How to run threads in parallel in Android

Question

I'm trying to run multiple threads in parallel. I tried to achieve this by having multiple instances of a thread. My assumption is that it will be executed simultaneously; however, the threads are being executed in a sequence.

Here's a simple code I tested:

new Thread(new Runnable() {
        @Override
        public void run() {
            for (int i=0; i<100; i++) {
                LMLog.info("THREAD", "Counting " + i + " in Thread A");
            }
        }
    }).start();
    new Thread(new Runnable() {
        @Override
        public void run() {
            for (int i=0; i<100; i++) {
                LMLog.info("THREAD", "Counting " + i + " in Thread B");
            }
        }
    }).start();
    new Thread(new Runnable() {
        @Override
        public void run() {
            for (int i=0; i<100; i++) {
                LMLog.info("THREAD", "Counting " + i + " in Thread C");
            }
        }
    }).start();

And the log shows that the for loops are being executed one after another

Counting from 0 to 99 in Thread A, followed by the same thing in Thread B and Thread C.

Based on this, I concluded that they were not being executed in parallel as I thought they would be, but rather in a sequence.

How can I achieve parallel execution in Android then?

Answer 1

I might be wrong, but I think your conclusion is faulty. The reason for it is that counting to 100 takes such a short time that it probably takes more time to instantiate a new Thread and start it. If you want to ensure that your test is "valid" you should first instantiate all 3 threads and then start them all, something like this (also increase counter to 10k or 100k):

Thread t1 = new Thread(new Runnable() {
    @Override
    public void run() {
        for (int i=0; i<10000; i++) {
            LMLog.info("THREAD", "Counting " + i + " in Thread A");
        }
    }
Thread t2 = new Thread(new Runnable() {
    @Override
    public void run() {
        for (int i=0; i<10000; i++) {
            LMLog.info("THREAD", "Counting " + i + " in Thread B");
        }
    }
Thread t3 = new Thread(new Runnable() {
    @Override
    public void run() {
        for (int i=0; i<10000; i++) {
            LMLog.info("THREAD", "Counting " + i + " in Thread C");
        }
    }
t1.start(); t2.start(); t3.start();

This is purely for making your 'test' work. You can also use AsyncTask or some other methods.