S0053_DuplicatesFrequencyRecord.java

import java.util.List;
import static java.util.Collections.frequency;

interface S0053_DuplicatesFrequencyRecord {

  static void main(String... args) {
    var numbers = List.of(1, 2, 3, 3, 4, 3, 1, 2);
    record Duplicate(int number, int frequency) {
      boolean isDuplicate() {
        return frequency > 1;
      }
    }
    var duplicates = numbers
        .stream()
        .map(n -> new Duplicate(n,frequency(numbers, n)))
        .filter(Duplicate::isDuplicate)
        .distinct()
        .toList();
    System.out.println(duplicates);
  }
}