world, there might be many objects which may belong to a single family.

Example: Bicycle is a class. (General Term) but

Jack’s Bicycle (which is a particular real world object ) is an instance (object )of

the class known as Bicycle.

An object refers to an existing particular thing. Now, the classification of the

type of object is called a class. We can also say that an object belongs to family

of objects, which is known as class.

Example of different Classes:

Car, Human Being, Cat, Dog, Bicycle etc.

Syntax:

class class_name {

}

Syntax Explanation:

Class – Class is a keyword

Class_name – Name of the class (An identifier)

{ } – Called a block. The block is defined by curly braces, which contains various statements. Example: methods, fields etc.


Code Example (Class):

class Bicycle {

int speed = 0;

int gear = 1;

void changeGear(int newValue) {

gear = newValue;

}

void speedup (int increment) {

speed += increment;

}

}

Fields (called variables in other

programming languages)

Methods (called functions in other

programming languages)


Inheritance

Object Oriented Programming allows to inherit commonly used state

and behaviour from other classes.

Example:

Now, here:

Animal - is the Super Class of all the classes

Mammal and Reptile – are the subclasses of class Animal.

Also Mammal and Reptile are the super classes for the classes Dog, Cat

and Frog, Snake respectively.

Dog , Cat - are the subclasses of class Mammal.

Frog , Snake - are the sub classes of class Reptile.

We can observe that the subclasses have inherited state and behaviour from

their respective super classes.

For example:

Mammals have hair. Hence both Dog and Cat have hair. (Talking about state)

Animal

Mammal Reptile

Dog Cat Frog Snake

Note :- A Super Class in Java has the potential of creating unlimited

subclasses.


Syntax:

class Sub_Class_Name extends Super_Class_Name {

}

Syntax Explanation:

class – keyword

Sub_Class_Name - name of sub class

extends – keyword

Super_Class_Name – name of the super class

Code Example (Inheritance):

class Dog extends Mammal {

}

Note :-

The Sub Class has all the same fields and methods as that of the

Super Class. In a Sub Class, you can define additional properties

(state and behaviour) too.

Multiple Inheritance is not allowed in Java. This means, you can not

have more than one Super Class of a single Class.


Interface

Methods from the object interface with the outside world. Example:

Power button on your TV, is the interface between you and the electrical

components.

Code Example:

interface Bicycle {

void changeGear();

void speedUp(); note the semicolon

}

To use this interface, you use implements keyword.

Code Example:

class MountainBicycle implements Bicycle {

// the methods changeGear() and speedUp() are defined here

}

Interface methods are public, abstract and never final.

Only prototypes are declared in the interface and no

implementations.


Package

Package is a namespace that organizes a set of related classes and

interfaces.

Package can be understood as similar to folders on a PC (Personal Computer).

The Java platform provides an enormous class library (a set of packages)

suitable for use in your own applications. This library is known as the

"Application Programming Interface", or "API" for short.

Java SE 7 API Specification:

http://docs.oracle.com/javase/7/docs/api/index.html

http://docs.oracle.com/javase/7/docs/api/index.html

Day 1 - OOPS Concepts - Java SE

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