Program 21.4 Side Effects

There are two times when this does become important though. The first is with side effects. If you create an object a and then set b equal to a then changing some property of b changes that property of a also. a and b refer to the same space in memory. For example

import java.awt.Point;

public class RefTest {

  public static void main(String[] args) {
    Point a = new Point(3, 2);
    Point b = a;
    b.x = 33;
    b.y = -7;
    System.out.println(a);
  }

}

And herešs the somewhat surprising output:

% java RefTest
java.awt.Point[x=33,y=-7]
%

This program tried to print a but it looks like you got b instead. Thatšs because a and b are references to the object, not the object itself. When a was assigned to b, b was made to point to the same location as a. Therefore when the value of the object at that location changes, both and b change. This is true for any object at all in Java including arrays. It is not true for primitive data types like int, float or char.

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