Question

I try hard to find the problem in this Java code, but I can’t find it – can you help me?

I hope the code I provide is enough, but I will post more if necessary.

Further I apologize, I didn’t make a minimal example.

game.getGroupPlayers().list();
MoverThread[] playerThread = game.getPlayers();
System.out.println(playerThread.length);
for (int i = 0; i < playerThread.length; i++) {
    try {
        System.out.println(i + " -> " +playerThread[i].toString());
        returnString += playerThread[i].toString() + "\n";
    } catch(NullPointerException e) {
        System.out.println("Problem at i = " + i);
        e.printStackTrace();
    }
    game.getGroupPlayers().list();
}

sometimes gives me the following output:

java.lang.ThreadGroup[name=Players,maxpri=10]
Player-0: 113
Player-1: 277
Player-2: 0
3
0 -> Player-0: 113
1 -> Player-1: 277
Problem at i = 2
java.lang.NullPointerException
at Referee.goalFound(Referee.java:70)
at DebugTestReferee.goalFound(DebugTestReferee.java:42)
at Player.checkGoal(Player.java:61)
at Player.run(Player.java:94)
at java.lang.Thread.run(Thread.java:636)
java.lang.ThreadGroup[name=Players,maxpri=10]
Player-0: 113
Player-1: 277
Player-2: 0

[edit]
here’s the source of getPlayers()

/*
 * post returns the games players as an array
 */
public MoverThread[] getPlayers() {
synchronized(movers) {
    MoverThread[] playerList = new MoverThread[players.activeCount()];
    players.enumerate(playerList);
    return playerList;
}
}

[edit]
here’s how players is generated

private ThreadGroup movers;
private ThreadGroup players;
private ThreadGroup ghosts;

private Observer observer;

/*
 * constructor
 */
 public Game(Maze maze, Referee referee) {
this.maze = maze;
this.referee = referee;
threadList = new ArrayList<MoverThread>();
movers = new ThreadGroup("Movers");
players = new ThreadGroup(movers, "Players");
ghosts = new ThreadGroup(movers, "Ghosts");
observer = null;
}

[edit]

Here’s how I call the method that generates the problem:

/*
 * post checks if the players thread was interrupted - if not if hostfield pretends to be a goal the game gets stopped and referee is called to perform "goal-found-actions"
 */
private void checkGoal() {
if (!getThread().isInterrupted()) {
    synchronized(getGame().getMovers()) {
        if (!getThread().isInterrupted()) {
            if (getHostField().isGoal()) {
                Field goal = getHostField();
                getGame().getReferee().goalFound(this, goal);
                getGame().setGameOver();
            }
        }
    }
}
}

and here’s the whole goalFound()

/*
 * post action to be performed if a player finds a goal
 * print some information
*/
public void goalFound(Player player, Field at) {
//FIXME get the Bug!!!
String returnString = "Game over - player " + player.getName() + " found a goal on (" + at.getPos()[0] + ", "  + at.getPos()[1] + ")!\n";
game.getGroupPlayers().list();
MoverThread[] playerThread = game.getPlayers();
System.out.println(playerThread.length);
for (int i = 0; i < playerThread.length; i++) {
    try {
        System.out.println(i + " -> " +playerThread[i].toString());
        returnString += playerThread[i].toString() + "\n";
    } catch(NullPointerException e) {
        System.out.println("Problem at i = " + i);
        e.printStackTrace();
    }
}
game.getGroupPlayers().list();
returnString += game.mazeString();
System.out.println(returnString);
}

Answer 1

I found a solution – or maybe more a workaround…

Beside using ThreadGroups I store my threads in an ArrayList aswell (maybe a Vector would be even better, but I’m fine with the ArrayList).

I don’t know why, but when I try to call all Threads in the ThreadGroup it often happens that some Threads are left out. However, with the ArrayList it works fine.

Would be interesting why ThreadGroups don’t work as supposed and what we need them for in this case.