Use of Static keyword

Static keyword in java is mainly used for memory management.this is mainly used for variable,methods and blocks.

• Java static variable : It is used to refer common property of all the objects. As name of city which is common for all the entity.
• Static variable gets memory at time of class load.

This explain you how static variable works , if you have some more time just see it.

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Simple Static example

public class studentdetails {

int age ;
String name;
static String city="fremont";
studentdetails(int age,String name)
{
this.age=age;
this.name=name;
}

public static void hello(){ //this is a static method
     city = "foster";
System.out.println("My city is "+city);// Static method can use only static variable
}

public void test(){
System.out.println("My Name is -->"+name);
System.out.println("My age is -->"+age);
System.out.println("City is -->"+city);

}
// static method can use only static variable 

public static void main(String[] args) {
studentdetails obj1 = new studentdetails(22,"Ram");
studentdetails obj2 = new studentdetails(19,"krishna");  
obj1.test();//variable city is loaded only once in class so memory
obj2.test();//second time also when object is called static variables memory remains same
hello();
}
}

Output:-

My Name is -->Ram
My age is -->22
City is -->fremont
My Name is -->krishna
My age is -->19
City is -->fremont
My city is foster

Static method is used by putting a static keyword after access modifier  in method as seen in above example.

• Static method uses static variable and can also change value of it.
• Static method can be call without creating instance of class,as it belongs to class

Static method is used in counter of website as when object is created the value is add rather creating instance of object.

Example : this program shows difference of creating object with static and non-static variable, expense of  room mates total and personal for different students.

program

public class Room {

static double totalExpense;

double localexpence=0;

Room(){

}

public void addExpense(double amount){

totalExpense=totalExpense+amount; // method use static variable to add total expense 

       System.out.println("total is "+totalExpense);

}

public void addnewexpence(double amount){

localexpence=localexpence+amount; // it uses non-static variable to add personal expense

System.out.println("personal expence is "+localexpence);

}

//every time when object is created new memory is allocated to non-static variable 

       // static variable shares common memory , so not invoked when instance is created

public static void main(String[] args) {

 
      Room obj1=new Room();

obj1.addExpense(20d);

obj1.addnewexpence(40d);

Room obj2=new Room();

obj2.addExpense(50d);

obj2.addnewexpence(60d);

Room obj3=new Room();

obj3.addExpense(40d);

obj3.addnewexpence(70d);

}

}

Output:-

total is 20.0

personal expence is 40.0

total is 70.0

personal expence is 60.0

total is 110.0

personal expence is 70.0

Object oriented programming

Object oriented programming is method by which you can program using classes and objects.Object is entity existing in real world.Some concepts of object oriented programming are as follows:

1. Object
2. Class
3. Inheritance
4. Encapsulation
5. Abstraction
6. polymorphism 

Object: Object is entity in existing real world with some properties. 

Class : In real world a class is an entity from which the object are created.Every car was constructed from same set of defined procedures,so contains same components. A class is an entity from where object are created.Class is a collection of object of similar type.

Inheritance: It is quality of class to acquire all the qualities and properties of parent class.It provides code re-usability.

Encapsulation: Process of binding code and data together in a single unit is know as encapsulation.Is is used to hide the implementation from the end user.

Abstraction: It refer to quality of hiding internal details and showing functionality.In is used to implement interface in java.

Polymorphism: It refers to the objects and methods having same name but different behavior.It also refer to methods having different parameters. 

Arrays in Java

An array is collection of values of same datatype.Its length is decided when it is created.It has index by which its element are accessed.
Index of array is always from 0 to n-1 ( for n size of array).

• How to declare array?

datatype[ ]  array_name; Example : ( int[] array_name; ) or (float[] array_name;)

• How to allocate memory to array?

array-name = new datatype[size];

 Example( array_name= new int[10];) or (array_name=new float[15]). 

size is number of elements you can store array.

• How you can initialize array?

array_name[0]= value;

array_name[1]=value1;

Example

array_name[0]=1;

array_name[2]=4;

Above example state that '1' value is stored at position 0 and '4' at position 2.

• How to access element of array at some position ?

System.out.println("Elements at position 1"+array_name[1]);

• How to do initialize and declare array in same step?

int array_name[] ={3,70,47,87,34,55};

Program Example in Eclipse

import java.util.Scanner;
public class arrayexp {

public static void main(String[] args) {
int i;
Scanner sc = new Scanner(System.in);
System.out.println("enter how much element u want in array ");
i=sc.nextInt();
int Arry[] = new int[i];
System.out.println("enter element s");
for(int a=0;a Arry[a]=sc.nextInt();
}
System.out.println("Elements in array are ");
for(int a=0;a System.out.print(" "+Arry[a]);
}

}


}

Output

enter how much element u want in array
4
enter element s
2
4
6
8
Elements in array are
 2 4 6 8