HashMap vs LinkedHashMap

• HashMap does not maintain any order
• LinkedHashMap maintains insertion order (or access order)

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Core Difference

Point	HashMap	LinkedHashMap
Ordering	No order guaranteed	Maintains insertion order
Internal structure	Hash table	Hash table + doubly linked list
Performance	Slightly faster	Slightly slower
Memory usage	Less	More (extra links)
Allows null key	✅ One	✅ One
Allows null values	✅ Yes	✅ Yes
Use case	Fast lookup	Ordered iteration
Introduced in	Java 1.2	Java 1.4

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HashMap

A HashMap stores key–value pairs using hashing, without maintaining any order.

Characteristics

• Fast access
• No predictable iteration order
• Most commonly used Map implementation

When to use

• Order does not matter
• Performance is more important than iteration order

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LinkedHashMap

A LinkedHashMap is a HashMap that also maintains a linked list of entries.

Characteristics

• Preserves insertion order by default
• Can also maintain access order (LRU behavior)
• Slightly slower due to extra bookkeeping

When to use

• When iteration order matters
• When implementing LRU cache

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HashMap provides fast key–value access without any order
LinkedHashMap maintains insertion or access order using a linked list.

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