Java Variables

To manipulate data, we have to reserve some space in memory and to access that memory space, that memory space should have some name. Variables names are memory blocks name, where data is stored. Data type of a variable defines what kind of data and what range of value can be stored in a variable.

Syntax of declaring a variable in java:

data-type variable-name;

We can also initialize when declaring a variable. For example:

int a = 5;

Types of Variables in Java

- Local Variable

- Instance Variable

- Class/Static Variable

Local Variable

Local variables can only be locally accessed. Local variables are declared in blocks, methods, constructors etc. Life time of a local variable is local to a block in which it is defined. Suppose a variable i is declared in a method, when this method is executing variable i reserves space in memory and destroys when this methods completes execution.

We can not give any access modifier to a local variable.

Programming Example of Local Variable in Java

public class LocalVariable

{

 void sum(int x, int y)

 {

  int a =x, b = y; //Local Variables

  int c = a + b;  //Local Variable

  System.out.println("Sum of "+a+" and "+b+" is "+c);

 }

    public static void main(String args[])

    {

  LocalVariable L = new LocalVariable();

        L.sum(1, 3);

 }

}

OUTPUT:

Sum of 1 and 3 is 4

Instance Variables

Instance variables are declared inside a class and outside all methods, constructors, or any bock. When an object is created, instance variables occupies space in heap memory. And when object destroys, instance variables also destroys. We can also give any access modifier to an instance variable. All methods, constructors or any block can access instance variables. It is a good practice to make instance variables private, so they are private to other classes. 

Programming Example of Instance Variable in Java

public class InstanceVariable

{

        private int roll_no;

        public String name;

        public InstanceVariable(String n)

        {

                name = n; 

        }

        public void setRoll_no(int r)

        {

                roll_no = r;

        }

        public void showDetail()

        {

             System.out.println("Name : "+name+"\nRoll no. : "+roll_no);

        }

    public static void main(String args[])

    {

                InstanceVariable I = new InstanceVariable("John");

                I.setRoll_no(32);

                I.showDetail();

    }

}

OUTPUT:

Name : John

Roll no. : 32

Class/Static Variable

Static variables are declared outside all instance member methods, constructors or any block with a static keyword. Static variables are also known as class variables. Static variable does not depends on object of a class. There is only one copy of a static variable for a class.

Static variable loads into memory when class is loading into JVM and removes from memory when class is removing from JVM.

Static variables can be accessed with the class name, 

Syntax:

class-name.variable-name;

Programming Example of Instance Variable in Java

public class StaticVariable

{

        public static String course;

        private int roll_no;

        public String name;

        public StaticVariable(String n)

        {

                name = n; 

        }

        public void setRoll_no(int r)

        {

                roll_no = r;

        }

        public void showDetail()

        {

       System.out.println("Name : "+name+"\nRoll no. : "+roll_no+"\nCourse : "+course);

        }

    public static void main(String args[])

    {

                StaticVariable I = new StaticVariable("John");

        I.setRoll_no(32);

                StaticVariable.course = "JAVA";

                I.showDetail();

        }

}

OUTPUT:

Name : John

Roll no. : 32

Course : JAVA