K&B 6.0 - chapter 9 - Question 15 - Thread

[Maggie Zhou]

This code is abstract from the K&B 6 chapter9, question15
this code will throw a java.lang.NullPointerException, the reason is o didn't initialized.

but in this code, go() assigned a new Test to o. Why still throw the exception?

    
class TestSam implements Runnable{
Test o;
public static void main(String[] arg){
new TestSam().go();
}
void go(){
o = new Test();
new Thread(new TestSam()).start();
}

public void run(){
o.yeak(Thread.currentThread().getId());
}
 
}
 
class Test {
void yeak(long id) {
System.out.println(id);
}
}
 

[Rekha Srinath]

Doraemon,

Its because when you call start() of a thread, its run() gets called in a new call stack. In your code, the new stack gets created in this line:
new Thread(new TestSam()).start();
In the new call stack thus created, the object "o" that you had created in main's call stack is not visible and hence you get NullPointerException in your run() method when you call a method with that invisible object.

Try including this line just before the line calling start() in run method.
Test o = new Test ();
You will not get an exception.

[Seema Gaurav]

Hi Doraemon,
This line-

 [quote]new Thread(new TestSam()).start();[/quote]

causes a new object of TestSam to be created. You havent overrident the default constructor to invoke go() or assign o a value. So o will still be null and hence causes the NullPointerException.
HTH,
Seema

[Christophe Verré]

"Doraemon Zhou",
Welcome to the ranch. You may not be aware of the ranch Naming Policy. Please read it carefully and change your name accordingly (obviously fake names are not allowed). Thank you.

[Maggie Zhou]

Thanks a lot! Now I get it.