Question 4

What will be the result of an attempt to compile and run the following program? (Choose 1 Answer)

public class ThreadPrediction {
    public ThreadPrediction() {
    }
    public static void main(String[] args){
      ThreadHelper th = new ThreadHelper();
      new Thread(th,"A").start();
      new Thread(th,"B").start();
    }
}
class ThreadHelper implements Runnable{  
  String str = new String("Running");
  public void run(){    
    synchronized(str){
      try{
        System.out.println(Thread.currentThread().getName());
        Thread.sleep(5000);
        System.out.println(Thread.currentThread().getName());
      }catch(InterruptedException ie){
        ie.printStackTrace();
      }
    }
  }
}
Options :
a. Result cannot be predicted 
b. Prints "ABAB" or "BABA" 
c. Prints "AABB" or "BBAA" 
d. Compiler Error 
e. An exception is thrown

Answer : C (Prints AABB or BBAA).

Thread prediction creates a single object ThreadHelper and shared it between both the threads. Both the Threads use the same instance variable str. If Thread A enters the synchronized block first then it will block the Thread B until completion of the synchronized block. Same is the case if Thread B enters first.