HashMap in Java vs. LinkedHashMap in Java: What's the Difference?
Edited by Harlon Moss || By Janet White || Published on February 29, 2024
HashMap in Java is a collection for storing key-value pairs unordered, while LinkedHashMap in Java maintains insertion order of these pairs.
Key Differences
In Java, a HashMap is a collection that stores elements as key-value pairs and uses hashing for fast retrieval, but it doesn't maintain any order of the elements. Conversely, LinkedHashMap is a subclass of HashMap which, in addition to hashing, maintains a doubly-linked list running through its entries, thus preserving the insertion order of the elements.
HashMap in Java offers constant-time performance for basic operations like get and put, assuming the hash function disperses elements properly across the buckets. On the other hand, LinkedHashMap, while providing similar performance, incurs a slight overhead due to the maintenance of a linked list, ensuring that iteration over the collection reflects the order in which the elements were inserted.
One key difference is in the iteration order; HashMap does not guarantee any specific order of iteration due to its hashing mechanism. In contrast, LinkedHashMap guarantees iteration in the order of insertion or the order in which keys were last accessed, depending on the constructor used.
HashMap is generally preferred when the order of elements is not important, and memory efficiency is a priority, as it consumes less memory than LinkedHashMap. Conversely, LinkedHashMap is suitable when a predictable iteration order is required, at the cost of additional memory overhead for maintaining the linked list.
Both HashMap and LinkedHashMap allow null elements and provide similar functionality, but the choice between them depends on specific requirements regarding ordering and memory usage. LinkedHashMap, while offering order, is less memory efficient compared to the more memory-efficient but unordered HashMap.
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Comparison Chart
Ordering of Elements
No guaranteed order.
Maintains insertion order.
Memory Usage
Less memory usage.
Additional memory for linked list.
Performance
Fast performance for 'get' and 'put'.
Slightly slower due to linked list maintenance.
Iteration Order
Unpredictable iteration order.
Iteration in insertion order or access order.
Use Case
Preferred when element order is irrelevant.
Used when element order is important.
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HashMap in Java and LinkedHashMap in Java Definitions
HashMap in Java
A HashMap is a map-based collection with key-value pairs, using hashing.
HashMap map = new HashMap<>(); creates a new HashMap for storing strings and integers.
LinkedHashMap in Java
Consumes more memory than HashMap due to the linked list.
A LinkedHashMap requires additional space for maintaining order, impacting its memory footprint.
HashMap in Java
HashMap is part of the Java Collections Framework.
Map anotherMap = new HashMap<>(); demonstrates HashMap's compatibility with the Map interface.
LinkedHashMap in Java
Inherits from HashMap, maintaining all functionalities with order.
Map.get(banana); in LinkedHashMap works similarly to HashMap, but with maintained order.
HashMap in Java
HashMap doesn't maintain any order of the stored key-value pairs.
Iterating over map.keySet() might return keys in an unpredictable order.
LinkedHashMap in Java
Offers predictable iteration order, either insertion-order or access-order.
Iterating over map.keySet() in a LinkedHashMap will follow the order of insertion.
HashMap in Java
Allows one null key and multiple null values.
Map.put(null, 1); is a valid operation in a HashMap.
LinkedHashMap in Java
A LinkedHashMap is a HashMap with a linked list maintaining insertion order.
LinkedHashMap map = new LinkedHashMap<>(); keeps elements in the order they were added.
HashMap in Java
It offers fast lookups, inserts, and deletes by using a hash table.
Map.put(apple, 5); quickly inserts 'apple' with value 5 into the HashMap.
LinkedHashMap in Java
It combines hashing with a doubly-linked list to store elements.
Map.put(banana, 3); and map.put(orange, 4); in LinkedHashMap will maintain this order during iteration.
FAQs
What is a LinkedHashMap in Java?
A HashMap subclass that maintains the insertion order of its entries.
What is a HashMap in Java?
A collection that stores key-value pairs in an unordered fashion using hashing.
What additional feature does LinkedHashMap have?
It maintains a doubly-linked list, preserving the insertion or access order.
What are the performance implications of LinkedHashMap?
Slightly slower than HashMap due to the maintenance of a linked list.
Are both HashMap and LinkedHashMap part of the Java Collections Framework?
Yes, both are part of the Java Collections Framework.
When should you use HashMap over LinkedHashMap?
When the order of elements is not a concern and memory efficiency is important.
What is the default capacity and load factor of a HashMap?
Default capacity is 16 and load factor is 0.75.
How do you choose between HashMap and LinkedHashMap?
Based on requirements for ordering and memory efficiency.
How does HashMap ensure quick access?
By using a hash table for storing its key-value pairs.
Can HashMap store null values?
Yes, it allows one null key and multiple null values.
Is the iteration order predictable in LinkedHashMap?
Yes, it iterates in either the order of insertion or the order of last access.
Do both HashMap and LinkedHashMap support iteration?
Yes, though iteration order differs.
Does LinkedHashMap use more memory than HashMap?
Yes, due to the additional linked list structure.
Can both HashMap and LinkedHashMap be synchronized?
Yes, using Collections.synchronizedMap().
Is LinkedHashMap faster than a regular HashMap for insertion and deletion?
It has a similar speed for insertion and deletion.
Can we replace HashMap with LinkedHashMap in existing code?
Yes, but consider the additional memory overhead.
How does LinkedHashMap handle collisions?
Similar to HashMap, through linked list or tree nodes in each bucket.
Can LinkedHashMap have null values?
Yes, it allows null values and a null key, like HashMap.
What happens when a HashMap's capacity is exceeded?
It is rehashed to a larger capacity.
What is the impact of load factor in HashMap?
It determines when the map is resized based on its capacity.
About Author
Written by
Janet WhiteJanet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Harlon MossHarlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
