JAVA 8 pass return value back into same method x number of times

Question

I have this code which I want to refactor using a functional style, using Java 8. I would like to remove the mutable object currentRequest and still return the filtered request.

    HttpRequest currentRequest = httpRequest;
    for (Filter filter : filters) {
        currentRequest = filter.doFilter(currentRequest);
    }

The aim is to pass a request to the filter.doFilter method, and take the output and pass it back into the filter.doFilter method, and continue to do this until all filters are applied.

For example in a more convoluted way to the for loop

    HttpRequest filteredRequest1 = filters.get(0).doFilter(currentRequest);
    HttpRequest filteredRequest2 = filters.get(1).doFilter(filteredRequest1);
    HttpRequest filteredRequest3 = filters.get(2).doFilter(filteredRequest2);
    ...

I think this is a case for composing functions, and the doFilter method should be a function like below:

    Function<HttpRequest, HttpRequest> applyFilter = request -> filters.get(0).doFilter(request);

But I know this is totally wrong, as I got stuck here.

The other way I was thinking was to use reduce, but I cannot see a way of using it in this case.

If you could help me out with a way of doing this, or point me to some resource that will be great.

Answer 1

It looks like you may want to do a reduce with your HttpRequest as its identity. Each step of the reduce will combine the intermediate result with the next filter, like so:

filters.stream().reduce(currentRequest,
                        (req, filter) -> filter.doFilter(req),
                        (req1, req2) -> throwAnExceptionAsWeShouldntBeHere());

Note: the last function is used to merge two HttpRequests together if a parallel stream is used. If that's the route you wish to go down, then proceed with caution.