java线程问题

线程等待通知报错：代码如下：
import java.util.Timer;
import java.util.TimerTask;public class ThreadA extends Thread { 
public static Calculator calculator = new Calculator();          public ThreadA(Calculator c) { 
                this.calculator = c; 
        }         public void run() { 
                synchronized (calculator) { 
                        try { 
                                System.out.println(Thread.currentThread() + "等待计算结果"); 
                                calculator.wait(); 
                        } catch (InterruptedException e) { 
                                e.printStackTrace(); 
                        } 
                        System.out.println(Thread.currentThread() + "11111111"); 
                } 
        }         public static void main(String[] args) { 
         Timer timer2=new Timer();
            timer2.schedule(new TimerTask() {    
               public void run() { 
                synchronized (this) { 
                System.out.println("88888888");
                    }              
                System.out.println("777777777");
                this.notifyAll();
                    System.out.println("777777777");//通知所有在此对象上等待的线程   
             
               }   
             },3000);                 
                new ThreadA(calculator).start(); 
                new ThreadA(calculator).start(); 
                new ThreadA(calculator).start(); 
                //启动计算线程 
                try{
 Thread.sleep(6000);
 }catch(InterruptedException e){
 
 }
                calculator.start(); 
        } 
}
class Calculator extends Thread { 
        public void run() { 
            synchronized (this) { 
                   
            } 
            //通知所有在此对象上等待的线程 
            notifyAll(); 
    } 
}报错：Exception in thread "Timer-0" 
java.lang.IllegalMonitorStateException: current thread not owner
at java.lang.Object.notifyAll(Native Method)
at ThreadA$1.run(ThreadA.java:31)
at java.util.TimerThread.mainLoop(Unknown Source)
at java.util.TimerThread.run(Unknown Source)
请问怎么解决？？？谢了