public static <T, C extends Collection<T>> Stream<T>
    ifExistIntoStream(C collection) {
        return getOptional(collection).map(Collection::stream)
                .orElseGet(Stream::empty);
    }
public static < T , C extends Collection < T > > Stream < T > ifExistIntoStream ( C collection ) { return getOptional ( collection ) . map ( Collection :: stream ) . orElseGet ( Stream :: empty ) ; }
If exist into stream stream.

@param <T>        the type parameter
@param <C>        the type parameter
@param collection the collection
@return the stream