AtticOverview
Key-value associations, architectural isolation from Collection, and live view mechanics
Map<K, V> represents a data structure that maps unique keys to values. Unlike List or Set, Map does not implement Collection or Iterable, because its contract concerns key-value associations rather than a single sequence of items.
Map Hierarchy
| Class | Key Ordering | Access / Insert | Null Key | Null Values | Primary Application |
|---|---|---|---|---|---|
HashMap | None | O(1) average | 1 allowed | Multiple allowed | Default general-purpose key-value lookup |
LinkedHashMap | Insertion or Access order | O(1) average | 1 allowed | Multiple allowed | Order-sensitive mapping; LRU caches |
TreeMap | Sorted (Key ordering) | O(log n) | Disallowed | Multiple allowed | Range queries, ceiling/floor lookups |
Live Collection Views
A map exposes three collection views backed directly by the map's internal data structures:
Map<String, Integer> map = new HashMap<>();
map.put("Alice", 90);
map.put("Bob", 80);
// 1. Key Set View
Set<String> keys = map.keySet(); // [Alice, Bob]
// 2. Values Collection View
Collection<Integer> scores = map.values(); // [90, 80]
// 3. Entry Set View (Key + Value pairs)
Set<Map.Entry<String, Integer>> entries = map.entrySet();
// Deleting through any view immediately mutates the map!
keys.remove("Alice");
System.out.println(map.containsKey("Alice")); // falseThe keySet() + get() Anti-Pattern: Never read values inside a for (String key : map.keySet()) { val = map.get(key); } loop! That pays for a second hash computation and bucket lookup for every iteration. Always iterate over map.entrySet() to receive both key and value in a single pass.
Detailed Guides
HashMap
Hash table bucket chaining, 8+ collision treeification, and null key handling
LinkedHashMap
Insertion vs access ordering and building a production LRU cache
TreeMap
NavigableMap contracts, sorted keys, range slices, and boundary lookups
Java 8+ Functional Methods
Atomic read-compute-write methods: computeIfAbsent, computeIfPresent, merge, and getOrDefault
Iteration Patterns
The 6 canonical ways to iterate maps: entrySet vs keySet performance, lambdas, safe removal, and Streams