another of Dan's question

[Anupam Sinha]

Question 14
class A extends Thread {
public A(Runnable r) {super(r);}
public void run() {System.out.print("A");}
}
class B implements Runnable {
public void run() {System.out.print("B");}
}
class C {
public static void main(String[] args) {
new A(new B()).start();
}
}

What is the result of attempting to compile and run the program?
a. Prints: A
b. Prints: B
c. Prints: AB
d. Prints: BA
e. Compiler error
f. Run time error
g. None of the above

The answer is a. But I don't understand why isn't B's run is being executed. However if i remove A's run method B's run method is run. I guess that A's run is overridding B's run. How this overriding takes place.
[ May 17, 2003: Message edited by: Anupam Sinha ]

[chi Lin]

changes in main() as following,

A a1 = new A(new B());
a1.start();

output will be A if run() in class A is active or B if run() is commented out.

class A extends Thread {
public A(Runnable r) {super(r);}
public A() {}
public void run() {System.out.print("A");} 
}
class B implements Runnable {
public void run() {System.out.print("B");}
}
class C1 {
public static void main(String[] args)
{
A a1 = new A(new B());
a1.start();

}
}

HTH
[ May 16, 2003: Message edited by: chi Lin ]

[Anupam Sinha]

Thanks Chi Lin for the reply but I was aware of that. What I am asking is that when the Thread's constructor has been supplied with a runnable object reference why isn't that runnable object's run method is being run.

[Maulin Vasavada]

hi anupama
okay. the code for the Thread's run() must be like,
public void run() {
if ( myRunnable != null ) {
myRunnable.run();
}
}
where myRunnable is the Runnable object which might have been passed in Thread()'s consturctor.
now, when u implement run() in class A, that default implementation gets OVERRIDDEN. so it doesn't matter that u passed the Runnable to the thread's constructor...
but when u remove the implementation of run() from A, it uses parent's run() method implementation and so if u had passed the Runnable object to the constructor then it would invoke run() on that...
hope i'm able to make a point.
regards
maulin

[Anupam Sinha]

Hi Maulin
Well you really really helped me explain this to me and that too in a nice manner. Thanks a lot. One more thing my name is not Anupama its Anupam Sinha.
[ May 17, 2003: Message edited by: Anupam Sinha ]

[Maulin Vasavada]

hey
sorry i misread ur name. i shd stop replying at 2 am in the morning (i 'm in CA)
regards
maulin