1.這種方法可以再新增多個執行緒，定義好執行的順序就可以了。喚醒的時候全部喚醒。不是當前執行緒執行時還說會休眠。

public class Main {

    private static boolean isFirst = true;

    static class First implements Runnable {
        private Object lock;

        public First(Object lock) {
            this.lock = lock;
        }
        @Override
        public
 
 void run() {
            while (true) {
                synchronized (lock) {
                    if (isFirst) {
                        System.out.println("1");
                        isFirst = false;
                        lock.notify();
                    } else {
                        try {
                            lock
 
.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        }
    }

    static class Second implements Runnable {
        Object lock;

        public Second
 
(Object lock) {
            this.lock = lock;
        }

        @Override
        public void run() {
            while (true) {
                synchronized (lock) {

                    if (!isFirst) {
                        System.out.println("2");
                        isFirst = true;
                        lock.notify();
                    } else {
                        try {
                            lock.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        }
    }

    public static void main(String[] args) {
        Object lock = new Object();
        new Thread(new First(lock)).start();
        new Thread(new Second(lock)).start();
    }
}

2.這種方式只是此問題的另一隻解法而已。

public class Main {

    public synchronized void print(String str) {
        notify();
        System.out.println(str);
        try {
            wait();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    class A implements Runnable {

        @Override
        public void run() {
            while (true) {
                print("A" + 1);
            }
        }
    }

    class B implements Runnable {

        @Override
        public void run() {
            while (true) {
                print("B" + 2);

            }

        }
    }

    public static void main(String[] args) {
        Main p = new Main();
        A a = p.new A();
        B b = p.new B();
        new Thread(a).start();
        new Thread(b).start();

    }

}