class calculate implements Runnable {

  private int    index, jump;
  private long   num_rects;
  private double width, mid, height;
  public  double sum;

  public calculate(long num_rects, int index, int jump) {
    this.num_rects = num_rects;
    this.index     = index;
    this.jump      = jump;
    this.width     = 1.0/(double)num_rects;
  }

  public void run() {
    for (int i = index; i < num_rects; i += jump) {
      mid = (i + 0.5) * width;
      height = 4.0/(1.0 + mid*mid);
      sum += height;
    }
  }
}

public class pi_threaded {

  private static long num_rects = 1000000000;
  private static double width = 1.0/(double)num_rects;

  public static void main(String[] args) {

    double sum = 0.0;
    double area = 0.0;

    int n = 8;

    calculate[] calculators = new calculate[n];
    Thread[]    threads     = new Thread[n];

    for (int i = 0; i < n; i++) { 
       calculators[i] = new calculate(num_rects, i, 8);
       threads[i]     = new Thread(calculators[i]);
       threads[i].start();
    }

    for (int i = 0; i < n; i++) { 
      try {
        threads[i].join();
        sum += calculators[i].sum;
      } catch (Exception ex) {
        ex.printStackTrace();
      }
    }

    area = width * sum;
    System.out.println("Computed pi = " + area);

  }

}