Explain the difference between HashMap, LinkedHashMap, and TreeMap.

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Java provides several Map implementations, and the most common ones are HashMap, LinkedHashMap, and TreeMap. They all store key–value pairs, but they differ in ordering, performance, and internal structure.

🔑 1. HashMap

• Ordering:

  • Does not guarantee any order of keys (insertion order is not preserved).

  • Iteration order may change if the map is modified.

• Implementation:

  • Based on hash table.

• Performance:

  • O(1) average time complexity for put(), get(), and remove() (constant time).

  • Worst case O(n) if too many hash collisions.

• Use case:

  • Best for fast lookups when order does not matter.

🔑 2. LinkedHashMap

• Ordering:

  • Maintains insertion order (keys are iterated in the order they were added).

  • Can also be configured to maintain access order (useful for caches, e.g., LRU cache).

• Implementation:

  • Extends HashMap but adds a doubly-linked list across all entries.

• Performance:

  • Similar to HashMap: O(1) average for basic operations.

  • Slightly slower than HashMap due to maintaining the linked list.

• Use case:

  • When you need fast lookups but also want predictable iteration order.

🔑 3. TreeMap

• Ordering:

  • Maintains sorted order of keys (ascending by default, or custom comparator).

• Implementation:

  • Based on a Red-Black Tree (self-balancing binary search tree).

• Performance:

  • O(log n) for put(), get(), and remove().

• Use case:

  • When you need a sorted map (e.g., finding the smallest/largest key, range queries).

⚡ Quick Comparison Table

Feature	HashMap	LinkedHashMap	TreeMap
Order	No order	Insertion (or access)	Sorted by key
Implementation	Hash table	Hash table + Linked list	Red-Black Tree
Performance (avg)	O(1)	O(1)	O(log n)
Null Keys	1 allowed	1 allowed	Not allowed
Null Values	Multiple allowed	Multiple allowed	Multiple allowed
Use Case	Fast lookup	Predictable iteration	Sorted map / range queries

👉 In short:

• Use HashMap if you just need fast lookups.

• Use LinkedHashMap if you care about iteration order.

• Use TreeMap if you need keys to be sorted.

Would you like me to also give you a Java code example showing how all three behave differently with the 

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