@Override
    public <R> Stream<R> map(Function<? super Map.Entry<K, V>, ? extends R> mapper) {
        return inner.map(requireNonNull(mapper));
    }
Returns a stream consisting of the results of applying the given
function to the elements of this stream.
<p>
This is an intermediate operation.

@param <R>    the element type of the new stream
@param mapper a non-interfering, stateless function to apply to each 
              element
@return       the new stream
@throws NullPointerException if any of the provided parameters are {@code null}
common-parent/mapstream/src/main/java/com/speedment/common/mapstream/MapStream.java