CODEWITHSUNDEEP
javajava-8java-stream
Adrian Krebs
Adrian Krebs

Reputation: 4387

What's the purpose of the IntStream.empty() method?

During a tutorial about the new JDK8 stream API I ran across the static .empty() method of IntStream, DoubleStream and LongStream.

So when does it make sense to use this methods?

Upvotes: 5

Views: 909

Answers (3)

Stuart Marks
Stuart Marks

Reputation: 132390

(You had asked about the empty() method on IntStream, LongStream, and DoubleStream, but this method is also on the Stream interface for reference types.)

The general answer is that empty() is useful as a stream source for passing to an API that takes a stream -- either as an argument or as a return value -- and when you have no values to pass. In most cases you can't pass null, you have to pass a stream of some sort. The way to get a stream that has no values is to use Stream.empty() and friends.

Here's an example that repeats even numbers and drops odd numbers and collects them into a list:

    List<Integer> list = 
        IntStream.range(0, 10)
                 .flatMap(i -> (i & 1) == 0 ? IntStream.of(i, i) : IntStream.empty())
                 .boxed()
                 .collect(Collectors.toList());

The result is

[0, 0, 2, 2, 4, 4, 6, 6, 8, 8]

as one would expect. The main point is that flatMap() passes in a single value and expects to receive an arbitrary number of values, including zero values. The way this is done is to to have the flat-mapping operation return a stream of values. To have it return zero values, it returns an empty stream.

Upvotes: 4

Tagir Valeev
Tagir Valeev

Reputation: 100209

A good example is to create the IntStream from the OptionalInt: you want a singleton stream if the optional is present and an empty stream if the optional is absent:

public static IntStream ofOptional(OptionalInt optional) {
    return optional.isPresent() ? IntStream.of(optional.get()) : IntStream.empty();
}

Actually such method is already added to JDK9.

Upvotes: 7

user5151012
user5151012

Reputation:

You can use to initialize an empty Stream, of any of the types mentioned by you. I see it as a another way how to construct a new object. Simple as that.

https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#empty--

Upvotes: 3

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