String a;
Point  b;

The null Value

A reference variable holds information about the location of an object. It does not hold the object itself. This code...

String a;
Point  b;

... declares two reference variables but does not construct any objects. The following constructs objects and puts references in the variables:

a = "Elaine the fair." ;
b = new Point( 23, 491 );

A special value called null is assigned to an object reference variable when it does not refer to an object.

The value null is a special value that means "no object." A reference variable is set to null when it is not referring to any object.