Nullpointerexception in Thread

[John Eipe]

Hi,

I was reading concept of Synchronized methods at this link:
http://download.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html
improvised the code there.

class Sample
{
	public static void main(String[] args)
	{
		Spawn s = new Spawn();					
		Thread t1 = new Thread(s,"John");		
		Thread t2 = new Thread(s,"Thomas");
	
		t1.start();
		t2.start();
					
	
		System.out.println(Thread.currentThread().getName()+"quits");	
	}
}
class SynCounter
{
	private int c = 0;

    public synchronized void increment() 
    {
        c++;
    }
    public synchronized void decrement() 
    {
        c--;
    }

    public synchronized int value() {
        return c;
    }
}
class Spawn implements Runnable
{
	private SynCounter obj;

	public void run()
	{
		if(Thread.currentThread().getName()=="John")
		{
			obj.increment();
			System.out.println(obj.value());
		}
		else
		{
			obj.decrement();
			System.out.println(obj.value());
		}
		try{
			Thread.sleep(100);
		}catch(InterruptedException e)
		{	
		}
	}
}

I'm getting the below error when I run,

Exception in thread "John" mainquits
java.lang.NullPointerException
at Spawn.run(s8.java:45)
at java.lang.Thread.run(Thread.java:636)
Exception in thread "Thomas" java.lang.NullPointerException
at Spawn.run(s8.java:45)
at java.lang.Thread.run(Thread.java:636)

[Henry Wong]

Your Spawn class never actually instantiated a SynCounter object -- so when it tries to use it, it gets a NPE.

Henry

[John Eipe]

yes, you are right!! (how embarrassing! Is it possible to delete this thread?)

[Rob Spoor]

Nope, afraid not. But don't worry, everybody makes this mistake at least once. Anyone who says differently is lying, or hasn't got to that point yet

[himanshu.harish agrawal]

Correct, not to worry, not everybody is lucky enough to become successful without relishing the taste of failure ..

same way, not everybody is lucky enough to become a good Java developer without getting null pointers.