1114. Print in Order

class Foo {

    private final CountDownLatch latchA;
    private final CountDownLatch latchB;

    public Foo() {
        this.latchA = new CountDownLatch(1);
        this.latchB = new CountDownLatch(1);
    }

    public void first(Runnable printFirst) throws InterruptedException {
        // printFirst.run() outputs "first". Do not change or remove this line.
        printFirst.run();
        latchA.countDown();
    }

    public void second(Runnable printSecond) throws InterruptedException {
        latchA.await();
        // printSecond.run() outputs "second". Do not change or remove this line.
        printSecond.run();
        latchB.countDown();
    }

    public void third(Runnable printThird) throws InterruptedException {
        latchB.await();
        // printThird.run() outputs "third". Do not change or remove this line.
        printThird.run();
    }

}

Reference

1114. Print in Order