Convert an for each loop with var inside to a Java Stream?

Question

I have a function:

String fun(List<Function<String, String>> pro, String x){

     for(var p: pro){
         x = p.apply(x);
     }
     return x; 
}

How can I convert this function to functional style instead of imperative style?

Answer 1

Assuming what you want is to apply each function to your string, passing along the result of each function to the next, you can do this with reduce .

String fun(List<Function<String, String>> functions, String x) {
    return functions.stream()
                    .reduce(s -> s, Function::andThen)
                    .apply(x);
}

Using reduce with andThen creates a combined function that chains your list of functions together. We then apply the combined function to x .

Alternatively, @Naman in the comments suggests the formulation:

functions.stream()
         .reduce(Function::andThen)
         .orElse(Function.identity())
         .apply(x)

which I believe performs one fewer andThen operation (when the list of functions is nonempty), but is functionally the same as the first version.

( Function.identity() is an another way to write s -> s .)

Answer 2

I believe you are already aware about those compilation errors. You can't just define List<Function<>> without having a common understanding about those list of functions. Maybe you can get some inspiration from below code snippet.

String fun(List<Function<String, String>> listOfFunctions, String commonInputStr){
    for (Function<String, String> function : listOfFunctions) {
        String tempValStr = function.apply(commonInputStr);
        if (tempValStr != null){
            return tempValStr;
        }
    }

    return null;
}

Or if you want to find the first result value like below:

Optional<String> fun(List<Function<String, String>> listOfFunctions, String commonInputStr){

    return listOfFunctions.stream()
            .map(stringStringFunction -> stringStringFunction.apply(commonInputStr))
            .findFirst();
}