Change part of code to functional paradigm

Question

I have few lines of code, which I'm trying to convert to functional paradigm. The code is:

private String test(List<String> strings, String input) {

    for (String s : strings) {
        input = input.replace(s, ", ");
    }

    return input;
}

I need to make this one instruction chain. It must replace all strings from given list with coma IN given String input. I tried to do it with map method, but with no success. I'm aware I can do it if I appended input string into list at beginning and call map then, but the list is immutable, so I cannot do that.

Why not use replaceAll with a regex based from strings? Like return input.replaceAll(String.join("|", strings), ", "); This may not have the same result depending on the strings though. — Tunaki
– Tunaki, Commented Feb 6, 2016 at 17:29

Community · Accepted Answer · 2017-05-23 11:45:11Z

2

I believe you can do this with a simple reduce:

strings.stream().reduce(input, (in, s) -> in.replace(s, ", "));

It takes the input, and replaces each occurence of the first string with ", ". Then it takes that result, and uses it as the input along with the next string, and repeats for every item in the list.

As Louis Wasserman points out, this approach cannot be used with parallelStream, so it won't work if you want parallelization.

edited May 23, 2017 at 11:45

CommunityBot

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answered Feb 6, 2016 at 17:29

resueman

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2 Comments

Louis Wasserman Over a year ago

Note that this won't work if you use parallelStream -- the input isn't actually an identity, and Bad Things will happen.

Holger Over a year ago

@Louis Wasserman: even worse, there’s actually no guaranty that the identity will be used when the stream is not empty, e.g. for a single element stream, returning the element would be a legal implementation, as identity op element is required to evaluate to element, so the implementation could just return element. Or return first op second for a two element stream…

Louis Wasserman · Accepted Answer · 2016-02-06 17:28:25Z

1

The only think I can think of -- which is pretty awkward -- is

 strings.stream()
     .map(s -> (Function<String, String>) (x -> x.replace(s, ", ")))
     .reduce(Function.identity(), Function::andThen)
     .apply(input)

edited Feb 6, 2016 at 17:28

answered Feb 6, 2016 at 17:26

Louis Wasserman

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2 Comments

user3212350 Over a year ago

Wow, that is indeed a bit awkward, although congratulations, wouldn't think of that

Daniel Over a year ago

Now that is functional programming. I'm both horrified and delighted.

Daniel · Accepted Answer · 2016-02-09 22:01:31Z

0

The following does pretty much the same thing.

private String test(List<String> strings, String input) {
    return input.replaceAll(
            strings
                .stream()
                .map(Pattern::quote)
                .collect(Collectors.joining("|"))
            , ", "
    );
}

The main difference is that it first combines all the search strings into a single regex, and applies them all at once. Depending on size of your input strings, this may perform even better than your original use case.

If the list of strings is fixed, or changes only rarely, you can get some more speed from precompiling the joined pattern.

answered Feb 9, 2016 at 22:01

Daniel

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