Building a proxy for http requests I came across an interesting generic problem: filter or mutate a map but only for the occurence of some given set of keys.

That's a lot of if - else - switch

The going approach is to have a specialized method with a switch:

  Map<String, String> transformMap(Map<String, String> input, String joker) {
    Map<String, String> result = new HashMap<>();

    input.entrySet().forEach(entry -> {
      String key = entry.getKey();
      String value = entry.getValue();
      switch (key) {
        case "a":
          result.put(key, "alpha");
          break;
        case "b":
          result.put(key, "beta");
          break;
        case "j":
          result.put(key, joker + "!");
        default:
          result.put(key, value);
      }
    });

    return result;
  }

This gets big and hard to maintain fast, so I was thinking of a more neutral approach entertaining Function and Optional, hear me out.