In which case is the internal array of a HashMap bucket transformed into a tree structure?

In Java, the internal array of buckets in HashMap is initially implemented as linked lists to handle collisions. When the number of elements in a single bucket exceeds a certain threshold (usually 8), and the overall size of the HashMap is sufficiently large (usually more than 64), the linked list within the bucket is transformed into a balanced tree (red-black tree). This improves the performance of search operations from O(n) to O(log n) in cases of many collisions.

Example of the transformation condition:

• Number of elements in a bucket > 8
• Total size of HashMap > 64

This helps avoid performance degradation when there are many collisions.