Double a stream

Question

I want to double a Stream (no DoubleStream). Meaning I start with a stream and want to get a new stream where each element of the old stream is streamed twice. So 1,2,3,4,4,5 gives us 1,1,2,2,3,3,4,4,4,4,5,5. Is there such a stream operation?

Answer 1

Create an inner stream which will contain current element two times and flatMap this stream.

stream.flatMap(e -> Stream.of(e,e))

If you want to multiply the number of elements by n you can create an utility method like this one:

public static <T> Stream<T> multiplyElements(Stream<T> in, int n) {
    return in.flatMap(e -> IntStream.range(0, n).mapToObj(i -> e));
    //  we can also use    IntStream.rangeClosed(1, n) 
    //  but I am used to iterating from 0 to n (where n is excluded)
}

(but try to use a better name for this method, since the current one may be ambiguous)

Usage example:

multiplyElements(Stream.of(1,2), 3).forEach(System.out::println);

Output:

1
1
1
2
2
2