public class Threads_WS1516 {
    public static int sum(int[] input, int numThreads) {
        class SumThread extends Thread {
            private int left, right;
            private int[] input;
            private int sum;

            // left und right inklusive
            public SumThread(int[] input, int left, int right) {
                this.left = left;
                this.right = right;
                this.input = input;
                sum = 0;
            }

            public void run() {
                for (int i = left; i <= right; i++) {
                    sum += input[i];
                }
            }

            public int getSum() {
                return sum;
            }
        }
        
        int groesse = input.length / numThreads;

        // erzeuge und starte Threads
        SumThread[] t = new SumThread[numThreads];
        for (int i = 0; i < numThreads-1; i++) {
            int left = i * groesse;
            int right = left + groesse - 1;
            t[i] = new SumThread(input, left, right);
            t[i].start();
        }
        // letzter Thread:
        t[numThreads-1] = new SumThread(input, (numThreads-1)*groesse, input.length-1);
        t[numThreads-1].start();
        
        // warte, bis alle fertig berechnet haben
        for (SumThread t0 : t) {
            while (true) {
                try {
                    t0.join();
                    break;
                } catch (InterruptedException ex) { }
            }
        }
        
        // aufsummieren und Ergebnis zurückgeben
        int sum = 0;
        for (SumThread t0 : t) {
            sum += t0.getSum();
        }
        return sum;
    }
    
}