我们提供了一个类：
public class Foo {
public void one() { print("one"); }
public void two() { print("two"); }
public void three() { print("three"); }
}
三个不同的线程将会共用一个 Foo 实例。
线程 A 将会调用 one() 方法
线程 B 将会调用 two() 方法
线程 C 将会调用 three() 方法
请设计修改程序，以确保 two() 方法在 one() 方法之后被执行，three() 方法在 two() 方法之后被执行。
示例 1:
输入: [1,2,3]
输出: "onetwothree"
解释:
有三个线程会被异步启动。
输入 [1,2,3] 表示线程 A 将会调用 one() 方法，线程 B 将会调用 two() 方法，线程 C 将会调用 three() 方法。
正确的输出是 "onetwothree"。
示例 2:
输入: [1,3,2]
输出: "onetwothree"
解释:
输入 [1,3,2] 表示线程 A 将会调用 one() 方法，线程 B 将会调用 three() 方法，线程 C 将会调用 two() 方法。
正确的输出是 "onetwothree"。
注意:
尽管输入中的数字似乎暗示了顺序，但是我们并不保证线程在操作系统中的调度顺序。
你看到的输入格式主要是为了确保测试的全面性。
方法1：volatile
class Foo {volatile int num = 0;public Foo() {}public void first(Runnable printFirst) throws InterruptedException {while(num!=0){}// printFirst.run() outputs "first". Do not change or remove this line.printFirst.run();num++;}public void second(Runnable printSecond) throws InterruptedException {while(num!=1){}// printSecond.run() outputs "second". Do not change or remove this line.printSecond.run();num++;}public void third(Runnable printThird) throws InterruptedException {while(num!=2){}// printThird.run() outputs "third". Do not change or remove this line.printThird.run();}
}方法2：synchronize wait notifyAll
class Foo {Object lock = new Object();boolean fisrtDone = false;boolean secondDone = false;public Foo() {}public void first(Runnable printFirst) throws InterruptedException {synchronized(lock){// printFirst.run() outputs "first". Do not change or remove this line.printFirst.run();fisrtDone = true;lock.notifyAll();}}public void second(Runnable printSecond) throws InterruptedException {synchronized(lock){while(!fisrtDone){lock.wait();}// printSecond.run() outputs "second". Do not change or remove this line.printSecond.run();secondDone = true;lock.notifyAll();}}public void third(Runnable printThird) throws InterruptedException {synchronized(lock){while(!secondDone){lock.wait();}// printThird.run() outputs "third". Do not change or remove this line. printThird.run();}}
}思路3：AtomInteger
class Foo {private AtomicInteger firstJobDone = new AtomicInteger(0);private AtomicInteger secondJobDone = new AtomicInteger(0);public Foo() {}public void first(Runnable printFirst) throws InterruptedException {// printFirst.run() outputs "first".printFirst.run();// mark the first job as done, by increasing its count.firstJobDone.incrementAndGet();}public void second(Runnable printSecond) throws InterruptedException {while (firstJobDone.get() != 1) {// waiting for the first job to be done.}// printSecond.run() outputs "second".printSecond.run();// mark the second as done, by increasing its count.secondJobDone.incrementAndGet();}public void third(Runnable printThird) throws InterruptedException {while (secondJobDone.get() != 1) {// waiting for the second job to be done.}// printThird.run() outputs "third".printThird.run();}
}思路4：ConutDownLatch
class Foo {CountDownLatch first = new CountDownLatch(1);CountDownLatch second = new CountDownLatch(1);public Foo() {}public void first(Runnable printFirst) throws InterruptedException {// printFirst.run() outputs "first".printFirst.run();first.countDown();}public void second(Runnable printSecond) throws InterruptedException {first.await();// printSecond.run() outputs "second".printSecond.run();second.countDown();}public void third(Runnable printThird) throws InterruptedException {second.await();// printThird.run() outputs "third".printThird.run();}
}