LinkedHashMap, TreeMap & Map Comparisons

HashMap is unordered O(1), LinkedHashMap is insertion/access-ordered O(1), and TreeMap is sorted O(log n) – each backed by a different internal structure with different ordering guarantees.

Use LinkedHashMap over HashMap when you need predictable iteration order (insertion or access order), LRU cache behavior, or deterministic output for testing and serialization.

HashMap is a hash table with O(1) unordered access; TreeMap is a Red-Black tree with O(log n) sorted access and NavigableMap range queries.

HashMap and LinkedHashMap allow one null key and multiple null values; TreeMap rejects null keys because compareTo cannot handle null; ConcurrentHashMap rejects both.

HashMap allows exactly one null key (stored in bucket 0 with hash 0) and any number of null values; a second null-key put overwrites the first.

TreeMap keeps keys sorted using a Red-Black tree; keys must implement Comparable or a Comparator must be provided, otherwise ClassCastException is thrown on the second put.