Question on multithreading from Sierra/Bates Chapter 9, Question 16

[Sandra Bachan]

Sierra/Bates Chapter 9, question 16:

class Chicks{
	synchronized void yack(long id){
		for(int x=1; x<3; x++){
			System.out.print(id + " ");
			Thread.yield();
		}
	}
}
public class ChicksYack implements Runnable{
	Chicks c;
	public static void main(String[] args){
		new ChicksYack().go();
	}
	void go(){
		c = new Chicks();
		new Thread(new ChicksYack()).start();
		new Thread(new ChicksYack()).start();
	}
	public void run(){
		c.yack(Thread.currentThread().getId());
	}
}

Even though the code compiles, it gives the following run-time error:


Exception in thread "Thread-0" java.lang.NullPointerException
at ChicksYack.run(ChicksYack.java:20)
at java.lang.Thread.run(Unknown Source)
Exception in thread "Thread-1" java.lang.NullPointerException
at ChicksYack.run(ChicksYack.java:20)
at java.lang.Thread.run(Unknown Source)

Explaination is that c should be assigned to an object. Isn't c already assigned to an object in go() on line 15?

[Neha Daga]

yes, but its initialised inside a method which means its limited to that method only.

[Shanky Sohar]

the code will compile because at compile time compiler is not able to find any errro.............

but as object are created at runtime..........so when JVM reaches
c.yack(Thread.currentThread().getId());
JVM not able to find any object to which "c" refers to.........
although you have done c = new Chicks();
within a go() method....but it will remain within a go method only......
but if you make
Chick c; as " static Chick c;" then below code will compile...................

or if you do c = new Chicks(); within a run() method then it will also compile.............

[Abimaran Kugathasan]

What about the ChicksYack object in the line 16 and 17? Do they have not null c object?

[Sandra Bachan]

Now I understand - the concept of multithreading must have overshadowed the simple concept of scope. Below is the corrected code:

class Chicks{
	synchronized void yack(long id){
		for(int x=1; x<3; x++){
			System.out.print(id + " ");
			Thread.yield();
		}
	}
}
public class ChicksYack implements Runnable{
	Chicks c = new Chicks();
	public static void main(String[] args){
		new ChicksYack().go();
	}
	void go(){
		//c = new Chicks();
		new Thread(new ChicksYack()).start();
		new Thread(new ChicksYack()).start();
	}
	public void run(){
		c.yack(Thread.currentThread().getId());
	}
}

[Shanky Sohar]

yeah you got it...........