hello Fork/Join for java

import java.util.concurrent.RecursiveTask;

public class Calculator extends RecursiveTask<Integer> {  
	  
    private static final int THRESHOLD = 4;  
    private int start;  
    private int end;  
  
    public Calculator(int start, int end) {  
        this.start = start;  
        this.end = end;  
    }  
  
    @Override  
    protected Integer compute() {  
        int sum = 0;  
        if((end-start) < THRESHOLD){  
            for(int i = start; i<= end;i++){  
                sum += i;  
            }
            System.err.println(">>>>>>>>");
        }else{  
            int middle = (start + end) /2;  
            Calculator left = new Calculator(start, middle);  
            Calculator right = new Calculator(middle + 1, end);  
            left.fork();  
            right.fork();  
  
            sum = left.join() + right.join();  
        }  
        return sum;  
    }  
  
}