Java: Consumer interface in a stream doesn't work as expected

Question

I've got 2 statements, I expected that they should "print" same result:

Arrays.stream("abc".split("")).forEach(System.out::println);//first
Arrays.stream("abc".split("")).peek(new Consumer<String>() {//second
    @Override
    public void accept(String s) {
        System.out.println(s);//breakpoint
    }
});

In fact, the first statement will print

a
b
c

Ok, but the second statement prints nothing. I tried to set a breakpoint in the line of "//breakpoint" inside IntelliJ, but it wasn't hit.

So how should I change the second statement to use "peek" as it create a new stream while processing every element using "Consumer"?

Thanks a lot.

Peek is an intermediate operation. – ema Nov 24 '18 at 07:12 — ema, Nov 24 '18 at 07:12

score 3 · Answer 1 · edited Dec 27 '18 at 03:54

3

peek() is not terminal operation, you need to add any terminal operation to make peek work, e.g.

Arrays.stream("abc".split("")).peek(new Consumer<String>() { //second
    @Override
    public void accept(String s) {
        System.out.println(s);//breakpoint
    }
}).count();

edited Dec 27 '18 at 03:54

Naman

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answered Nov 24 '18 at 07:07

Evgeniy Dorofeev

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not a good idea to perform short-circuiting operations like `count `with peek, as when the operation is optimized the `action` would not be invoked. – Naman Nov 24 '18 at 07:27

Naman · Accepted Answer · 2018-11-24T07:34:48.543

Stream.peek, as stated in the javadocs of the API as well, is meant mainly for debugging purposes and performing any update operations on the stream during the peek operation is not recommended.

For example, you can verify the intermediate stream state with the following code and what it eventually results in:

Arrays.stream("acb".split(""))
      .peek(System.out::println) // print a  c  b 
      .sorted()
      .forEach(System.out::println); // print a  b  c

In general, this operation is an intermediate operation wouldn't be executed unless and terminal operation is performed on the stream as mentioned in the Stream operations and pipelines section of the docs and that is exactly the reason why your first statement will print.

Note: Though as suggested in a few other answers, the action within peek is not invoked in the cases when its able to optimize the result for some short-circuiting operations like findFirst etc.

In cases where the stream implementation is able to optimize away the production of some or all the elements (such as with short-circuiting operations like findFirst, or in the example described in count()), the action will not be invoked for those elements.

Suggested Read: [Answer: In Java streams is peek really only for debugging?](https://stackoverflow.com/questions/33635717/in-java-streams-is-peek-really-only-for-debugging/33636377#33636377) — Naman, Nov 24 '18 at 07:49

score 1 · Answer 3 · answered Nov 24 '18 at 07:06

The peek() is not a terminal operation, it produces an intermediate stream. Your stream would be executed only when it finds a terminal operation.

For eg: if you add the count() terminal operation to your second stream, you will get the expected output.

Note - You got an output for the first stream because forEach() is a terminal operation.

score 1 · Answer 4 · answered Nov 24 '18 at 07:15

Stream operations are divided into intermediate (Stream-producing) operations and terminal (value- or side-effect-producing) operations. Intermediate operations are always lazy. So, Steam will start executing the operation pipeline once it gets any terminal operation. In your first case forEach is the terminal operation, so the stream executed. But in the second ca,se the last operation in the pipeline is peek() which is not a terminal operation.

Java: Consumer interface in a stream doesn't work as expected

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