HashMap in Java vs. TreeMap in Java: Know the Difference
By Dua Fatima & Shumaila Saeed || Published on July 21, 2026
HashMap in Java offers faster O(1) access and insertion times using hashing, without order, whereas TreeMap maintains a sorted order of keys based on their natural ordering or a custom Comparator, with O(log n) access and insertion times.
Key Differences
HashMap in Java utilizes a hash table, providing constant time complexity, O(1), for basic operations like add, remove, and contains, assuming the hash function disperses elements properly across buckets. However, it doesn't guarantee any order of the key-value pairs. TreeMap, on the other hand, implements the Map interface using a red-black tree structure, ensuring that the elements are sorted according to their natural ordering or by a Comparator provided at map creation. This results in O(log n) time complexity for the same operations due to the tree structure.
Dua Fatima
Jul 21, 2026
HashMap is efficient for scenarios where order is not a concern, TreeMap is preferable when a sorted order of keys is beneficial, such as in range searches or when displaying data in a sorted manner. TreeMap also provides additional methods like firstKey(), lastKey(), headMap(), and tailMap(), which are useful for ordered map operations, unlike HashMap.
Shumaila Saeed
Jul 21, 2026
HashMap allows one null key and multiple null values, making it more flexible for storing values associated with keys that might not be explicitly defined. TreeMap, in contrast, does not allow null keys (if natural ordering is used or the Comparator does not support null) but can have null values. This distinction makes TreeMap slightly less flexible in terms of key values but beneficial for sorting purposes.
Shumaila Saeed
Jul 21, 2026
The choice between HashMap and TreeMap typically depends on the requirements for ordering and performance. HashMap is generally preferred for lookup-intensive scenarios, whereas TreeMap is chosen for its sorted map capabilities and when operations like subMap are needed.
Dua Fatima
Jul 21, 2026
HashMap's iteration order over key-value pairs is unpredictable and can change with every insertion or deletion, reflecting its non-order. TreeMap guarantees a consistent order, following the natural ordering or the specified Comparator, which is crucial for applications requiring sorted data.
Dua Fatima
Jul 21, 2026
ADVERTISEMENT
Comparison Chart
Ordering
No guaranteed order
Sorted according to natural ordering or Comparator
Shumaila Saeed
Jul 21, 2026
Null Values
Allows one null key and multiple null values
Does not allow null keys (if using natural ordering), allows null values
Dua Fatima
Jul 21, 2026
ADVERTISEMENT
Use Case
Best for non-ordered entries and when performance is the priority
Best when a sorted order of keys is needed or for range-based operations
Dua Fatima
Jul 21, 2026
HashMap in Java and TreeMap in Java Definitions
HashMap in Java
Allows one null key and multiple null values.
Map.put(null, nullKey); map.put(2, null).
Shumaila Saeed
Feb 26, 2024
TreeMap in Java
Maintains keys in sorted order.
TreeMap<Integer, String> sortedMap = new TreeMap<>().
Dua Fatima
Feb 26, 2024
HashMap in Java
Iteration order can vary with modifications.
For (Integer key : map.keySet()) { ... }.
Dua Fatima
Feb 26, 2024
TreeMap in Java
Provides ordered navigation methods.
SortedMap.firstKey(); sortedMap.lastKey().
Shumaila Saeed
Feb 26, 2024
ADVERTISEMENT
HashMap in Java
Offers O(1) access and insertion time.
Map.put(1, A); map.get(1).
Shumaila Saeed
Feb 26, 2024
TreeMap in Java
No null keys if natural ordering or Comparator does not support null.
SortedMap.put(null, throwException); // Error if natural ordering.
Dua Fatima
Feb 26, 2024
HashMap in Java
Stores key-value pairs with no guaranteed order.
HashMap<Integer, String> map = new HashMap<>().
Dua Fatima
Feb 26, 2024
TreeMap in Java
Access and insertion operations are O(log n).
SortedMap.put(3, C); sortedMap.get(3).
Shumaila Saeed
Feb 26, 2024
TreeMap in Java
Ideal for range-based operations.
SortedMap.subMap(1, true, 3, true).
Hifza Nasir
Feb 26, 2024
Repeatedly Asked Queries
Can TreeMap handle null values?
TreeMap can handle null values but not null keys if it relies on natural ordering.
Dua Fatima
Jul 21, 2026
Is TreeMap performance always O(log n)?
Yes, TreeMap operations like get, put, and remove have O(log n) time complexity due to the red-black tree structure.
Shumaila Saeed
Jul 21, 2026
Can both maps contain duplicate keys?
No, both HashMap and TreeMap cannot contain duplicate keys as per the Map interface contract.
Dua Fatima
Jul 21, 2026
What are the memory implications of using TreeMap over HashMap?
TreeMap generally consumes more memory than HashMap due to the overhead of maintaining a tree structure.
Dua Fatima
Jul 21, 2026
When is TreeMap preferred?
TreeMap is preferred when a sorted order of keys is necessary or for efficient range queries.
Shumaila Saeed
Jul 21, 2026
What makes TreeMap sorted?
TreeMap uses a red-black tree that automatically orders keys based on their natural ordering or a provided Comparator.
Dua Fatima
Jul 21, 2026
Do TreeMap and HashMap support thread-safe operations?
Neither TreeMap nor HashMap is thread-safe. Use Collections.synchronizedMap or ConcurrentHashMap for thread-safe operations.
Shumaila Saeed
Jul 21, 2026
How do the null handling capabilities of HashMap and TreeMap differ?
HashMap allows null keys and values, whereas TreeMap typically does not allow null keys but does allow null values.
Dua Fatima
Jul 21, 2026
How does ordering affect iteration in HashMap and TreeMap?
HashMap's iteration order is unpredictable, while TreeMap iterates in ascending key order.
Hifza Nasir
Jul 21, 2026
Can TreeMap's sorting order be customized?
Yes, by providing a custom Comparator at the time of TreeMap initialization.
Shumaila Saeed
Jul 21, 2026
Why choose HashMap over TreeMap?
Choose HashMap for its higher performance in non-ordered access and insertion operations.
Shumaila Saeed
Jul 21, 2026
How does HashMap handle collisions?
HashMap handles collisions by linking entries within the same bucket, potentially impacting its O(1) performance.
Dua Fatima
Jul 21, 2026
What interfaces do HashMap and TreeMap implement?
HashMap implements Map, while TreeMap implements NavigableMap and SortedMap.
Shumaila Saeed
Jul 21, 2026
How do HashMap and TreeMap fit into Java's Collections Framework?
Both are part of the Collections Framework, with HashMap providing a hash table implementation and TreeMap providing a red-black tree implementation for sorted map functionality.
Shumaila Saeed
Jul 21, 2026
How do you choose between HashMap and TreeMap?
The choice depends on the need for ordering. Use HashMap for speed and TreeMap for sorted data operations.
Dua Fatima
Jul 21, 2026
Share this page
Link for your blog / website
HTML
Link to share via messenger
About Author
Written by
Dua Fatima
Co-written by
Shumaila SaeedShumaila Saeed, an expert content creator with 6 years of experience, specializes in distilling complex topics into easily digestible comparisons, shining a light on the nuances that both inform and educate readers with clarity and accuracy.
Hifza Nasir
Shumaila Saeed
Dua Fatima
