这段代码执行时,为什么报空指针异常?(有关同步)

[Sleeping0804]

class xt{		
			
	public static void main(String in[]){
		
		l lo=new l();

		Thread t1=new Thread(lo);
		Thread t2=new Thread(lo);

		t1.start();
		t2.start();

	}	
	
}		


class l implements Runnable{

	Object o1;

	public void run(){
		
		synchronized(o1){
			for(int i=1; i<4; i++){
			
			
				try{
					Thread.sleep(1000);
				}catch(InterruptedException ie){}

				System.out.println(Thread.currentThread().getName()+" "+i);
			}
		}		
	}	
}

我好晕,请达人指点一下,谢谢!!!

[swandragon]

Object o1;
没有初始化
Object o1 = new Object();

[Sleeping0804]

Oh, how stupid I am!

谢谢!

[o_range]

学习啦

[lsdb]

学习啦

[qingyunzhuimeng]

太有才了