Active objects

Here is a classic problem given a piece of legacy code written without any threading consideration. How do we make it thread-safe?

The following class illustrates this problem:

private class LegacyCode {
  int x;
  int y;

    public LegacyCode(int x, int y) {
      this.x = x;
      this.y = y;
    }
    // setters and getters are skipped
    // they are there though 

There are two methods, m1() and m2(), which change the instance state in some way. Here is the m1() method:

public void m1() {
  setX(9);
  setY(0);
}

It sets the x field to 9 and the y field to 0:

public void m2() {
  setY(0);
  setX(9);
}

 The m2() method does the opposite: it sets x to 0 and y to 9. 

If we try to make this class concurrent, using threads, you know we need to carefully synchronize access to all shared states.  Of course, any legacy code has many other ramifications...