HashMap vs. TreeMap in Java: Key Differences & Performance Tips
HashMap stores keys without order and allows one null key; TreeMap keeps keys sorted and forbids nulls.
Both look like dictionaries, so devs pick one at random. Then their leaderboard crashes because “first” suddenly isn’t first—HashMap shuffles while TreeMap alphabetizes, and deadlines slip.
Key Differences
HashMap uses hashing for O(1) average get/put; TreeMap uses red-black trees for O(log n) but guarantees ascending order. HashMap accepts null keys; TreeMap throws NullPointerException. Iteration over HashMap is unpredictable; TreeMap gives sorted traversal.
Which One Should You Choose?
Need speed and don’t care about order? HashMap. Building a leaderboard or range queries? TreeMap. If memory is tight, HashMap wins; if thread safety matters, wrap either with Collections.synchronizedMap.
Can TreeMap replace HashMap?
Only if you need ordering; otherwise the extra log n cost is wasted.
Why did my HashMap reorder after rehash?
Resizing recalculates bucket positions, so iteration order changes—expected behavior, not a bug.
Is TreeMap slower than HashMap always?
Yes, but the gap shrinks for small maps and becomes negligible when sorted data is required.