具有三个可从方法调用的参数的Java函数

Question

I have this method that calls multiple APIs and aggregates all the results and returns them in a Map where each set of results is matched to the target API so that they can be grouped and displayed properly in a service.

public AggregateResults search(QueryParams params) {
    Stream<APITarget> targets = params.getTargets().parallelStream();

    Function<APITarget, APIResults> func = (APITarget target) -> {
        if (params.length() >= MIN_LENGTH) {
           switch (target) {
               case api1:
                    return searchAPI1(params);
               case api2:
                   return searchAPI2(params);
               .
               .
               case apin:
                    return searchAPIn(params);
            }
        }
        return new APIResults.Builder().build();
    };

    Map<APITarget, APIResults> results = targets.collect(Collectors.toMap(t -> t, func));
    return AggregateResults;
}

I have now have to refactor this code so that for example API 1 - 3 can be called in one function and then API 4 - N can be called from another function. The functions will be called from different methods so I need to move this function out of this method and then pass in the QueryParams object in as another parameter to the function but then I run into the issue of the Function not being able to accept more than one parameter. Eg

Function<APITarget, APIResults> exampleFunc = (APITarget target, QueryParams params) -> {
    if (params.length() >= MIN_LENGTH) {
        switch (target) {
            case api1:
                return searchAPI1(params);
            case api2:
                return searchAPI2(params);            
        }
    }
    return new APIResults.Builder().build();
};

I have seen something similar mentioned here: Can a java lambda have more than 1 parameter? But the examples of BiFunctions only show them being used as lambdas and so wont work when being called by an external method, from what I have been able to find. Also when it talks about creating a new FunctionalInterface I'm not sure what exactly is needed to be able to use the new interface in my required functions.

Any ideas would be great.

Answer 1

Modified to reflect example

First, define some required BiFunction

  BiFunction<Integer, int[], Integer> func = (a, b) ->
      {
         for (int i = 0; i < b.length; i++) {
            a *= b[i];
         }
         return Integer.valueOf(a);
      };

Now define some data.

      //Stream source
      int[] data = { 10, 20, 30
      };
      // parameter source
      int[] params = { 3, 5, 11
      };
      Convert data to stream
      IntStream targets = Arrays.stream(data);

Now apply the mapping function.

     Map<Integer, Integer> results = targets.boxed().collect(
            Collectors.toMap(t -> t, t -> func.apply(t, params)));
      System.out.println(results);

The above takes the stream and creates a map with the data acting as the keys and then takes those same keys and applies them to function with the parameter list. That becomes the value of the map.

You will probably need to use mapToObj or map in place of the boxed() method above depending on the initial stream type.

Answer 2

A functional interface is (in simple terms) an interface with one method.

You'd want to annotate the interface with @FunctionalInterface to document its purpose, and to ensure nobody accidentally adds another method.

The name of the method doesn't really matter, but should of course be appropriate for what it's supposed to do.

So, if you need a function with 3 parameters, create your own functional interface. You can make it specific or or make it general purpose using generic type arguments.

@FunctionalInterface
interface TripleString {
    String doStringOp(String a, String b, String c);
}

@FunctionalInterface
interface TriFunction<T, U, V, R> {
    R apply(T t, U u, V v);
}