Learn

The four core interfaces in java.util:

• List<T> — ordered, allows duplicates. ArrayList, LinkedList.
• Set<T> — unique elements, no order guarantee. HashSet, LinkedHashSet, TreeSet.
• Map<K, V> — key-value. HashMap, LinkedHashMap, TreeMap.
• Queue<T> / Deque<T> — ArrayDeque, LinkedList, PriorityQueue.

List<String> list = new ArrayList<>();
list.add("a"); list.add("b");

Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 1));   // {1,2,3}

Map<String, Integer> ages = new HashMap<>();
ages.put("Alice", 30);
ages.put("Bob", 25);
ages.getOrDefault("Carol", -1);    // -1

for (Map.Entry<String, Integer> e : ages.entrySet()) {
    System.out.println(e.getKey() + ": " + e.getValue());
}

Variants worth knowing:

• LinkedHashMap — preserves insertion order
• TreeMap — sorted by key
• ConcurrentHashMap — thread-safe

Immutable factories (Java 9+): List.of(1, 2, 3), Map.of("a", 1, "b", 2). Cannot be modified — add() throws.