Code requiring "synchronized" works OK without it

Question

This is a program from Herbert Schildt's Java Fundamentals to demonstrate synchronization.The method sumArray(int[]) is synchronized, and that is a requirement for the program to work correctly. However, when I removed the synchronized keyword, unexpectedly, the program showed almost the same output, and still gave correct results. This is the program.

class SumArray {
        private int sum;

        int sumArray(int[] nums) {
            sum = 0; // Reset sum.
            for(int i = 0; i < nums.length; i++) {
                sum += nums[i];
                System.out.println("Running total for " + Thread.currentThread().getName() + " is " + sum);
            }
            return sum;
        }
    }

    class SumThread implements Runnable {
        static SumArray sa = new SumArray();
        Thread thrd;
        int[] a;
        int answer;

        public SumThread(String name, int[] nums) {
            thrd = new Thread(this, name);
            a = nums;
            thrd.start();

        }

        @Override public void run() {
            System.out.println(thrd.getName() + " starting.");
            answer = sa.sumArray(a);
            System.out.println("Sum for " + thrd.getName() + " is " + answer);
            System.out.println(thrd.getName() + " terminating.");
        }

    }

    public class SyncExample {

        public static void main(String[] args) {
            int[] a = new int[] {1, 2, 3, 4, 5, 6};

            SumThread mt1 = new SumThread("Thread #1", a);
            SumThread mt2 = new SumThread("Thread #2", a);

            try {
                mt1.thrd.join();
                mt2.thrd.join();
            } catch(InterruptedException e) {
                e.printStackTrace();
            }
        }

    }

Output.

Thread #1 starting.
Running total for Thread #1 is 1
Running total for Thread #1 is 3
Running total for Thread #1 is 6
Running total for Thread #1 is 10
Running total for Thread #1 is 15
Running total for Thread #1 is 21
Sum for Thread #1 is 21
Thread #1 terminating.
Thread #2 starting.
Running total for Thread #2 is 1
Running total for Thread #2 is 3
Running total for Thread #2 is 6
Running total for Thread #2 is 10
Running total for Thread #2 is 15
Running total for Thread #2 is 21
Sum for Thread #2 is 21
Thread #2 terminating.

Is there something wrong with my computer, or are threads supposed to be this strange?

Answer 1

You problem is that you have a very small array int[] a = new int[] {1, 2, 3, 4, 5, 6};.

Thread 1 finishes before even thread 2 starts.

If you try to run with array of 10000 or more elements you might see the difference.

I've run your code with 10000 elements:

Thread #1 starting.

Thread #2 starting.

Running total for Thread #1 is 1

Running total for Thread #2 is 1

Running total for Thread #1 is 3

Running total for Thread #2 is 5

Running total for Thread #2 is 11

Running total for Thread #2 is 15

Running total for Thread #1 is 8

..........

So even if your machine shows threads executing sequentially - there is no guarantee that it will this way on every machine and on every program launch.

Also, this way one more mistake is found:

EDIT

You've declared sum as a field which should be in fact a local variable. You can see in my output how this effects calculation (sum from one thread is modified by another and so on.)