1ae9.md

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Solution to 1ae9: Reusing immutable value objects

See code at solutions/code/tutorialquestions/question1ae9

I have implemented makePoint as follows:

public static Point makePoint(int coordX, int coordY, int coordZ) {
  Point p = new Point(coordX, coordY, coordZ);
  if (pool.get(p) != null) {
    return pool.get(p);
    // p will now be garbage collected
  }
  pool.put(p, p);
  return p;
}

The first thing this method does is calls the constructor of Point to make a new point. So the approach does not prevent lots of identical points from being created. However, it does avoid lots of identical points being stored forever. The test: if(pool.get(p) != null) checks whether pool contains an entry of the form (first, second) such that p.equals(first) holds.

If this is the case, the method returns second. As a result, there are no references to the point that was referred to by p, and eventually (though, as confirmed in question b4a5, we do not know when) the point will be garbage-collected.

Otherwise, the statement pool.put(p, p) causes the entry (p, p) to be inserted into the pool. This means that in future, if we call have a point t with t.equals(p), the test pool.get(t) != null will hold. As a result, we will never permanently create another point that is identical to p.

See class objectpool.Demo for a demonstration that this solution really works.