References in Java :

Reference :  Reference is an abstract identifier for a block of memory in the heap.

Special Reference: There are three special references in Java source code.

1. null
2. this
3. super

null reference : The null reference is an invalid reference. It has no type , and may be assigned to a variable of any reference type. When a reference variable is declared but not constructed , it is initially equal to null.

this reference : The this reference always refers to the current object, method .

Example:     int j = this.x;

super reference : The super reference refers to the methods and fields of the immediate super class. You need to use super prefix only if a field or method in the subclass has same name as a field or method in super class. 

If  you use super() as the first statement in a constructor, it calls the matching constructor in the immediate super class based on parameters passed to super() .

If you do not include an explicit call to super() as the first statement in your constructor, then the compiler will insert such a call into the byte code. The compiler always chooses to no  arguments super() constructor if you don't explicitly choose a different one.




    Example:     
                               public class SuperClass{

                                   public SuperClass(int i){

                              }
                     }
                         class SubClass extends SuperClass{

                                  public SubClass(){

                                 }
                           }


Note: If you try to compile this program you get the error message "no constructor matching SuperClass() found in class SuperClass. Because constructor chaining is mandatory in super-sub hierarchy.If you provide a parameterised constructor in super class then always put a noargs constructor in super class.