Quiz yourself: Streams and flatMap operations in Java

The powerful peek() function can be tricky to use correctly in Java streams.

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Given the code fragment

String[][] arr = {{"a", "d"}, {"n", "d"}, {"a", "x"}};

Arrays.stream(arr)

  .peek(v -> v.equals(new String[] {"a", "d"}))

  .flatMap(u -> Arrays.stream(u))

  .forEach(System.out::print);

What is the output? Choose one.

A. ad

B. [a,d]

C. anaddx

D. adndax

E. [[a,d][n,d][a,x]]

F. Compilation fails due to the argument to peek.

Answer. This question investigates the behavior of arrays and the flatMap operation in streams. Notice first that the array arr is an array containing arrays of Strings. When an array is passed to Arrays.stream, the resulting stream will contain the elements of that array in the order of low to high index values. So, the initial stream contains three elements, {"a", "d"}, {"n", "d"}, and {"a", "x"} in left-to-right order.

The peek method itself never alters the stream, although it’s possible for the argument to do so if it includes a side effect. In this question case, the argument has no side effects and does not modify anything in the stream. In fact, the argument does nothing useful whatsoever. Don’t confuse peek with a filter operation merely because the argument is a lambda that yields a Boolean result. Of course, a filter operation would potentially remove some elements, but that’s not what is shown here.

Does the lambda passed to peek cause a compilation error? The peek method requires an argument that is a Consumer of the stream element type. The lambda provided as argument is correctly formed but appears to return a Boolean result from the equals method. Incidentally, that return will always be false. However, it is acceptable to return a value in this way in a void-compatible lambda; the returned value is simply ignored. This is closely parallel to calling the add method on a list object but ignoring the Boolean value returned by that method. Thus, option F is incorrect because this code does not cause a compilation error.

The flatMap invocation has a lambda expression that expands the String[] elements to Stream<String> elements. The effect of a flatMap is to take the elements of each stream that the argument lambda returns and concatenate them into a single Stream<String>. Again, the substreams returned from the Arrays.stream invocations will be processed from low to high index values. Consequently, the result will be the sequence adndax. From this, you can see that option D is correct, and the remaining options—A, B, C, and E—are incorrect.

Conclusion. The correct answer is option D.

Source: oracle.com