synchronized block - lock more than one object

Question

I\'m modelling a game where multiple players (threads) move at the same time. The information of where a player is located at the moment is stored twice: the player has a variab

Answer 1

You should not feel bad about your modelling - this is only a two way navigable association.

If you take care (as in the other answers told) to manipulate atomic, e.g. in the Field methods, thats fine.

public class Field {

  private Object lock = new Object();

  public removePlayer(Player p) {
    synchronized ( lock) {
      players.remove(p);
      p.setField(null);
    }
  }

  public addPlayer(Player p) {
    synchronized ( lock) {
      players.add(p);
      p.setField(this);
    }
  }
}

It would be fine if "Player.setField" were protected.

If you need further atomicity for "move" semantics, go one level up for board.