guava/src/com/google/common/collect/Streams.java
public static LongStream concat(LongStream... streams) {
    // TODO(lowasser): optimize this later
    return Stream.of(streams).flatMapToLong(stream -> stream);
  }
public static LongStream concat(LongStream... streams) {
    // TODO(lowasser): optimize this later
    return Stream.of(streams).flatMapToLong(stream -> stream);
  }
Returns a {@link LongStream} containing the elements of the first stream, followed by the
elements of the second stream, and so on.

<p>This is equivalent to {@code Stream.of(streams).flatMapToLong(stream -> stream)}, but the
returned stream may perform better.

@see LongStream#concat(LongStream, LongStream)