public class ThreadTest {
    
    private Thread child = null;

    /**
     * @param args
     */
    public static void main(String[] args) {
        ThreadTest test = new ThreadTest();
        test.work();
    }
    
    public void work() {
        myThread1 thread1 = new myThread1();
        myThread2 thread2 = new myThread2();
        
        //将第二个线程的引用赋值给成员变量child
        this.child = thread2;
        
        thread1.start();
        thread2.start();
    }
    
    class myThread1 extends Thread {
        
        public void run() {
            synchronized(child) {
                try {
                    child.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                
                System.out.println("我睡醒了，现在开始打印信息");
            }
        }
    }
    
    class myThread2 extends Thread {

        public void run() {
            synchronized(this) {
                try {
                    sleep(5 * 1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                
                System.out.println("唤醒另一个线程吧");
                this.notifyAll();
            }
        }
        
    }

}

------解决方案--------------------
忘记加注释了

Java code

public class ThreadTest {
    
    private Thread child = null;

    /**
     * @param args
     */
    public static void main(String[] args) {
        ThreadTest test = new ThreadTest();
        test.work();
    }
    
    public void work() {
        myThread1 thread1 = new myThread1();
        myThread2 thread2 = new myThread2();
        
        //将第二个线程的引用赋值给成员变量child
        this.child = thread2;
        
        thread1.start();
        thread2.start();
    }
    
    class myThread1 extends Thread {
        
        public void run() {
            synchronized(child) {
                try {
                    //取得第二个Thread object reference，然后调用wait()
                    child.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                
                System.out.println("我睡醒了，现在开始打印信息");
            }
        }
    }
    
    class myThread2 extends Thread {

        public void run() {
            synchronized(this) {
                try {
                    sleep(5 * 1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                
                System.out.println("唤醒另一个线程吧");
                this.notifyAll();
            }
        }
        
    }

}