Understanding the Differences Between HashMap, LinkedHashMap, and TreeMap in Java

HashMap, LinkedHashMap, and TreeMap are all implementation of the Map interface in Java, but they differ in their behavior and use cases. Let's explore their key differences.

1. Iteration Order

• HashMap: No guaranteed iteration order; keys and values can be returned in any order.
• TreeMap: Keys and values are sorted in ascending order by their natural ordering or by the provided Comparator.
• LinkedHashMap: Keys and values are returned in the order they were inserted.

2. Performance

• Get/Put/Remove/ContainsKey:

  • HashMap: O(1)
  • TreeMap: O(log(n))
  • LinkedHashMap: O(1)

3. Interfaces

• HashMap: Only implements the Map interface.
• TreeMap: Implements the NavigableMap, Map, and SortedMap interfaces.
• LinkedHashMap: Only implements the Map interface.

4. Null Values/Keys

• HashMap: Allows both null keys and values.
• TreeMap: Allows only null values.
• LinkedHashMap: Allows both null keys and values.

5. Fail-Fast Behavior

• HashMap: Fail-fast behavior is not guaranteed for an iterator in the presence of concurrent modification.
• TreeMap: Same as HashMap.
• LinkedHashMap: Same as HashMap.

6. Implementation

• HashMap: Uses buckets for storing key-value pairs.
• TreeMap: Uses a Red-Black Tree for maintaining sorted keys.
• LinkedHashMap: Uses double-linked buckets to preserve insertion order.

7. Synchronization

• HashMap: Implementation is not synchronized.
• TreeMap: Implementation is not synchronized.
• LinkedHashMap: Implementation is not synchronized.

8. Hashtables

Hashtables are a legacy implementation of the Map interface that is strongly synchronized but less efficient than HashMap. It is generally recommended to use HashMap over Hashtables for most use cases.