public class SumV2
{
  // corruption occurs on this shared variable sum:
  private long sum;

  private Object lock;   // this will give us the ability
                         //  to achieve "mutual exclusion" to the sum var

  public static void main( String[] args )
  {
    SumV2 a = new SumV2();
    a.doit();
  }


  public void doit()
  {
    sum = 0L;
    lock = new Object();   // generic
    int n = 1000000; // assume evenly divisble by 100,000
    SumThread[] t = new SumThread[ n / 100000 ];

    // divide the summation task into 10 threads (subtasks)
    for ( int j = 0 ; j < t.length ; j++ ) {
      t[j] = new SumThread( j * 100000 + 1, ( j + 1 ) * 100000 );
    }

    //        1 - 100,000
    //  100,001 - 200,000
    //  200,001 - 300,000
    //        etc.

    for ( int j = 0 ; j < t.length ; j++ ) {
      t[j].start();
    }

    try
    {
      for ( int j = 0 ; j < t.length ; j++ ) {
        t[j].join();
      }
    }
    catch( InterruptedException ex )
    {
      System.err.println( "ERROR: thread interrupted" );
    }

    System.out.println( "SUM 1.." + n + " IS " + sum );
  }

  class SumThread extends Thread
  {
    private int m;    // sum up values m..n
    private int n;

    public SumThread( int m, int n )
    {
      System.out.println( "ADDING " + m + ".." + n + " TO SUM" );
      this.m = m;
      this.n = n;
    }

    public void run()
    {
      for ( int i = m ; i <= n ; i++ )
      {
        synchronized( lock )   // ensure mutually exclusive access to sum
        {
          sum += i;   // each of the 10 threads accesses the shared sum var
            // sum = sum + i;
            // this statement is being interrupted partway through
        }
      }
    }
  }
}