Unexpected output in multithreaded program

Question

class ThreadSafe implements Runnable {
  int arr[]=new int[]{1,2,3,4,5};
  int sum=0;
  public void run() {
    int result=sum();
    System.out.println("for "+Thread.currentThread().getName()+"the value        is"+result);
  }

  public int sum() {
    for(int i=0;i<5;i++) {
      sum=sum+arr[i];
      System.out.println("calculating sum for      thread"+Thread.currentThread().getName()+"sum ="+sum);
      try {
        Thread.sleep(10);
      } catch(Exception e) {}
    }
    return sum;
  }

  public static void main(String...d) {
    ThreadSafe ts=new ThreadSafe();
    ThreadSafe ts1=new ThreadSafe();
    Thread t=new Thread(ts);
    Thread t1=new Thread(ts1);
    t1.start();
    t.start();
  }
}

I was expecting the output not to come 15. because the sum method is not synchronized so more then one thread can execute the sum method at the same time

What I was expecting that because the 2 thread's will execute the sum method instantly so the output should not be 15 because the first thread will update the value of sum to some value which will be read by the another thread.

So my question is why the output of the program come out the way I'm expecting even though i haven't synchronized the sum() method?

add volatile to sum. It indicats that it is concurrently changed. — gregory561
– gregory561, Commented Aug 27, 2012 at 13:18
arr is not static, each ThreadSafe is getting its own copy. Neither is sum. — Wug
– Wug, Commented Aug 27, 2012 at 13:19
What's wrong with the code? -> For starters, the formatting (or the lack of it)... ;) — brimborium
– brimborium, Commented Aug 27, 2012 at 13:19
@Wug: Your point is well made (see my answer), but he's modifying the sum variable, not the source array. — Adam Robinson
– Adam Robinson, Commented Aug 27, 2012 at 13:20

Adam Robinson · Accepted Answer · 2012-08-27 13:26:36Z

7

You're creating two instances of ThreadSafe, each with their own sum instance variable.

If you want them to overwrite each other (I'm assuming this is just playing around), create two Threads on the same instance of ThreadSafe. Also, mark your sum variable as volatile to indicate that it can be changed by another thread (to avoid internal caching of the value if the compiler detects that it is not changed elsewhere within the method).

For example, change your main method to this:

public static void main(String...d) {
    ThreadSafe ts=new ThreadSafe();
    Thread t=new Thread(ts);
    Thread t1=new Thread(ts);
    t1.start();
    t.start();
  }

And the beginning of your ThreadSafe class definition to this:

class ThreadSafe implements Runnable {
  int arr[]=new int[]{1,2,3,4,5};
  volatile int sum=0;

edited Aug 27, 2012 at 13:26

answered Aug 27, 2012 at 13:19

Adam Robinson

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9 Comments

kTiwari Over a year ago

that means if i will declare sum as a local variable in the method then the result will come as i'm expecting

Adam Robinson Over a year ago

@krishnaChandra: No, the opposite. To keep your code as it is now (with two instances of ThreadSafe, you'd need to make the sum variable static. But why not just create two Thread objects off of the same ThreadSafe instance?

brimborium Over a year ago

@krishnaChandra No, but you can make it static for instance.

kTiwari Over a year ago

@adam :sir Can you please provide the code you are explaining in your answer.

kTiwari Over a year ago

@AdamRobinson: ya after doing what you suggest i got 30 by each of the thread.but still confusing with the code i declared the sum method as synchronized got 15 for the first thread ......ok ...but why 30 is the output by the second thread it should be the 15 again

|

Wug · Accepted Answer · 2012-08-27 15:07:43Z

0

Because you have two instances of ThreadSafe being handled by different threads. For producing your desired result it should be like one instance and multiple threads working on that same instance

edited Aug 27, 2012 at 15:07

Wug

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answered Aug 27, 2012 at 13:20

Pokuri

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3 Comments

brimborium Over a year ago

Or the two instances sharing a single variable.

Adam Robinson Over a year ago

@brimborium: It seems more idiomatic (to me) to have two threads operating on the same instance, though obviously either would work in this case.

brimborium Over a year ago

@AdamRobinson I agree, though sharing a single variable might be easier to understand for a beginner...

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