Threading in Java. Creating each figure in new thread, loop not working

Question

I want to create a separate thread for each of my created rectangles. I need to pass arguments to run the thread, and it's not allowed. I can't figure out how to do this. This is what I already wrote:

    int number_of_cubes = 10;
    Rect[] r1 = new Rect[number_of_cubes];
    for(int i=0; i <number_of_cubes;i++){
        Thread myThread = new Thread(new Runnable()
        {
            public void run(Rect[] r1,int i){
                Random rn = new Random();
                Random rand = new Random();
                float r = rand.nextFloat();
                float g = rand.nextFloat();
                float b = rand.nextFloat();
                Color randomColor = new Color(r, g, b);
                r1[i] = new Rect(rn.nextInt(600), rn.nextInt(400), 15, 15, randomColor);
            }

        });
    }

Answer 1

As for your immediate question, use

final Rect[] r1 = new Rect[number_of_cubes];
for (int i = 0; i < number_of_cubes; i++) {
  final int targetIndex = i;
  new Thread(new Runnable() { public void run() {
    ...
    r1[targetIndex] = ...
  }}).start();

And a few notes:

• the overhead of thread creation is large enough for this idiom to start making sense only if you have a substantial amount of work for it. Say, at least 10,000 rectangles;

• you are redundantly creating two Random instances. Use just one per thread;

• watch out for visibility issues: you may use the rectangle array only after all threads have finished ( join on each thread from the main method);

• you'll experience performance gain only with a moderate number of threads, usually equal to the number of available CPU cores;

• a much better approach would be to use an Executor Service.

Answer 2

Do this, It will create a function that runs the thread you want with the parameters. Then, in the for loop you can call it however you would like:

nt number_of_cubes = 10;
Rect[] r1 = new Rect[number_of_cubes];

for(int i=0; i <number_of_cubes;i++){
    //call the function here if you want
}
public void runThread(final Rect[] r1,final int i){
    new Thread(new Runnable(){
        @Override
        public void run(){
            Random rn = new Random();
            Random rand = new Random();
            float r = rand.nextFloat();
            float g = rand.nextFloat();
            float b = rand.nextFloat();
            Color randomColor = new Color(r, g, b);
            r1[i] = new Rect(rn.nextInt(600), rn.nextInt(400), 15, 15, randomColor);
        }

    }).start();
}