Why Do Hash Maps in Java 8 Use Binary Trees Instead of Linked Lists?

What are the reasons behind the use of binary trees in hash maps in Java 8 instead of linked lists?

In Java 8, hash maps introduced a significant performance enhancement by replacing linked lists with binary trees for managing collisions. This change drastically improves the efficiency of lookup operations in cases of high collision rates, which can occur when many keys hash to the same bucket.

// Example of entering data into a HashMap (Java 8)
HashMap<String, Integer> map = new HashMap<>();
map.put("key1", 1);
map.put("key2", 2);
// If collisions occur, binary trees replace linked lists for performance.

Causes

• The performance of operations such as get and put in a hash map can degrade from O(1) to O(n) when using a linked list in cases of high collision.
• A binary tree structure, specifically a balanced binary search tree (BST), allows for logarithmic time complexity for search and insertion operations, improving over the O(n) of linked lists.

Solutions

• The introduction of the TreeNode class implements binary trees within hash map buckets when the number of entries in a bucket exceeds a certain threshold (8 by default).
• This allows for maintaining a more efficient data structure, enhancing performance when there are many collisions.

Mistake: Assuming all hash maps use binary trees regardless of the number of collisions.

Solution: Understand that binary trees are only used in hash maps when the bucket exceeds a specific load factor.

Mistake: Neglecting the impact of poor hash functions leading to more collisions.

Solution: Implement good hash functions to minimize collisions and maximize efficiency.