SOS： i have some questions about Thread

[kevin saber]

 public class Test5 implements Runnable{
	Chicks c;

	public static void main(String[] args) {
		new Test5().go();
	}
	
	void go(){
		c = new Chicks();
		new Thread(new Test5()).start();
		new Thread(new Test5()).start();
	}
	
	public void run(){
		c.yack(Thread.currentThread().getId());		
	}
}

class Chicks{
	synchronized void yack(long id){
		for(int i = 1; i < 3; i++){
			System.out.println(id+" ");
			Thread.yield();
		}
	}
}
   

this will cause NullPointerException when i run it, but i don't understand why...

[Ankit Garg]

In the go method, the two Test5 objects with which you create 2 threads both have Chicks as null, so calling yack in the run method causes NPE. Try this

public class Test5 implements Runnable{
	Chicks c;

	public static void main(String[] args) {
		new Test5().go();
	}
	
	void go(){
		c = new Chicks();
		Test5 t1 = new Test5();
		t1.c = new Chicks();
		Test5 t2 = new Test5();
		t2.c = new Chicks();
		new Thread(t1).start();
		new Thread(t2).start();
	}
	
	public void run(){
		c.yack(Thread.currentThread().getId());		
	}
}

class Chicks{
	synchronized void yack(long id){
		for(int i = 1; i < 3; i++){
			System.out.println(id+" ");
			Thread.yield();
		}
	}
}

[kevin saber]

oh~~ i would have simply thought that c have pointed to an object by mistake as line 9 showed.
thank you!!!