Question

I wanted to measure, how long 2 different programs need to perform 1 task. One program used threads, the other didn’t.The task was to count up to 2000000.

Class with threads:

public class Main {
    private int res1 = 0;
    private int res2 = 0;

    public static void main(String[] args) {
        Main m = new Main();

        long startTime = System.nanoTime();
        m.func();
        long endTime = System.nanoTime();

        long duration = endTime - startTime;
        System.out.println("duration: " + duration);
    }

    public void func() {
        Thread t1 = new Thread(new Runnable() {

            @Override
            public void run() {
                for (int i = 0; i < 1000000; i++) {
                    res1++;
                }
            }
        });

        Thread t2 = new Thread(new Runnable() {

            @Override
            public void run() {
                for (int i = 1000000; i < 2000000; i++) {
                    res2++;
                }
            }
        });

        t1.start();
        t2.start();

        System.out.println(res1 + res2);
    }
}

Class without threads:

public class Main {

    private int res = 0;

    public static void main(String[] args) {
        Main m = new Main();

        long startTime = System.nanoTime();
        m.func();
        long endTime = System.nanoTime();

        long duration = endTime - startTime;
        System.out.println("duration: " + duration);

    }

    public void func() {

        for (int i = 0; i < 2000000; i++) {
            res++;
        }
        System.out.println(res);
    }
}

After 10 measurement the average results (in nanoseconds) were:

With threads:    1952358
Without threads: 7941479

Am I doing it right?
How come, with 2 threads it’s 4x faster and not only 2x?

Answer 1

In the lines

    t1.start();
    t2.start();

you are starting the thread execution, but you aren’t actually waiting for them to finish before you take your time measurement. To wait until the threads are finished, call

   t1.join();
   t2.join();

The join method will block until the thread is finished. Then measure the execution time.