Sets and Maps — cheat sheet

Set

Like a List, but: no defined order (no indexing/.get(i)), and every element is unique — adding a duplicate is a silent no-op, not an error.
Mutable (new HashSet<>()) or immutable (Set.of(...), throws on mutation attempts), same split as List/ArrayList.
Checking whether an element is contained (contains) is significantly faster than in a List, especially at scale — this is usually the reason to reach for a Set instead of a List.

Stores key/value pairs (Map<K, V>). Like List/Set, can't hold primitives directly, and has no defined order.
Keys are unique (like Set elements); values may repeat.
put(key, value) — inserts, or overwrites if the key already exists.
get(key) — retrieves the value for a key.
remove(key) — removes an entry; removing a non-existent key is a silent no-op.
keySet() — iterate over keys (commonly paired with get inside the loop).
Mutable (new HashMap<>(...)) or immutable (Map.of(...), throws on mutation attempts).
Alternate terminology: Map = "Dictionary"; HashMap = a concrete "Hash Table" implementation. Checking whether a key exists is very fast, same underlying mechanism as Set.

Set over List: you only care about membership/uniqueness, not order or duplicates, and especially when checking "is X already in here?" happens often on a large collection.
Map over List: you need to look values up by some key (not by position), e.g. "the exam score for this student," "the capital city of this country."