How do I list, map and "print if count>0" with Java 8 / stream API?

Question

Here's my code as per now.

List<Cat> cats = petStore.getCatsForSale();

if (!cats.empty) 
    logger.info("Processing for cats: " + cats.size());

for (Cat cat : cats) {
    cat.giveFood();
}

My colleague writes realy nice code using the Java stream API. I tried to rewrite it as one streaming statement, but I got stuck.

petStore.getCatsForSale().stream.forEach(cat -> cat.giveFood)
    .countTheCats().thenDo(logger.info("Total number of cats: " + x)); // Incorrect... is this possible?

How can I do this? Ideally I want a single streaming statement...

What is `count`? Do you expect a log statement for every non empty cat? — Tim Biegeleisen, Feb 05 '19 at 08:26

Naman · Answer 1 · 2020-06-29T01:38:32.793

18

Your current code is much better without a stream and can further be cutshort to:

if (!cats.isEmpty()) {
    logger.info("Processing for cats: " + cats.size());
}
cats.forEach(Cat::giveFood); // Assuming giveFood is a stateless operation

edited Jun 29 '20 at 01:38

answered Feb 05 '19 at 08:36

Naman

27,789
26
218
353

Hi and thx for answer. Can this be done in a one liner? Of course I can write `"if (!cats.isEmpty()) logger.info("...")` but the purpose of this question is to write it with streams – vikingsteve Feb 05 '19 at 08:43
3

In my opinion, is this the only acceptable solution. *Don't* do everything with `Stream`s just because a tutorial told you so, just by looking at every answer here it's clear what the intentions of **this** one here is, but for every other? It's just a mess – Lino Feb 05 '19 at 08:59

score 11 · Accepted Answer · edited Feb 05 '19 at 17:46

11

I am not sure why you want to use streams as the current loop solutions works, but you may as well use a Stream<List<Cat>>:

Stream.of(petStore.getCatsForSale())
    .filter(cats -> !cats.isEmpty())
    .flatMap(cats -> {
        logger.info("Processing for cats: " + cats.size());
        return cats.stream();
    })
    .forEach(Cat::giveFood);

Maybe an optimization:

Stream.of(petStore.getCatsForSale())
    .filter(cats -> !cats.isEmpty())
    .peek(cats -> logger.info("Processing for cats: " + cats.size()))
    .flatMap(Collection::stream)
    .forEach(Cat::giveFood);

Or use this other variant:

Stream.of(petStore.getCatsForSale())
    .filter(cats -> !cats.isEmpty())
    .mapToInt(cats -> {
        cats.forEach(Cat::giveFood);
        return cats.size();
    })
    .findAny()
    .ifPresent(count -> logger.info("Processing for cats: " + count));

edited Feb 05 '19 at 17:46

Peter Mortensen

30,738
21
105
131

answered Feb 05 '19 at 08:36

Lino

19,604
6
47
65

`petStore.getCatsForSale()` is a `List` – Naman Feb 05 '19 at 08:37
@nullpointer I know and with this ugly piece of code consisting of Java8 streams I think it works, correct me if I'm wrong of course :) – Lino Feb 05 '19 at 08:39
3

@CommonMan not really, its empty vs non-empty here. I doubt `Optional` would be useful here much. – Naman Feb 05 '19 at 08:47
Thank you sir! Works :) Is there a way to improve the contents of the "flatMap", so it's one line instead of two? I notice it returns "cats.stream()" there, is that necessary? – vikingsteve Feb 05 '19 at 08:48
An opinion based tempted No to the latter solution. I feel like its getting clumsy with the stream approach. (Didn't really downvote though.) – Naman Feb 05 '19 at 08:50
@vikingsteve see the second variant – Lino Feb 05 '19 at 08:50
@vikingsteve Actually, there's **one line solution**. – Oleg Cherednik Feb 05 '19 at 08:51
3

