[Java] Problem with a Queue

[cdiem]

I'm trying to make a simple snake clone for fun. I got stuck when trying to update the drawing of the snake on the screen. The problematic part is adding a node to a queue (similar to the following):



int[] test = new int[2];
Queue<int[]> dummyQueue = new LinkedList<int[]>();
for (int i = 0; i < 3; i++) {
test[1] += 50;
dummyQueue.add(test);

// show me some results:
for (int[] node : dummyQueue) {
System.out.print(Arrays.toString(node));
}
System.out.println();
}


This outputs:



[0, 50]
[0, 100][0, 100]
[0, 150][0, 150][0, 150]


Why does adding an element to the queue change every other element in the queue? Is this an expected behaviour? How would you rewrite it, so that it shows:


[0, 50]
[0, 50][0, 100]
[0, 50][0, 100][0, 150]


Basically I need a queue of tuples of (x,y) coordinates - but the above outputs surprised me. Please, help.

[Zugzwang]

Ok, the problem here is that you keep appending the same tuple over and over again to the Queue. When adding the arrray to the queue, it is *not* copied. You will need to allocate a *new* one each time.

[raffaele181188]

zugzwang was right. You'd better use a Point obj, consider


class Point {
// Add constructor
int x, y;
}

public class DrawRoutine {
int lastX = 0, lastY = 0;

public void draw(...args...) {
for (...) {
dummyQueue.add(new Point(lastX, lastY =+ 50));
}
}
}

[cdiem]

Thank you very much; now I understood my mistake. The "test" array mutates inside the queue, not being copied - hence the results. Thanks once again.