Instance Members in Java





  • All non-static members (variables , methods) are called instance members.

  • Instance members are encapsulated into object.

  • Every object has its own copy of instance members.

A class can have the following instance members :
  • Instance data fields (either variable or constants)
  • Instance methods
  • Non-static initializer block (instance block)
  • Inner class

Non-static/Instance Variable :

  • Instance variables are declared in a class, but outside a method.

  • When a space is allocated for an object in the heap, a slot for each instance variable value is created.

  • Instance variables have default values. For numbers the default value is 0, for Booleans it is false and for object references it is null.

Example

class Demo
{
 int x;
 public static void main(String []arg)
 {
  Demo ob1 = new Demo();
  Demo ob2 = new Demo();
 }
}

 Here x is non-static variable of integer type in class Demo.
 There will be separate copy of x created for all instance of Demo class.
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Non-static Method

  • Usually create to operate instance data of the class.

  • A non-static method can access other static & non-static members (variables, methods) in the same class directly.

  • A non-static method implicitly provides 2 references :

    • this : Reference to the current object.

    • super : Reference to the super object.

Example

class Demo
{
 int x;
 static int y ;
 void show()
 {
/* direct accessing of both static
   and non-static */
  System.out.println("x :"+x);
  System.out.println("y :"+y);
 }
 public static void main(String []arg)
 {
  Demo ob1 = new Demo();
/*since show is non-static object is
required to access in static method */
      ob1.show();
 }
}
            

Non-Static Block :

  • Non-static block of code provided in the class, execute every time an object of the class is constructed.

  • It automatically executes right after the allocation of non-static data fields and just before the invocation of constructor.

  • It can be used to initialized the non-static data fields with some dynamic value.

Example

class Demo
{
 // non-static block
 {
System.out.println("Non-Static Block");
 }
 public static void main(String []arg)
 {
  Demo ob1 = new Demo();
  Demo ob2 = new Demo();
  //Anonumous Object
  new Demo();
 }
}           
OUTPUT :
Non-Static Block
Non-Static Block
Non-Static Block

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