The Collection family

Key points

• Three questions decide everything: do duplicates matter, does order matter, do you look things up by key?
• List keeps order, allows duplicates. Set rejects duplicates. Deque adds both ends.
• Map is not a Collection. It stores key-value pairs, so it sits beside the hierarchy, not inside it.
• A Map still exposes collection views: keySet(), values() and entrySet().
• Declare the interface, assign the class: List<String> x = new ArrayList<>(). Swaps then cost one word.

Example

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Collection<String> list = new ArrayList<>(List.of("a", "b", "a"));
        Collection<String> set = new LinkedHashSet<>(list);

        Map<String, Integer> map = new LinkedHashMap<>();
        for (String s : list) map.merge(s, 1, Integer::sum);

        System.out.println("List (ordered, duplicates ok) : " + list);
        System.out.println("Set  (no duplicates)          : " + set);
        System.out.println("Map  (key -> value)           : " + map);

        System.out.println();
        System.out.println("list is a Collection ? " + (list instanceof Collection));
        System.out.println("map  is a Collection ? " + (map instanceof Collection));
        System.out.println("map.keySet()  is a Set        : " + map.keySet());
        System.out.println("map.values()  is a Collection : " + map.values());
    }
}

Pick the interface that matches your data, not the class that feels familiar.