How to synchronize the work of threads for incrementing a loop variable?

Question

Does the ability to synchronize the increment of a variable exist, so that picking up the values of i the thread will work with it further along the code, and the next thread would use the variable i with the next value (i + 1). Ie there is a loop and the threads in it should work with ArrayList elements:

for ( int i = 0; i < arrayList.size(); i++) { System.out.println(arrayList.get(i)); newArrayList.add(arrayList.get(i)); }

The problem is that it is impossible to force the threads to miss the unique values of i, and the threads work with the same data ...

If you need to synchronize a variable of type int, then use the class java.concurrent.atomic.AtomicInteger

Accepted Answer · 2018-09-16T18:17:21

The threads themselves can increment the variable and select the desired element of the array.
If for simplicity, the workflows are disposable, and the array in the process does not change, then you can do it this way:

  // создаем массив с данными (например с числами 1-100) ArrayList<Integer> list = new ArrayList<>(); for (int i=1; i<=100; i++) list.add(i); // объявляем и инициализируем потокобезопасную переменную счетчика final AtomicInteger n = new AtomicInteger(-1); // описываем поток который будет работать с элементом массива Runnable r = new Runnable() { @Override public void run() { int element = list.get(n.incrementAndGet()); // поток будет самостоятельно инкрементировать счетчик System.out.println(Thread.currentThread().getName() + " get element " + element); // и что то делать с элементом } }; // запускаем потоки по количеству элементов массива ExecutorService service = Executors.newFixedThreadPool(5); for (int i=0; i<list.size(); i++){ service.submit(r); } service.shutdown(); // даем всем потокам доработать

How to synchronize the work of threads for incrementing a loop variable?

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