guava/src/com/google/common/collect/Streams.java
public static DoubleStream concat(DoubleStream... streams) {
    // TODO(lowasser): optimize this later
    return Stream.of(streams).flatMapToDouble(stream -> stream);
  }
public static DoubleStream concat(DoubleStream... streams) {
    // TODO(lowasser): optimize this later
    return Stream.of(streams).flatMapToDouble(stream -> stream);
  }
Returns a {@link DoubleStream} 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).flatMapToDouble(stream -> stream)}, but the
returned stream may perform better.

@see DoubleStream#concat(DoubleStream, DoubleStream)