/**
 * This thread finds the sum of a subsection of an array.
 */
public class SumThread extends Thread {
    private int lo, hi;
    private int[] arr;
    private int ans = 0;
    
    public SumThread(int[] arr, int lo, int hi) {
        this.lo = lo;
        this.hi = hi;
        this.arr = arr;
    }
    
    @Override
    public void run() {
        for (int i = lo; i < hi; i++) {
            ans += arr[i];
        }
    }
    
    /**
     * Sum the elements of an array.
     * 
     * @param arr array to sum
     * @return sum of the array's elements
     * @throws InterruptedException shouldn't happen
     */
    public static int sum(int[] arr) throws InterruptedException {
        int len = arr.length;
        int ans = 0;
        
        // Create and start 4 threads.
        SumThread[] ts = new SumThread[4];
        for (int i = 0; i < 4; i++) {
            ts[i] = new SumThread(arr, (i * len) / 4, ((i + 1) * len / 4));
            ts[i].start();
        }
        
        // Wait for the threads to finish and sum their results.
        for (int i = 0; i < 4; i++) {
            ts[i].join();
            ans += ts[i].ans;
        }
        return ans;
    }
    
    public static void main(String[] args) throws InterruptedException {
        int[] arr = new int[100];
        for (int i = 0; i < arr.length; i++) {
            arr[i] = i;
        }
        int sum = sum(arr);
        System.out.println("Sum: " + sum);
    }
}