HashMap in Java vs. TreeMap in Java — What's the Difference?

HashMap in Java is a collection that stores key-value pairs and uses hashing to compute an index where these values are stored. This makes most operations such as get() and put() run in constant time (O(1)). The elements in a HashMap are not sorted in any order. Conversely, TreeMap in Java is a map implementation that guarantees the keys are sorted according to their natural order or by a specified Comparator. As a result, operations like get() and put() have O(log n) time complexity because it uses a red-black tree structure.

The internal structure of a HashMap is an array of nodes, where each node represents a key-value pair. The keys are not stored in any particular order, and there is no guarantee that the order will remain constant over time. TreeMap, on the other hand, is a navigable map and sorts its entries based on the natural ordering of its keys or by a specified comparator provided at the map's creation time.

In terms of null keys, HashMap allows one null key and multiple null values, which is often sufficient for applications where sorting is not necessary. TreeMap does not allow null keys (as it cannot compare null with other keys), but it does allow multiple null values. This distinction is important when keys are expected to be comparable and ordered.

Collision handling in HashMap is a consideration since hashing can lead to different keys producing the same index. HashMap addresses this by using a linked list or binary tree (from Java 8 onwards) to store multiple items at the same index. TreeMap, however, does not have this problem because it stores each key-value pair in a separate node within a balanced tree structure, with each node connected to two other nodes, ensuring the balance of the tree.

Performance-wise, HashMap is generally faster than TreeMap for inserting and accessing elements because it provides constant time performance for the basic operations when the hash function disperses the elements properly among the buckets. TreeMap, though slower with logarithmic time complexity for basic operations, has the advantage of maintaining a consistent order and offering several handy methods like firstEntry(), lastEntry(), headMap(), tailMap(), which makes it the preferred choice when sorted data is required.