A DESCRIPTION OF THE PROBLEM :
When performing sift-down operation, the moved "t' may not be a child of the target place, causing the min-heap structure being broken.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute the test code.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Tasks are executed in the ascending order of their respective delay, outputting the following content:

Delay 200
Delay 310
Delay 420
Delay 430
Delay 400
Delay 500
Delay 700
Delay 800
Delay 900
ACTUAL -
The output is as following. Note the 400-delayed task is executed after the 420 and 430 ones.

Delay 200
Delay 310
Delay 400
Delay 420
Delay 430
Delay 500
Delay 700
Delay 800
Delay 900

---------- BEGIN SOURCE ----------
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;

public class Application {

    private static final long[] DELAYS = new long[] {400, 900, 800, 700, 600, 430, 420, 310, 500, 200};
    private static final int DELAY_OFFSET = 5000;
    private static final CountDownLatch LATCH = new CountDownLatch(DELAYS.length);

    public static void main(String[] args) throws Exception {

        ScheduledFuture<?>[] tasks = new ScheduledFuture[DELAYS.length];
        ForkJoinPool pool = ForkJoinPool.commonPool();
        for (int i = 0; i < DELAYS.length; ++i) {
            TestRunnable runnable = new TestRunnable(DELAYS[i]);
            tasks[i] = pool.schedule(runnable, runnable.delay + DELAY_OFFSET, TimeUnit.MILLISECONDS);
        }
        Thread.sleep(50);
        tasks[4].cancel(true);
        LATCH.countDown();
        LATCH.await();
    }

    private static class TestRunnable implements Runnable {

        private final long delay;

        public TestRunnable(long delay) {
            this.delay = delay;
        }

        @Override
        public void run() {
            synchronized (TestRunnable.class) {
                System.out.println("Delay " + delay);
            }
            LATCH.countDown();
        }

        @Override
        public String toString() {
            return "toString called for delay " + delay;
        }
    }
}

---------- END SOURCE ----------