CODEWITHSUNDEEP
javamultithreading
Dheeraj Joshi
Dheeraj Joshi

Reputation: 1582

Thread working unexpectedly

I have written a simple java program and I was hoping for an output like

go
hello 
bye //pause for 2 seconds
hello

But it is giving an output like

go
bye
hello //pause for 2 sec
hello

Programme:

class Tread{
      public static void main(String[] args){
           Hello h= new Hello();
           Thread t= new Thread(h);
           System.out.println("go");
           t.start();
           System.out.println("Bye");
      }
}

class Hello implements Runnable{
     public  void run(){
           System.out.println("hello");
           try{
                Thread.sleep(2000);
           }
           catch(Exception e){ }
           System.out.println("hello");
    }
 }

Please, can anybody tell me why i am not getting the desired otput?

Upvotes: 0

Views: 69

Answers (2)

Rado
Rado

Reputation: 39

I guess because it is on another thread, independent from main program. Try this:

public static void main(String[] args){
        Hello h= new Hello();
        Thread t= new Thread(h);
        System.out.println("go");
        t.start();
        try {
            Thread.sleep(1000);
        }
        catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
        System.out.println("Bye");
    }
}

class Hello implements Runnable{
    public  void run(){
        System.out.println("hello");
        try{
            Thread.sleep(2000);
        }
        catch(Exception e){}
        System.out.println("hello");
    }
}

Upvotes: 0

Alexandre Fenyo
Alexandre Fenyo

Reputation: 4819

The result is unpredictable since 2 threads try to write something on the output simultaneously. The JVM specs do not specify the order of the operations:

  • you have the main thread that starts another thread
  • after that, the main thread and the other thread both print something.

So, all of this is implementation dependant: somebody may use another JVM, one day, and/or another operating system, and he could then have another behaviour. With your JVM, on your environment, you may encounter millions of run of this program writing output in the same order. But it does not mean that it will never change in the future.

If you want the sequence of operations to be done according to a particular order, you need to use some locks, or synchronized keyword to access resources.

Thinking it will always behave the same often leads to mistakes, that happen only occasionally, and that we call race conditions.

Upvotes: 1

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