Variables in Java

In java a variable is nothing but name, which can be used to store the values. A variable is like a container which can hold the different type contents. Internally a variable is represented as a memory location name.

Types of variables in Java

Based on the value holding a variable are divide into two types.

• Primitive Variable
• Reference Variable

Primitive Variable

Primitive Variables only allow to store single value and hold single value.

Example of primitive variable

	
    //primitve variable..
    int a = 10;
    char ch = 'b';
    float a = 2.5f;
    

Reference Variable

Reference Variable allow to store multiple values in a single variable.

Example of reference variable

	
    //reference variable..
    String str = new String();
    String st = new String();    
    

Based on the position of declaration and based on the position all variables divide in three types.

• Instance Variable
• Static Variable
• Local Variable

Instance Variable

• When we go for instance variable?

If the value of the variable has to be different from one object to another object then we should go instance variable.

• Why this variable called Instance Variable?

As these variables are created and available with the object that's why these variable are called instance variable or object variable.

• Position of Instance Variable

Instance variable directly declared inside the class not inside the method area or constructor.

• Memory Allocation

Instance variable allocate the memory when we declare or create the object. Memory will be allocated for these variable inside the object and object created in heap area.

• Initialization of Instance Variable

Initialization of instance variable is optional. If we don't initialized instance variable then JVM will automatically assign default value.

• Scope and Lifetime

Scope and Lifetime of instance variable same as a object. If object gone then instance variable will destroy.

• Every object will have separate copy of instance variable.
• These variable are called object level members.
• These variable are called object level members.

Static Variable

• When we should go for static variable?

If the value of variable has to be same from one object to another object.

• Declaration of Static variable

Static Variable are declared directly within the class but using static modifier. But not inside any method or block or constructor.

	
    int i; \\instance variable
    
    static int i; \\static variable
    

• Memory Allocation

Memory will be allocated for these variables at the time of loading the class.

• These variables are created inside method area.
• Initialization is optional, same as a instance variable.
• The Scope and Lifetime of static variable is same as class.

When class is loaded, variable created.

When class is unloaded, variable destroy.

• A single copy of static variable is created in mehtod area as class is loaded only once. Which is shared by all the objects.
• This variable are called class level member.

Local Variable

• When we go for local variable?

If the requirement for the programmer is to use the same variable for temporary usage then should go for local variable.

• Other name of local variable is temporary variable or automatic variable.
• Declaration of Static variable

These variable created inside the stack memory. Which is temporary memory.

	
    class Book
    {
    	void pen()
        {
        	//local varialbe initialization..
        	String color  = "red";
        }
    }
    

• Position of Local Variable

These variable are declared within the method or a block or a constructor, but inside the class.

• Memory Allocation

Memory will we allocated for these variable when method or a block execution starts.

• Initialization of local variable

Initialization of local variable is mandatory.

• Scope and Lifetime

The Scope and Lifetime of local variable is within the method or block. Lifetime of local variable is same as method or block.

• Local variable destroy when method execution complete.