How can i find duplicates in List repeated x times?-CodePudding

I would like to retrieve duplicates from the list that only repeat x times. I don't know how to do it,I only managed to get all the duplicates.

CodePudding user response：

Count the frequencies for each element into a map Map<MyObject, Integer> and then select the entries from the map by the required frequency.

Methods hashCode and equals MUST be properly implemented in the class MyObject:

public static List<MyObject> findDuplicates(int frequency, List<MyObject> input) {
    return input
        .stream()
        .collect(Collectors.groupingBy(x -> x, Collectors.summingInt(x -> 1))) // Map<MyObject, Integer>
        .entrySet()
        .stream()              // Stream<Map.Entry<MyObject, Integer>>
        .filter(e -> e.getValue() == frequency)
        .map(Map.Entry::getKey) // Stream<MyObject>
        .collect(Collectors.toList());
}

Non-stream implementation would be to build the frequency map and then iterate it to populate the filtered list and/or print the matching duplicated element:

public static List<MyObject> findDuplicates(int frequency, List<MyObject> input) {
    Map<MyObject, Integer> frequencies = new LinkedHashMap<>();

    for (MyObject mo : input) {
        frequencies.merge(mo, 1, Integer::sum);
    }

    List<MyObject> result = new ArrayList<>();
    frequencies.forEach((mo, freqValue) -> {
        if (freqValue == frequency) {
            result.add(mo);
            // System.out.printlnm("Found: "   mo);
        }
    });

    return result;
}

CodePudding user response：

Here is a 3-line algorithm in pseudocode:

FOR each element "e" of collection "x"
    INCREMENT the count for "e"
ENDFOR

We can implement this in Java many ways. For example, with a Map:

public static <T> Map<T, Integer> countDuplicates(Collection<T> collection) {
  Map<T, Integer> duplicateCounts = new HashMap<>();

  for(T element : collection) {
    // Map#putIfAbsent will put the entry (element, 1) iff the map does NOT already contain the key "element".
    // If the map already contains the key "element", it returns the current value (element, ?).
    Integer duplicateCount = duplicateCounts.putIfAbsent(element, 1);

    if(duplicateCount != null) {
      duplicateCounts.put(element, duplicateCount   1);
    }
  }

  return duplicateCounts;
}