OOP-Java.md

Class - Named group of properties and functions

Objects - Instance of a Class

Class is logical construct and objects are Physical reality

Objecta are categorised by state,identity,behaviour.

Object are created in heap memory and variables in stack .

Dynamic memory allocation (new)

    Student student = new Student();
-------------------   ---------------
     Complile time.      Runtime

Java Constructor

A special function that runs when we create a object and it allocates some variables. Allocate values that we need when creation of objects.

" this " keyword

access the reference variable of object

cannot use inside static blocks.

By Default Constructor

a constructor without argument is acually a default constructor.

Constructor Overloading

Same as function overloading

We can call a constructor inside another constructor using the this keyword.

In java primitives are not implemented as objects so we dont use new keyword to initialise them.

Wrapper Class

Integer num =45 it is now created as object

When we pass objects reference values are passed.

Final Keyword

final int INCREASE = 2; always initialise when using final keyword

`final Student akshay = new Student();

when a non primitive is final we cannot reassign it but we can change the values.

Garbage Collection

finalise method

Distructing ..java executes this method before garbage collection.sort of like a clean up method.

@Override
protected void finalise() throws Throwable {
System.out.println(" Object is distroyed ");
}

Static in java

properties common to all the object of a class

To acess static variable inside constructor we use Class name

Inner class

Outside class cannot be static

inner class can be static

Singleton Class

Just a class that we can create only one object of

public class Singleton{
private Singleton(){
private Singleton instance;

public static Singleton getInstance(){
if(instance == null){
instance = new Singleton():
}
}

}

Inheritance

extends keyword

super () used to initialise values in parent class.

Types of Inheritance

Single Inhertance : one class extends another class.

Multilevel Inheritance : java doesnt support

Hierachical Inhertance

Hibrid Inheritance - Combination of single and multiple inheritance

Polimorphism

Compile Time or static polymorphishm --- bY METHOD OVERLOADING

Runtime or Dynamic polymorphism ----by method overiding. @Override keyword

CHILD CLASS OVERRIDES PARENT METHOD

Interfaces

You can inherit but you cant override static methods

Encapsulation

wrapping up implemention of data members & methods in a class

Abstraction

Hiding unnessasary details and showing valuable information.

Data Hiding