public class Sum
{
  private long sum;

  public void doit()
  {
    sum = 0L;
    int n = 1000000;  // assume this is evenly divisible by 100000
    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
       // etc.

    // Start all of the threads
    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.exit( 1 );
    }

    System.out.println( "SUM 1.." + n + " IS " + sum );
        // sum is a global variable, accessible by all threads
  }


  class SumThread extends Thread
  {
    private int m;    // sum 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++ )
      {
          sum += i;  // each of the 10 threads accesses the shared
                     //  global variable sum
          // how is sum+=i compiled down....?
          //
          //    sum = sum + i;
          //    ===pretend this is bytecode)====>   load  sum
          //                                        add   i
          //                                        ----thread-context-switch
          //                                        store sum
      }
    }
  }


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