public class ThreadPrint {

    public static void main(String[] args) throws Exception {
    String lock = "lock";
    Thread t1 = new Thread(new Thread1(lock));
    t1.start();
    Thread.sleep(1000);
    t1.stop();
    Thread t2 = new Thread(new Thread2(lock));
    t2.start();
    Thread.sleep(1000);
    t2.stop();
    }

}

class Thread1 implements Runnable {
    String lock;
    public Thread1(String lock) {
    this.lock = lock;
    }
    public void run() {
    synchronized (lock) {
        for (int i = 1; i < 53; i++) {
        System.out.println("The first thread.-->" + i);
        if (i % 2 == 0) {
            try {
            lock.notify();
            lock.wait();
            
            } catch (InterruptedException e) {
            e.printStackTrace();
            }
        }
        }
    }
    }

}

class Thread2 implements Runnable {
    String lock;

    public Thread2(String lock) {
    this.lock = lock;
    }

    public void run() {
    synchronized (lock) {
        for (char i = 'A'; i < 'Z'; i++) {
        try {
            System.out.println("The second thread.-->" + i);
            lock.notify();
            lock.wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        }
    }

    }

}

------解决方案--------------------
Thread-0:0
Thread-1:A
Thread-0:1
Thread-0:2
Thread-1:B
Thread-0:3
Thread-0:4
Thread-1:C
Thread-0:5
Thread-0:6
Thread-1:D
Thread-0:7
Thread-0:8
Thread-1:E
Thread-0:9
Thread-0:10
Thread-1:F
Thread-0:11
Thread-0:12
Thread-1:G
Thread-0:13
Thread-0:14
Thread-1:H
Thread-0:15
Thread-0:16
Thread-1:I
Thread-0:17
Thread-0:18
Thread-1:J
Thread-0:19
Thread-0:20
Thread-1:K
Thread-0:21
Thread-0:22
Thread-1:L
Thread-0:23
Thread-0:24
Thread-1:M
Thread-0:25
Thread-0:26
Thread-1:N
Thread-0:27
Thread-0:28
Thread-1:O
Thread-0:29

从运行的结果可以知道，在Thread1 运行到30的时候，不会进入到循环了。但是，你并没有释放lock锁，而Thread2还在等待lock锁，这样就导致了，线程一直等待了。注：当调用wait()方法后，只有调用了notify()方法后才会释放锁的