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Quiz your self: Dealing with unintended effects in Java

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This query exemplifies a mode that’s well-liked with take a look at creators. It’s much less well-liked with candidates.

Think about that your colleague is prototyping new enterprise logic that should work in a multithreaded utility and has created the next class:

class MyRunnable implements Runnable {

    public void run() {

        synchronized (MyRunnable.class) {

            System.out.print(“good day “);

            System.out.print(“bye “);




To check the category, your colleague wrote the next methodology after which invoked the tactic, passing a Stream object containing two MyRunnable situations:

public static void testMyRunnable(Stream<Runnable> s) {

        i -> {

            new Thread(new MyRunnable()).begin();

            return i;




A. The output shall be precisely good day bye good day bye.

B. The output will at all times begin with good day adopted by both good day or bye.

C. No output shall be produced.

D. Not one of the above.

Which assertion is appropriate? Select one.

Reply. This query exemplifies a mode that’s well-liked with take a look at creators, however maybe it’s much less well-liked with candidates. The setup makes the query seem like on one subject, when in truth it’s actually about one thing else. On this case, the query most likely seems to be about threading and mutual exclusion utilizing synchronization. It’s actually concerning the Stream API.

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Take a look at the tactic and its invocation. The take a look at methodology receives a Stream as an argument, calls a map() operation on that stream, after which executes the depend() terminal operation on the ensuing stream. from the query that the Stream argument has two gadgets in it, so the depend() methodology should return 2.

Right here is the element that issues most: If the Stream object is one for which the scale is thought with out having to attract components to exhaustion, the depend() methodology may really return that measurement with out ever processing the physique of the stream. Certainly, the documentation for the depend() methodology states the next:

An implementation could select to not execute the stream pipeline (both sequentially or in parallel) whether it is able to computing the depend instantly from the stream supply. In such circumstances no supply components shall be traversed and no intermediate operations shall be evaluated. Behavioral parameters with side-effects, that are strongly discouraged apart from innocent circumstances akin to debugging, could also be affected.

In different phrases, if the argument stream has a identified measurement, there shall be no output in any respect. If, nonetheless, the argument stream has a measurement that’s not identified till it runs, some output shall be produced.

The unintended effects of printing “good day ” and “bye ” are subsequently not not possible however are additionally not assured. Choices A, B, and C are subsequently incorrect, and choice D have to be the right reply.

To dig deeper, let’s examine this concept of a stream having a identified or unknown ingredient depend. The next streams have precisely two components:

Checklist.of(1, 3).stream()

Stream.of(1, 3)

Nevertheless, as a result of a number of the components could be eliminated, the next stream has a component depend that have to be decided dynamically:

Checklist.of(1, 3).stream.filter(x -> 3 * Math.random())

On condition that this sort of facet impact could be ignored—the documentation calls it elided—how must you write code supposed for use within the map methodology and associated strategies? The steerage is that the operations handed as arguments to the strategies of a stream ought to typically be pure capabilities. A key (however not the one) function of a pure operate in programming (as distinct from mathematical idea) is that it doesn’t have observable unintended effects. (Printing a message is usually thought of to be a visual facet impact, although logging messages may not be thought of seen. It’s sophisticated and what’s seen relies upon a bit on perspective.)

On this subject, the documentation has extra to supply.

The eliding of side-effects may additionally be shocking. Apart from terminal operations forEach and forEachOrdered, side-effects of behavioral parameters could not at all times be executed when the stream implementation can optimize away the execution of behavioral parameters with out affecting the results of the computation.

As talked about earlier, this query seems as if it’s about synchronization. So, within the curiosity of completeness, think about how this facet will behave if the map methodology’s argument is invoked with every ingredient of the stream.

The physique of the run() methodology is synchronized on the java.lang.Class object that describes MyRunnable within the working VM (that’s, MyRunnable.class). That is, in impact, a static ingredient and, subsequently, irrespective of what number of situations of this explicit MyRunnable class may exist, just one thread could be within the strategy of executing the sequence of print statements. That tells you that if any thread manages to print “good day ” it should proceed to print “bye ” earlier than every other thread can print something. This may imply that, if the stream really processed its components by way of the map operation, the output can be as proven in choice A.

Conclusion. The proper reply is choice D.




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