实现两个线程交替执行

今天看到一道面试题，要求实现两个线程交替打印A、B，作为练习，总结一下 代码

public class Info {

    String msg = "A";
    boolean flag = false;//交替执行标志
}

线程A

**
 * 线程A，负责打印A
 * Created by DONGSHILEI on 2017/10/17
 */
public class ThreadA implements Runnable{
    private Info info;

    public ThreadA(Info info) {
        this.info = info;
    }

    @Override
    public void run() {
        while (true){
            synchronized (info){
                if (!info.flag){
                    try {
                        //线程等待
                        info.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                } else {
                    System.out.println(Thread.currentThread().getName()+"--"+info.msg);
                    info.flag = false;
                    info.msg = "B";
                    //唤醒等待info的线程，选择一个
                    info.notify();
                }
            }
        }
    }
}

线程B

**
 * 线程B 负责打印B
 * Created by DONGSHILEI on 2017/10/17
 */
public class ThreadB implements Runnable{
    private Info info;

    public ThreadB(Info info) {
        this.info = info;
    }

    @Override
    public void run() {
        while (true){
            synchronized (info){
                if (info.flag){
                    try {
                        info.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                } else {
                    System.out.println(Thread.currentThread().getName()+"--"+info.msg);
                    info.msg = "A";
                    info.flag = true;
                    info.notify();
                }
            }
        }
    }
}

测试程序

**
 * 两个线程，交替打印A、B
 * Created by DONGSHILEI on 2017/10/17
 */
public class LockTest {

    public static void main(String[] args) {
        //公共对象，在线程获得该对象时加锁
        Info info = new Info();
        ThreadA a = new ThreadA(info);
        ThreadB b = new ThreadB(info);
        Thread t1 = new Thread(a);
        Thread t2 = new Thread(b);
        t1.start();
        t2.start();

    }
}

执行结果

结果
-0--A
Thread-1--B
Thread-0--A
Thread-1--B
Thread-0--A
Thread-1--B
Thread-0--A
Thread-1--B
Thread-0--A
Thread-1--B
Thread-0--A
Thread-1--B
Thread-0--A
Thread-1--B
Thread-0--A