Thread tricky questions.

[Appu Mehta]

I could not understand the logic behind the answers for the following two questions.Please help.
Answers are provided in BOLD.
Source Dan Chisholm's Mock test.
Thanks and Regards,
Appu.

Question

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. Compile-time error
f. Run-time error
g. None of the above

--------------------------------------------------------------------------------

Question

class A implements Runnable {
  public void run() {System.out.print(Thread.currentThread().getName());}
}
class B implements Runnable {
  public void run() {
    new A().run();
    new Thread(new A(),"T2").run();
    new Thread(new A(),"T3").start();
}}
class C {
  public static void main (String[] args) {
    new Thread(new B(),"T1").start();
}}

What is the result of attempting to compile and run the program?

a. Prints: T1T1T1
b. Prints: T1T1T2
c. Prints: T1T2T2
d. Prints: T1T2T3
e. Prints: T1T1T3 f. Prints: T1T3T3
g. Compile-time error
h. Run-time error
i. None of the above

[Ankit Garg]

Appu, the first one is a fairly simple one. You see when you initialize a thread with a runnable, then something like this happens inside the Thread class

class Thread implements Runnable
{
   Runnable runnable;
   Thread(Runnable other)
   {
      runnable = other;
   }
}

Now when you call start on the Thread instance, the run method of Thread class is called. The run method looks like this

public void run()
{
   if(runnable != null)
   {
      runnable.run();
   }
}

So basically the run method of the concrete Thread class is called who has the responsibility to call the run method of the runnable passed to the Thread constructor. In the example given by you, an instance of A class is created to instantiate the thread. Now when the start method is called, the run method of A class is called. It is the responsibility of run method of A class to call run method of the Runnable object passed to it during initialization. Since it doesn't fulfill that responsibility, so the run method B class is not called. A correct way to implement the run method would have been

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

[Ankit Garg]

The second question is also fairly simple. When you call run on a Thread instance, then a new thread is not started to execute the run method. Instead the run method is called in the current thread. A new thread is started only when you call start on a Thread instance. So in the run method of class B

new A().run();

and

new Thread(new A(),"T2").run();

will both call run method in the thread started in main method

new Thread(new B(),"T1").start();

Only the third call in run method of class B will a new thread be started

new Thread(new A(),"T3").start();

This is why the first two calls return T1 i.e. the name of the thread started in the main method...

[geeta vemula]

In the first example,

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

Here we are calling start of class A and to its constructor you are passing B. So A's run mothod will run which prints A.


In the second example,

new Thread(new B(),"T1").start(); //Will call run method of B as follows

new A().run(); // will call run of class A and prints name of current thread T1

new Thread(new A(),"T2").run(); // will call run of A but will not start the new thread of execution(see run is calling directly) so prints the same name name T1
new Thread(new A(),"T3").start(); //will call run of A but will start the new thread of execution and print its name as T3.Hope it clears to you.

[Jesper de Jong]

Please use code tags when you post source code.

[James Tharakan]

@ Ankit,
No wonder you got 98%.
I had read similar question before . the explaination given there was not clear. you have explained it wonderfully.

[Abhi vijay]

Ankit, your explanations are so simple and to the point. You explain the most difficult topic with so much ease.....

Great work!!! Kudos 2 you man!!

[Ankit Garg]

Thanks! It was my pleasure . Good work by Geeta too who is approaching the 100 mark

[Appu Mehta]

i am able to get second question. But 1st question i still have doubts.
As per kathy sierra study guide, we can start a thread by two ways.
1.

Thread t = new Thread();
t.start()

2.

Thread t = new Thread(runnable instance);
t.start();

So, in ques 2

new Thread(new B(),"T1").start();

starts a thread and executes run() method of class B. I got it.

But in ques 1,

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

should start run of B() because it is the runnable target passed as parameter to a new thread instance A as per point 2 mentioned above from Kathy Sierra.

i am confused .Please help

[Punit Singh]

new Thread(new B(),"T1").start();  

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

Appu both works in same way, just try to think more, and do one thing open the code of Thread class and see its run method.

new Thread(new B(),"T1").start();: for this first run() method of Thread class is called, that run() has some coding that will call B.run() method.

new A(new B()).start();: again first run() method of A class is called, but this run() has no code that is calling B.run() method.

Try to think for these two lines. Ask if any doubt.

[Appu Mehta]

thanks puneet, Ankit, Geeta and all.