A Threadpool has a Queue of tasks, of which each will be executed on one these Threads.

The following example shows how to add two int arrays using a Threadpool.

int[] firstArray = { 2, 4, 6, 8 };
int[] secondArray = { 1, 3, 5, 7 };
int[] result = { 0, 0, 0, 0 };

ExecutorService pool = Executors.newCachedThreadPool(); 

// Setup the ThreadPool:
// for each element in the array, submit a worker to the pool that adds elements
for (int i = 0; i < result.length; i++) {
    final int worker = i;
    pool.submit(() -> result[worker] = firstArray[worker] + secondArray[worker] );
}

// Wait for all Workers to finish:
try {
    // execute all submitted tasks
    pool.shutdown();
    // waits until all workers finish, or the timeout ends
    pool.awaitTermination(12, TimeUnit.SECONDS);
}
catch (InterruptedException e) {
    pool.shutdownNow(); //kill thread
}

System.out.println(Arrays.toString(result));

Notes:

1. This example is purely illustrative. In practice, there won’t be any speedup by using threads for a task this small. A slowdown is likely, since the overheads of task creation and scheduling will swamp the time taken to run a task.
2. If you were using Java 7 and earlier, you would use anonymous classes instead of lambdas to implement the tasks.