Question

For the given program, can you explain because I didn’t get what I expected:

package javaapplication14;

class Mythread implements Runnable{
    String myname;
    int myage;

    public Mythread( String par_name, int par_age){
        myname = par_name;
        myage = par_age;
    }
    public void run(){
        try{
          for(int i=1; i<=myage; i++)  {
              System.out.println("Happy birthday "+myname);
              Thread.sleep(2000);
          }
        }catch(Exception e){
            System.out.println();
        }
    }
}
public class JavaApplication14 {

    public static void main(String[] args) {
      Mythread m = new Mythread("Mutinda ", 2);
      Mythread p = new Mythread("Boniface", 2);
      Thread k = new Thread(m);
      Thread q = new Thread(p);
      k.start();
      q.start();
      Thread t = new Thread(m);
      try{
        for( int i=1; i<=5; i++){
          System.out.println("Main thread executing");
          Thread.sleep(1000);          
        }  
      }catch(Exception e){
          System.out.println("Thread interrupted");
      }

    }
}

And this was my output:

Main thread executing
Happy birthday Boniface
Happy birthday Mutinda 
Main thread executing
Main thread executing
Happy birthday Boniface
Happy birthday Mutinda 
Main thread executing
Main thread executing

I expected this:

Main thread executing
Happy birthday Mutinda 
Happy birthday Boniface
Main thread executing
Main thread executing
Happy birthday Mutinda
Happy birthday Boniface
Main thread executing
Main thread executing

I need someone to explain to me the priorites set up for the two threads k and q such that the output of q becomes the first one than k, regardless of the sleep time.

My argument: since k.start() was called before the q.start(), is expect my output to start with k, since I called it first and the two takes the same sleep time.

Answer 1

When you start threads, its because you have independant tasks which have little or no inter-dependence. As such you shouldn’t expect a particular order of execution between the two threads, in fact most multi-threaded bugs come from making such assumptions.

If you want things to happen in a particular order use a single thread. If you can assume there is no particular order for tasks, only then use multiple threads.