经典

线程交替打印：

private int count = 0;
private final Object lock = new Object();

public void turning() throws InterruptedException {
    new Thread(new TurningRunner(), "偶数").start();
    // 确保偶数线程线先获取到锁
    Thread.sleep(1);
    new Thread(new TurningRunner(), "奇数").start();
}

class TurningRunner implements Runnable {
  @Override
  public void run() {
      while (count <= 100) {
          // 获取锁
          synchronized (lock) {
              // 拿到锁就打印
              System.out.println(Thread.currentThread().getName() + ": " + count++);
              // 唤醒其他线程
              lock.notifyAll();
              try {
                  if (count <= 100) {
                      // 如果任务还没有结束，则让出当前的锁并休眠
                      lock.wait();
                  }
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
          }
      }
  }
}