Java task: given an int[] array, form two lists — only unique and only duplicates.

To split an int[] array into lists of unique elements and duplicates, you can use a HashMap to count occurrences:

import java.util.*;

public class UniqueAndDuplicates {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 2, 4, 5, 5, 6};
        Map<Integer, Integer> counts = new HashMap<>();

        for (int num : arr) {
            counts.put(num, counts.getOrDefault(num, 0) + 1);
        }

        List<Integer> unique = new ArrayList<>();
        List<Integer> duplicates = new ArrayList<>();

        for (Map.Entry<Integer, Integer> entry : counts.entrySet()) {
            if (entry.getValue() == 1) {
                unique.add(entry.getKey());
            } else {
                duplicates.add(entry.getKey());
            }
        }

        System.out.println("Unique: " + unique);
        System.out.println("Duplicates: " + duplicates);
    }
}

In this example, we first count the occurrences of each number, then create two lists: one with elements that occur once, and another with repeating elements.