Can you tell about the key differences of HashMap in Java 8 compared to previous versions?

In Java 8, improvements in HashMap implementation include significant enhancements for performance and collision resistance:

• When many collisions occur (many elements in one bucket), instead of a simple linked list, a balanced tree (red-black tree) is used. This reduces search time from O(n) to O(log n).
• The threshold for converting a chain to a tree is 8 elements in one bucket.
• This improvement helps avoid performance degradation with poor hash distribution.

Example:

Map<String, String> map = new HashMap<>();
// When many keys with the same hash are added,
// HashMap switches to using a tree for fast search.

Earlier, in versions before Java 8, all collisions were handled only with linked lists, which could lead to significant performance issues.