weird Integer's moniter state.

HI there,I ran into a really weird problem today,I wrote the code when
I'm trying to get my Producer-Consumer-model running.

I got the running error exception like this:
Exception in thread "Thread-0" java.lang.IllegalMonitorStateException
        at java.lang.Object.notify(Native Method)
        at Lock$Increaser.run(Lock.java:22)
Exception in thread "Thread-1" java.lang.IllegalMonitorStateException
        at java.lang.Object.notify(Native Method)
        at Lock$Decreaser.run(Lock.java:39)

Seems like is the Integer class that caused the weird problem,can
anyone tell me how this happens,maybe the autoboxing or something
else?

public class Lock{
    Integer i=new Integer("3");
    Increaser in=new Increaser();
    Decreaser de=new Decreaser();
    public static void main(String[] args){
        Lock l=new Lock();
        l.in.start();
        l.de.start();
    }
class Increaser extends Thread{
    public void run(){
        while(true){
            synchronized(i){
                if(i>10){
                    try{
                        i.wait();
                    }catch(InterruptedException e){
                    }
                    System.out.println("increaser waiting");
                }
                i++;
                i.notify();
            }
        }
    }
}
class Decreaser extends Thread{
    public void run(){
        while(true){
            synchronized(i){
                if(i<=0){
                    try{
                        i.wait();
                    }catch(InterruptedException e){
                    }
                    System.out.println("decreaser waiting");
                }
                i--;
                i.notify();
            }
        }
    }
}
}