[size=3]import java.util.*;
class Pair{
    private int x,y;
    public Pair(int x,int y){
        this.x = x;
        this.y = y;
    }
    public Pair(){ this(0,0);}
    public int getX(){return x;}
    public int getY(){return y;}
    public void incrementX(){x++;}
    public void incrementY(){y++;}
    public String tostring(){
        return "x:"+ x +", y:"+y;
    }
    class PairValuesNotEqualException extends RuntimeException{
        public PairValuesNotEqualException(){
        super("这一对值不相等："+ Pair.this );    
        }
    }
    public void checkstate(){
        if(x!=y)
            throw new PairValuesNotEqualException();
    }
}
    abstract class PairManager{
        protected Pair p = new Pair();
        private List storage=new ArrayList();
        public synchronized Pair getPair(){
            return new Pair(p.getX(),p.getY());
        }
        protected void store(){storage.add(getPair());
        }
        public abstract void dotask();
    }
    class PairManager1 extends PairManager{
        public synchronized void dotask(){
            p.incrementX();
            p.incrementY();
            store();
        }
    }
    class PairManager2 extends PairManager{
        public void dotask(){
            synchronized(this){
                p.incrementX();
                p.incrementY();
            }
            store();
        }
    }
    class PairManipulator extends Thread{
        private PairManager pm;
        private int checkCounter=0;
        private class PairChecker extends Thread{
            PairChecker(){start();}
            public void run(){
                while(true){
                    checkCounter++;
                    pm.getPair().checkstate();
                }
            }
        }
        public PairManipulator(PairManager pm){
            this.pm=pm;
            start();
            new PairChecker();
        }
        public void run(){
            while(true){
                pm.dotask();
            }
        }
        public String toString(){
            return"一对数:"+pm.getPair()+"检查="+checkCounter;
        }
    }
public class CriticalSection  {
    public static void main(String[] args) {
        final PairManipulator
        pm1 = new PairManipulator(new PairManager1()),
      pm2 = new PairManipulator(new PairManager2());
        new Timer(true).schedule(new TimerTask(){
            public void run(){
                System.out.println("pm1:"+ pm1);
                System.out.println("pm2:"+ pm2);
                System.exit(0);
            }
        },500);
    }
}

以上这段代码是书上的一个例子，可是我运行了好多次都出错了，每次都出现以下提示，请大家帮我看看究竟是什么问题呢？
Exception in thread "Thread-0" java.lang.NoSuchMethodError: Pair.incrementX()V
    at PairManager1.dotask(CriticalSection.java:39)
    at PairManipulator.run(CriticalSection.java:72)
Exception in thread "Thread-1" java.lang.NoSuchMethodError: Pair.getX()I
    at PairManager.getPair(CriticalSection.java:31)
    at PairManipulator$PairChecker.run(CriticalSection.java:61)
Exception in thread "Thread-2" java.lang.NoSuchMethodError: Pair.incrementX()V
    at PairManager2.dotask(CriticalSection.java:47)
    at PairManipulator.run(CriticalSection.java:72)
Exception in thread "Thread-3" java.lang.NoSuchMethodError: Pair.getX()I
    at PairManager.getPair(CriticalSection.java:31)
    at PairManipulator$PairChecker.run(CriticalSection.java:61)[/size]