IntStream.empty（）方法的目的是什么？

Question

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?

Answer 1

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.

Answer 2

(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.

Answer 3

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--