How can I make this rxjava zip to run in parallel? : <p>I have a sleep method for simulating a long running process.</p>

<pre><code>private void sleep() {
    try {
        Thread.sleep(2000);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}
</code></pre>

<p>Then I have a method returns an Observable containing a list of 2 strings that is given in the parameters. It calls the sleep before return the strings back.</p>

<pre><code>private Observable&lt;List&lt;String&gt;&gt; getStrings(final String str1, final String str2) {
    return Observable.fromCallable(new Callable&lt;List&lt;String&gt;&gt;() {
        @Override
        public List&lt;String&gt; call() {
            sleep();
            List&lt;String&gt; strings = new ArrayList&lt;&gt;();
            strings.add(str1);
            strings.add(str2);
            return strings;
        }
    });
}
</code></pre>

<p>Then I am calling the getStrings three times in Observalb.zip, I expect those three calls to run in parallel, so the total time of execution should be within <strong>2 seconds</strong> or maybe 3 seconds the most because the sleep was only 2 seconds. However, it's taking a total of <strong>six</strong> seconds. <strong>How can I make this to run in parallel so it will finish within 2 seconds?</strong></p>

<pre><code>Observable
.zip(getStrings("One", "Two"), getStrings("Three", "Four"), getStrings("Five", "Six"), mergeStringLists())
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Observer&lt;List&lt;String&gt;&gt;() {
    @Override
    public void onCompleted() {

    }

    @Override
    public void onError(Throwable e) {

    }

    @Override
    public void onNext(List&lt;String&gt; strings) {
        //Display the strings
    }
});
</code></pre>

<p>The mergeStringLists method</p>

<pre><code>private Func3&lt;List&lt;String&gt;, List&lt;String&gt;, List&lt;String&gt;, List&lt;String&gt;&gt; mergeStringLists() {
    return new Func3&lt;List&lt;String&gt;, List&lt;String&gt;, List&lt;String&gt;, List&lt;String&gt;&gt;() {
        @Override
        public List&lt;String&gt; call(List&lt;String&gt; strings, List&lt;String&gt; strings2, List&lt;String&gt; strings3) {
            Log.d(TAG, "...");

            for (String s : strings2) {
                strings.add(s);
            }

            for (String s : strings3) {
                strings.add(s);
            }

            return strings;
        }
    };
}
</code></pre>
