Hashmap vs. Treemap — What's the Difference?
By Fiza Rafique & Maham Liaqat — Updated on May 2, 2024
Hashmap provides unordered data access with average constant time complexity, while TreeMap maintains a sorted order with logarithmic time operations.
Difference Between Hashmap and Treemap
Table of Contents
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Key Differences
Hashmap implements the Map interface and stores elements in key-value pairs without any inherent order. Whereas, TreeMap also implements the Map interface but sorts the elements based on their natural ordering or by a comparator provided at map creation.
Hashmap uses hashing for storing its entries, which allows it to achieve constant time complexity for basic operations like get and put under typical conditions. On the other hand, TreeMap uses a red-black tree structure, resulting in logarithmic time complexity for these operations.
Hashmap does not guarantee any order of its elements; the order can change when resizing occurs. Conversely, TreeMap guarantees that the elements will always be in sorted order, whether it be natural ordering or an order specified by a custom comparator.
Hashmap is generally faster than TreeMap for insertion and lookup if order is not a concern, making it suitable for scenarios where performance is crucial. Whereas, TreeMap is preferable when applications require ordered traversal, such as displaying sorted data.
Hashmap may use more memory than TreeMap because of the need for a larger array size to maintain an efficient load factor. Whereas, TreeMap tends to be more memory-efficient by using tree nodes linked to each other without requiring pre-allocated larger arrays.
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Comparison Chart
Ordering
Unordered
Sorted according to natural ordering or comparator
Time Complexity
Average O(1) for get and put
O(log n) for get, put, and remove operations
Implementation
Uses a hash table
Uses a red-black tree structure
Memory Usage
Can be high due to initial capacity and load factor
Typically lower, depends on tree structure
Suitable Scenarios
When order doesn't matter and speed is essential
When sorted data is required for operations
Compare with Definitions
Hashmap
A data structure that uses a hash function to compute an index into an array of buckets, from which it can retrieve or store data.
A Hashmap can store user IDs as keys and user details as values.
Treemap
Does not allow null keys and has a predictable iteration order.
Inserting null as a key in TreeMap will throw a NullPointerException.
Hashmap
Ideal for applications requiring quick lookup of data.
Hashmap is used to count the frequency of words in a document.
Treemap
Suitable for applications that require a consistently ordered view.
TreeMap is often used where the data needs to be displayed sorted as per the natural order of keys.
Hashmap
It allows null values and one null key.
In a Hashmap, putting a null key with any value is possible and retrieves the value using just 'get(null)'.
Treemap
Generally, offers lower performance for add and remove operations compared to Hashmap.
Inserting or removing elements in TreeMap involves rebalancing the tree, which can be slower than Hashmap operations.
Hashmap
Does not maintain any order of the elements stored in it.
Iterating over a Hashmap might return elements in a different order each time.
Treemap
A map implemented using a red-black tree.
A TreeMap can be used to store items in which every key has a natural ordering, like alphabetical order.
Hashmap
Resizing Hashmap may affect its performance temporarily.
When a Hashmap grows too large, it needs to resize, which can slow down performance during the resizing process.
Treemap
Maintains elements in sorted order, either natural or through a comparator.
A TreeMap storing dates as keys would sort them from earliest to latest automatically.
Hashmap
Alternative spelling of hash map
Treemap
A representation of hierarchical data as a set of nested rectangles
Common Curiosities
How does TreeMap sort its keys?
TreeMap sorts its keys based on natural ordering or using a comparator provided at the time of map creation.
Can I store a null key in a TreeMap?
No, TreeMap does not allow null keys and will throw a NullPointerException.
What is the primary difference between Hashmap and TreeMap?
Hashmap offers faster operations in an unordered manner, while TreeMap maintains a sorted order with slower logarithmic operations.
Is Hashmap synchronized?
No, Hashmap is not synchronized. If multiple threads access a Hashmap concurrently, and at least one of the threads modifies the map, it must be synchronized externally.
Which is faster, Hashmap or TreeMap?
Hashmap is generally faster for adding and searching elements as it provides constant time complexity for these operations.
Does TreeMap allow null values?
Yes, like Hashmap, TreeMap also allows null values associated with keys.
Can TreeMap handle high-volume data as efficiently as Hashmap?
While TreeMap can handle high volumes of data, its performance may lag behind Hashmap due to the time complexity of red-black tree operations.
What happens when a Hashmap resizes?
When a Hashmap resizes, it rehashes all of its contents, which can lead to a temporary decrease in performance.
What kind of data structure underpins a TreeMap?
TreeMap is built around a red-black tree, which is a type of self-balancing binary search tree.
Can I use primitives as keys in Hashmap and TreeMap?
While primitives cannot be used directly as keys, their wrapper classes (like Integer, Double) can be used.
What are typical uses of a TreeMap?
TreeMap is typically used in applications where an ordered representation of data is needed, such as in sorting and displaying ordered data.
What is the impact of load factor in Hashmap?
The load factor in a Hashmap affects its space efficiency and performance; a higher load factor reduces space overhead but increases the likelihood of collisions, impacting performance.
Is it possible to customize the order in a TreeMap?
Yes, TreeMap can be customized with a comparator at creation to control the order of the keys.
What does resizing in Hashmap entail?
Resizing in Hashmap involves creating a new array of buckets and rehashing all existing entries, which is computationally expensive.
How do I choose between using a Hashmap and a TreeMap?
Choose Hashmap when order is not a concern and performance is critical; opt for TreeMap when a sorted order of keys is necessary.
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Written by
Fiza RafiqueFiza Rafique is a skilled content writer at AskDifference.com, where she meticulously refines and enhances written pieces. Drawing from her vast editorial expertise, Fiza ensures clarity, accuracy, and precision in every article. Passionate about language, she continually seeks to elevate the quality of content for readers worldwide.
Co-written by
Maham Liaqat
