After Java 8, there was a significant change in the HashMap implementation to improve its performance in scenarios with high hash collisions. Previously, when multiple keys generated the same hash code, HashMap would use a linked list to store these keys, resulting in a worst-case time complexity of O(n) for operations like search, insert, and delete due to the need to traverse the list.
Java 8 introduced a new approach to handling such collisions. When the number of items in a bucket reaches a certain threshold, instead of using a linked list, the HashMap dynamically replaces the linked list with a balanced tree (specifically, a red-black tree). This change reduces the worst-case time complexity from O(n) to O(log n) for operations in the case of high hash collisions.
Main Reasons for This Change:
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Improved Performance in Worst-case Scenarios: By switching to a balanced tree structure when a bucket contains too many items,
HashMapavoids the performance degradation that occurs with linked lists in cases of high hash collisions. -
Efficiency with Large Data Sets: As the size of the data increases, especially in a situation with many collisions, the efficiency of operations in a tree structure (O(log n)) becomes significantly better than that of a linked list (O(n)).
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Maintaining Overall Performance: The balanced tree is only used when necessary (i.e., when the number of elements in a bucket exceeds a threshold). For buckets with fewer elements, the
HashMapstill uses a linked list, maintaining the general performance benefit of O(1) in average cases. -
Backward Compatibility: This change was made in a way that existing implementations using
HashMapwould not be affected in terms of how they use theHashMapAPI.
It's important to note that this change is an internal implementation detail of HashMap in Java 8 and onwards. It enhances the performance in specific worst-case scenarios without altering the fundamental way developers use HashMap in Java.
