Multiple Objects

It is possible for two different reference variables to point to the same object.

When an object is no longer pointed to by any reference variable (including references stored deep inside the runtime or class library) it will be marked for garbage collection.

For example, the following program declares two TwoDPoint reference variables, creates one two-d point object, and assigns that object to both variables. The two variables are equal.

class EqualPointPrinter {
  public static void main(String[] args) {
    TwoDPoint origin1; // only declares, does not allocate
    TwoDPoint origin2; // only declares, does not allocate
    // The constructor allocates and usually initializes the object
    origin1 = new TwoDPoint();    
    origin2 = origin1;
    // set the fields
    origin1.x = 0.0;
    origin1.y = 0.0;
    // print
     "origin1 is at " + origin1.x + ", " + origin1.y);
     "origin2 is at " + origin2.x + ", " + origin2.y);
  }  // end main
} // end EqualPointPrinter

origin1 and origin2 are two different reference variables referring to the same point object.

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Copyright 1997-2006 Elliotte Rusty Harold
Last Modified September 18, 2006