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
– Tim Biegeleisen, Commented Feb 5, 2019 at 8:26

Naman · Accepted Answer · 2020-06-29 01:38:32Z

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, 2020 at 1:38

answered Feb 5, 2019 at 8:36

Naman

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2 Comments

vikingsteve Over a year ago

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

Lino Over a year ago

In my opinion, is this the only acceptable solution. Don't do everything with Streams 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

Peter Mortensen · Accepted Answer · 2019-02-05 17:46:55Z

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 5, 2019 at 17:46

Peter Mortensen

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answered Feb 5, 2019 at 8:36

Lino

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13 Comments

Naman Over a year ago

petStore.getCatsForSale() is a List<Cat>

Lino Over a year ago

@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 :)

Naman Over a year ago

@CommonMan not really, its empty vs non-empty here. I doubt Optional would be useful here much.

Lino Over a year ago

@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 Over a year ago

@Spara so that I have a "wrapper"-stream around the Cat-list (Stream<List<Cat>>) 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<Cat>)

|

Oleg Cherednik · Accepted Answer · 2019-02-05 08:48:10Z

4

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

edited Feb 5, 2019 at 8:48

answered Feb 5, 2019 at 8:38

Oleg Cherednik

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8 Comments

ETO Over a year ago

Starting from java-9 you may leave your cats hungry ))). Execution of peek is not guaranteed anymore. Please read javadoc for more details.

Oleg Cherednik Over a year ago

@ETO Please look at tags. It says java-8!

Lino Over a year ago

@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 :)

ETO Over a year ago

@oleg.cherednik Yes, I know. Using peek for non-debugging purposes is considered harmful anyway.

ETO Over a year ago

Moreover, starting from February 2019 there is no long-term support 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.

|

Peter Mortensen · Accepted Answer · 2019-02-05 17:49:25Z

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 5, 2019 at 17:49

Peter Mortensen

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answered Feb 5, 2019 at 8:48

David Pérez Cabrera

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2 Comments

Lino Over a year ago

Why not use an IntStream.range(1)? No need to use cats.stream().limit(1) that way

David Pérez Cabrera Over a year ago

@Lino because of empty list

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