Java FAQ: Top Questions

HashMap , LinkedHashMap , and TreeMap are implementations of the Map interface in java.util , differing in ordering, performance, and use cases.

• HashMap:
  • Unordered key-value pairs.
  • Fast for lookups, inserts, and deletes (O(1) average).
  • Allows one null key and multiple null values.
• LinkedHashMap:
  • Maintains insertion order (or access order with configuration).
  • Slightly slower than HashMap due to linked list maintenance.
  • Useful for LRU caches.
• TreeMap:
  • Sorted by keys (natural order or custom Comparator ).
  • Slower operations (O(log n)) due to red-black tree.
  • No null keys; null values allowed.