Efficient way to iterate and copy the values of HashMap

user_1337007 0
Viewed 325

I want to convert:

Map<String, Map<String, List<Map<String, String>>>> inputMap

to:

Map<String, Map<String, CustomObject>> customMap

inputMap is provided in the config and is ready but I need to customMap Format. CustomObject will be derived from List<Map<String, String>> using few lines of code in a function.

I have tried a normal way of iterating input map and copying key values in customMap. Is there any efficient way of doing that using Java 8 or some other shortcut?

Map<String, Map<String, List<Map<String, String>>>> configuredMap = new HashMap<>();
Map<String, Map<String, CustomObj>> finalMap = new HashMap<>();


for (Map.Entry<String, Map<String, List<Map<String, String>>>> attributeEntry : configuredMap.entrySet()) {
    Map<String, CustomObj> innerMap = new HashMap<>();
    for (Map.Entry<String, List<Map<String, String>>> valueEntry : attributeEntry.getValue().entrySet()) {
        innerMap.put(valueEntry.getKey(), getCustomeObj(valueEntry.getValue()));
    }
    finalMap.put(attributeEntry.getKey(), innerMap);
}

private CustomObj getCustomeObj(List<Map<String, String>> list) {
    return new CustomObj();
}
java-stream java-8 collections java
4 Answers

One solution is to stream the entrySet of inputMap, and then use Collectors#toMap twice (once for the outer Map, and once for the inner Map):

Map<String, Map<String, CustomObj>> customMap = inputMap.entrySet()
        .stream()
        .collect(Collectors.toMap(Function.identity(), entry -> {
            return entry.getValue()
                        .entrySet()
                        .stream()
                        .collect(Collectors.toMap(Function.identity(), 
                            entry -> getCustomeObj(entry.getValue())));
        }));
user_72946470

You could stream, but that ain't going to look readable; at least to me. So if you have a method:

static CustomObject fun(List<Map<String, String>> in) {
    return .... // whatever processing you have here
}

you could still use the java-8 syntax, but in a different form:

    Map<String, Map<String, CustomObject>> customMap = new HashMap<>();

    inputMap.forEach((key, value) -> {

        value.forEach((innerKey, listOfMaps) -> {

            Map<String, CustomObject> innerMap = new HashMap<>();
            innerMap.put(innerKey, fun(listOfMaps));
            customMap.put(key, innerMap);

        });
    });

If you can make the inner map immutable, you could make that even shorter:

inputMap.forEach((key, value) -> {
      value.forEach((innerKey, listOfMaps) -> {
          customMap.put(key, Collections.singletonMap(innerKey, fun(listOfMaps)));
      });
});
user_10593720

IMHO streaming is not so bad idea. There're no bad tools. It depends on how you're using them.

In this particular case I would extract the repeating pattern into an utility method:

public static <K, V1, V2> Map<K, V2> transformValues(Map<K, V1> map, Function<V1, V2> transformer) {
    return map.entrySet()
              .stream()
              .collect(toMap(Entry::getKey, e -> transformer.apply(e.getValue())));
}

The method above can be implemented using any approach, though I think Stream API fits pretty well here.

Once you defined the utility method, it can be used as simple as follows:

Map<String, Map<String, CustomObj>> customMap = 
    transformValues(inputMap, attr -> transformValues(attr, this::getCustomObj));

The actual transformation is effectively one liner. So with proper JavaDoc for transformValues method the result code is pretty readable and maintainable.

user_105846050

How about Collectors.toMap for the entries both at an outer and inner level such as:

Map<String, Map<String, CustomObj>> finalMap = configuredMap.entrySet()
        .stream()
        .collect(Collectors.toMap(Map.Entry::getKey,
                attributeEntry -> attributeEntry.getValue().entrySet()
                        .stream()
                        .collect(Collectors.toMap(Map.Entry::getKey,
                                valueEntry -> getCustomeObj(valueEntry.getValue())))));
user_17461180
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