Java 8 streams: count values

Question

I am trying to get the number of ocurrences of an object in the set of values in a list using Java 8 streams, but I can't get the hang of it yet.

This is what I am trying to do:

int threshold = 5;
for (Player player : match) { // match is a Set<Player>
    int count = 0;
    for (Set<Player> existingMatch : matches)
        if (existingMatch.contains(player))
            count++;
    if (count >= threshold )
        throw new IllegalArgumentException("...");
}

I know I can group with collect and groupingBy, and use a filter saying that the operation to apply is contains with the new method reference operator. But I am still too green with these new Java 8 features and cannot put it all together.

So how could I extract the number of ocurrences of player in all set of values in a list, using Stream?

I don't see any Map in the posted code. Why would you use a Map in a stream-based equivalent? — JB Nizet
– JB Nizet, Commented May 2, 2016 at 19:34
Not quite sure what you're trying to achieve here. Can you share an example of such a map and the output you're trying to get for it? — Mureinik
– Mureinik, Commented May 2, 2016 at 19:42

Misha · Accepted Answer · 2016-05-02 19:57:49Z

7

Lambda expressions can help separate the different bits of logic and then compose them together.

As I understand from your code, you are testing players for whether they are contained in at least threshold elements of matches. We can write the test logic as follows:

Predicate<Player> illegalTest = player -> matches.stream()
        .filter(m -> m.contains(player))
        .count() >= threshold;

Then we want to apply this test to see if any of the players matches:

boolean hasIllegal = match.stream().anyMatch(illegalTest);

And finally:

if (hasIllegal) {
    throw new IllegalArgumentException("...");
}

answered May 2, 2016 at 19:57

Misha

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2 Comments

JB Nizet Over a year ago

Would be more efficient with a limit(threshold) after the filter.

Misha Over a year ago

@JBNizet I debated whether to put that in, but decided to keep it simple.

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