Passing the address itself rather than passing the value is 
called ?Pass By Reference? and  Passing a copy of the value 
to be passed is called as ?Pass by Value?.

***In Java, primitives and object references are passed by 
value.*** 
The variable represents the bit information which is copied 
to the parameter. 

eg:primitive****
Say for primitives x=2, the value of 2 is represented in 
bits and it is copied to the parameter variable.

eg:object reference*** 
Button b = new Button("hello");
Button c = b;
Similarly for object reference the bit value in the 
reference is copied to the parameter reference. Thus both 
references have the same value representing an object. The 
object reference(bit representation) represents a way to 
get to the object.

Pass-by-value means, Passing a copy of the value.
Pass-by-reference means, instead of passing the actual
value, the address(of the value) is passed.