Set, Map & Deque

Set — no duplicates, no order promises (HashSet):

Set<Integer> seen = new HashSet<>();
seen.add(1);
seen.add(1);                    // ignored: add returns false
System.out.println(seen.size()); // 1

Membership tests are the point: contains on a HashSet is effectively instant regardless of size — it buckets by hash code. "Have I processed this id before?" is a Set question, not a List scan.

Map — key → value pairs (HashMap):

Map<String, Integer> stock = new HashMap<>();
stock.put("pen", 12);
stock.put("book", 3);
stock.get("pen");               // 12
stock.getOrDefault("ink", 0);   // 0 — no missing-key surprise
stock.containsKey("book");      // true
stock.remove("pen");

for (var entry : stock.entrySet()) {          // both halves
    System.out.println(entry.getKey() + "=" + entry.getValue());
}
for (String key : stock.keySet()) { /* keys */ }
for (int qty : stock.values())    { /* values */ }

get on a missing key returns null — with primitive-like wrapper types (Integer here) that null can NPE on unboxing later. getOrDefault and containsKey are the defensive reads. Note var (Java 10+): local type inference when the right-hand side makes the type obvious.

Choosing, in one table:

| Need | Structure | |---|---| | ordered sequence, index access | ArrayList | | unique members, fast contains | HashSet (or LinkedHashSet to keep insertion order) | | key → value lookup | HashMap (or TreeMap for sorted keys) | | first-in-first-out processing | ArrayDeque as a queue | | undo / recent items | ArrayDeque as a stack (push/pop) |

Next: what the <…> actually means.