HashMap in Java vs. LinkedHashMap in Java — What's the Difference?
By Tayyaba Rehman — Published on January 17, 2024
HashMap in Java offers no order guarantees, while LinkedHashMap in Java maintains insertion order.
Difference Between HashMap in Java and LinkedHashMap in Java
Table of Contents
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Key Differences
HashMap in Java is a data structure that stores key-value pairs with no inherent order. LinkedHashMap in Java extends HashMap, maintaining the order of elements as they were inserted.
In HashMap, keys and values are stored in a hash table, offering fast lookups and updates. LinkedHashMap achieves this too but also links entries to preserve order.
HashMap optimizes for speed and efficiency, not caring about the order of entries. LinkedHashMap, while slightly slower, ensures that iteration order is predictable as per insertion sequence.
A key feature of HashMap is that it allows one null key and multiple null values. LinkedHashMap inherits this feature but also maintains a doubly-linked list running through its entries.
HashMap is typically used when order is not a concern, making it a go-to for faster operations. LinkedHashMap is preferred when a predictable iteration order is required, such as in caching scenarios where least-recently accessed items are removed.
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Comparison Chart
Ordering
No ordering; unpredictable iteration
Maintains insertion order
Performance
Generally faster due to no ordering overhead
Slightly slower due to maintaining order
Iteration
Iteration order is not fixed
Iterates in the order of insertion
Use Case
Ideal for situations where data is accessed randomly
Used where order is important, like caches
Data Structure
Based on hash table
Extends HashMap with a linked list
Compare with Definitions
HashMap in Java
It provides constant-time performance for basic operations.
Retrieving an employee's name from a HashMap using their ID is very fast.
LinkedHashMap in Java
A HashMap with a predictable iteration order.
In a LinkedHashMap, entries appear in the order they were added.
HashMap in Java
It is unsynchronized and subject to concurrent modification.
A HashMap used in multi-threaded applications can lead to data inconsistency.
LinkedHashMap in Java
Ideal for cache implementations.
LinkedHashMap can be used for LRU caches, where oldest entries are removed first.
HashMap in Java
It doesn’t guarantee the order of the map.
Iterating over a HashMap doesn't follow the insertion or natural ordering.
LinkedHashMap in Java
It maintains a doubly-linked list across all entries.
LinkedHashMap uses extra memory for each entry to maintain order.
HashMap in Java
A collection that maps keys to values, not allowing duplicate keys.
In a HashMap, employee IDs can be keys mapping to employee names.
LinkedHashMap in Java
Inherits features from HashMap, like allowing one null key.
A LinkedHashMap, like HashMap, can have a null key representing a default value.
HashMap in Java
It allows one null key and multiple null values.
A HashMap can have a null key representing a generic account.
LinkedHashMap in Java
It's slightly slower than HashMap due to order maintenance.
Iterating over a LinkedHashMap is slower than HashMap due to its linked nature.
Common Curiosities
What is a LinkedHashMap in Java?
A HashMap variant that maintains the insertion order of entries.
Can a HashMap in Java contain duplicate keys?
No, it cannot have duplicate keys.
Is iteration order in a HashMap predictable?
No, HashMap does not guarantee any specific iteration order.
Does LinkedHashMap in Java allow null values?
Yes, like HashMap, it allows null values and one null key.
How does LinkedHashMap maintain element order?
It maintains a doubly-linked list running through its entries.
What is a HashMap in Java?
A data structure for storing key-value pairs without inherent ordering.
Can I use a LinkedHashMap as a LRU cache?
Yes, its order maintenance feature makes it suitable for LRU caches.
Why would one use LinkedHashMap over HashMap?
For scenarios where maintaining the order of entries is important.
Are LinkedHashMap operations slower than HashMap?
Yes, slightly, due to the overhead of maintaining order.
Is HashMap synchronized in Java?
No, HashMap is not synchronized.
Does LinkedHashMap maintain access order or insertion order by default?
By default, it maintains insertion order.
Are there any special considerations when using HashMap in multi-threaded environments?
Yes, external synchronization is needed as it is not thread-safe by default.
Can the order in LinkedHashMap be changed to access order?
Yes, by using a constructor that enables access-order.
Is there a performance benefit to using HashMap?
Yes, especially in scenarios where order is not a concern.
What happens if I modify a HashMap while iterating over it?
It can result in a ConcurrentModificationException.
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Written by
Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.