@nullpointer I don't really like any of the solutions here except the one from you, I think using Streams for everyday logic is just overkill and makes it extremly hard to understand the simplest of logic – Lino Feb 05 '19 at 08:53
Having side-effects in non-terminal operations is very ugly. @nullpointer's solution is way much cleaner. – ETO Feb 05 '19 at 08:53
Why did you use Stream.of(cats) instead of cats.stream ? – Mohsen Feb 05 '19 at 08:54
and maybe an `Optional` version with Java-11 syntax could be `Optional.of(cats) .filter(Predicate.not(List::isEmpty)) .stream() .peek(cat -> logger.info("Processing for cats: " + cats.size())) .flatMap(Collection::stream) .forEach(Cat::giveFood);` – Naman Feb 05 '19 at 08:55
1

@Spara so that I have a "wrapper"-stream around the `Cat`-list (`Stream>`) which allows me to operate on the whole list at once, but still inside the stream. Using `cats.stream()` though will give you a stream over all cats (`Stream`) – Lino Feb 05 '19 at 08:56
@nullpointer I like your oneliner there, thank you. It works too :) – vikingsteve Feb 05 '19 at 09:00

Oleg Cherednik · Answer 3 · 2019-02-05T08:48:10.163

4

cats.stream()
    .peek(Cat::giveFood)
    .findAny().ifPresent(cat -> logger.info("Processing for cats: " + cats.size()));

edited Feb 05 '19 at 08:48

answered Feb 05 '19 at 08:38

Oleg Cherednik

17,377
4
21
35

6

Starting from `java-9` you may leave your cats **hungry** ))). Execution of `peek` is not guaranteed anymore. Please read [javadoc](https://docs.oracle.com/javase/9/docs/api/java/util/stream/Stream.html#peek-java.util.function.Consumer-) for more details. – ETO Feb 05 '19 at 09:00
1

@ETO Please look at tags. It says **java-8**! – Oleg Cherednik Feb 05 '19 at 09:06
4

@oleg.cherednik I think ETO wanted to say that this solution may only work for java-8 which makes it not a favorable solution when one wants to migrate the version upwards :) – Lino Feb 05 '19 at 09:11
2

@oleg.cherednik Yes, I know. Using `peek` for non-debugging purposes is considered harmful anyway. – ETO Feb 05 '19 at 09:11
3

Moreover, starting from February 2019 there is [no long-term support](https://java.com/en/download/release_notice.jsp) of `java-8` for cemmercial users. So if you write `java-8` code, consider avoiding potential migration bugs. One may not notice that `java-9`'s `peek` is not working as it did before. Thus your code will have a hidden unobvious bug. – ETO Feb 05 '19 at 09:17
Execution of `peek()` is not guaranteed in Java 8, either. Certainly not when followed by `findAny()`, which is a short-circuiting operation. I wrote this code in Java 8: https://stackoverflow.com/a/50880849/507761 – Matthew Read Feb 05 '19 at 19:24
Stop spamming. I know all these changes from JVM9. This example works under JVM8 – Oleg Cherednik Feb 05 '19 at 19:26
1

This does _not_ work in Java 8. Run this and count the "meow"s: http://tpcg.io/VWHXlA – Matthew Read Feb 05 '19 at 19:37

score 2 · Answer 4 · edited Feb 05 '19 at 17:49

2

I agree with @Lino. This is another alternative based on his idea:

List<Cat> cats = petStore.getCatsForSale();

cats.stream().limit(1)
    .flatMap(c -> {
        logger.info("Processing for cats: " + cats.size());
        return cats.stream();
    }).forEach(Cat::giveFood);

edited Feb 05 '19 at 17:49

Peter Mortensen

30,738
21
105
131

answered Feb 05 '19 at 08:48

David Pérez Cabrera

4,960
2
23
37

Why not use an `IntStream.range(1)`? No need to use `cats.stream().limit(1)` that way – Lino Feb 05 '19 at 08:54
1

@Lino because of empty list – David Pérez Cabrera Feb 05 '19 at 08:55

How do I list, map and "print if count>0" with Java 8 / stream API?

4 Answers4