Reduce/Collect `List<Map<String, Set<String>` to `Map<String, Set<String>>`

Question

After performing a parallelStream() on a List, I end up with a List<Map<String, Set<String>. I want to unify this into a Map<String, Set<String>> (which will only keep uniques across the List of Maps).

I am unfamiliar with the collect and reduce functions, so don't have anything to go ahead with.

Existing code:

private val TYPES = listOf("string", "integer")

private fun getLinesOfEachTypeAcrossMultipleFiles(files: List<File>): Map<String, Set<String>> {
  return files
    .parallelStream()
    .map { file ->
      TYPES.associate {
        it to getRelevantTypeLinesFromFile(file)
      }
    }
// Converted into a Stream<String, Set<String>>
// .reduce() / collect() ?
}

private fun getRelevantTypeLinesFromFile(it: File): Set<String> {
  // Sample code
  return setOf()
}

Naman · Accepted Answer · 2019-04-05 02:27:26Z

1

If you're looking for an equivalent Java code, you can stream all the entries using flatMap and then collect them as a Map with a merge function as :

Map<String, Set<String>> some(List<Map<String, Set<String>>> listOfMap) {
    return listOfMap.stream()
            .flatMap(a -> a.entrySet().stream())
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                    (s1, s2) -> {
                        s1.addAll(s2);
                        return s1;
                    }));
}

answered Apr 5, 2019 at 2:27

Naman

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Comments

Daksh · Accepted Answer · 2019-04-05 20:49:17Z

I figured out and implemented a Kotlin-specific solution of using the fold operator (instead of reduce or collect):

private val TYPES = listOf("string", "integer")

private fun getLinesOfEachTypeAcrossMultipleFiles(files: List<File>): Map<String, Set<String>> {
  return files
    .map { file ->
      TYPES.associate { it to getRelevantTypeLinesFromFile(file) }
    }

    .fold(mutableMapOf<String, MutableSet<String>>()) { acc, map ->
      acc.apply {
        map.forEach { key, value ->
          acc.getOrPut(key) { mutableSetOf() }.addAll(value)
        }
      }
    }
}

private fun getRelevantTypeLinesFromFile(it: File): Set<String> {
  // Sample code
  return setOf()
}

A benefit of using fold is that we don't need to change the type of the data from Map to MutableMap and Set to MutableSet.

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