// Use synchronize to control access.

class SumArray {
  private int sum;

  synchronized 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);
      try {
        Thread.sleep(10); // allow task-switch
      }
      catch(InterruptedException exc) {
        System.out.println("Thread interrupted.");
      }
    }
    return sum;
  }
}

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

  // Construct a new thread.
  MyThread(String name, int[] nums) {
    thrd = new Thread(this, name);
    a = nums;
    thrd.start(); // start the thread
  }

  // Begin execution of new thread.
  public void run() {
    int sum;

    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.");
  }
}

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

    MyThread mt1 = new MyThread("Child #1", a);
    MyThread mt2 = new MyThread("Child #2", a);

    try {
      mt1.thrd.join();
      mt2.thrd.join();
    }
    catch(InterruptedException exc) {
      System.out.println("Main thread interrupted.");
    }
  }
}

Results

C:\ece538\java_thread>java Sync
Child #1 starting.
Child #2 starting.
Running total for Child #1 is 1
Running total for Child #1 is 3
Running total for Child #1 is 6
Running total for Child #1 is 10
Running total for Child #1 is 15
Running total for Child #2 is 1
Sum for Child #1 is 15
Child #1 terminating.
Running total for Child #2 is 3
Running total for Child #2 is 6
Running total for Child #2 is 10
Running total for Child #2 is 15
Sum for Child #2 is 15
Child #2 terminating.