How do I transform a Map<K,V1> to another Map<K,V2> using a map of functions in Java 8?

Question

I want to transform a Map<String, Integer> to another Map<String, Long> using a map of functions in Java 8, matching the data with the functions by key in both maps. You can assume that both maps have the same keys.

I tried the following approach:

Map<String, Integer> inputData = new HashMap<>();
inputData.put("A",8);
inputData.put("B",7);
inputData.put("C",6);

Map<String, Function<Integer, Long>> transformers = new HashMap<>();
transformers.put("A", x -> x*2L);
transformers.put("B", x -> x+3L);
transformers.put("C", x -> x+11L);

Map<String, Long> mappedData = inputData.entrySet().stream()
    .map(entry -> new AbstractMap.SimpleEntry<>(
         entry.getKey(),
         transformers.get(entry.getKey()).apply(entry.getValue())))
    .collect(toMap(Map.Entry::getKey, Map.Entry::getValue));

Expected result: {A=16, B=10, C=17}.

Is there any simpler way of expressing "apply map of transformers to the map of inputData matching by key" in Java Streams API?

Answer 1

You may transform directly in the collector:

Map<String, Long> mappedData = inputData.entrySet().stream()
    .collect(toMap(Map.Entry::getKey, 
            entry -> transformers.get(entry.getKey()).apply(entry.getValue())));

Such solution is shorter and does not create intermediate objects.

Were your inputData had the same value type (Map<String, Long>), you could perform the transformation in-place:

inputData.replaceAll((key, value) -> transformers.get(key).apply(value));

Answer 2

You can also start from transformers map itself, which looks slightly easier:

    Map<String, Long> collect = transformers.entrySet().stream()
            .collect(toMap(Map.Entry::getKey, 
                    e -> e.getValue().apply(inputData.get(e.getKey()))));