What is the Difference Between HashMap and TreeMap?

The main difference between HashMap and TreeMap in Java lies in their underlying data structures and the order of elements. Here are the key differences between HashMap and TreeMap:

1. Data Structure: HashMap is implemented using a hash table, while TreeMap is implemented using a Red-Black Tree, which is a self-balancing binary search tree.
2. Order of Elements: HashMap does not provide any order for its elements, while TreeMap provides a sorted order for its elements.
3. Speed: HashMap is faster than TreeMap, as it provides constant-time performance (O(1)) for basic operations like get() and put(). TreeMap has a lookup complexity of O(log n).
4. Null Keys and Values: HashMap allows one null key and multiple null values, while TreeMap does not allow null keys but may contain multiple null values.
5. Memory Consumption: HashMap consumes more memory space compared to TreeMap.

In summary, HashMap is faster and does not maintain any order for its elements, while TreeMap provides a sorted order for its elements at the cost of slightly slower performance. The choice between HashMap and TreeMap depends on the specific requirements of your application, such as the need for sorted elements or faster insertion and lookup operations.

Comparative Table: HashMap vs TreeMap

Here is a table comparing the differences between HashMap and TreeMap:

HashMap does not provide any order for elements, has a faster speed, and allows one null key and multiple null values. It is implemented using hashing and consumes more memory space. In contrast, TreeMap provides an order for elements, has a slower speed, and does not allow null keys but allows multiple null values. It is implemented using a Red-Black Tree and consumes less memory space. The complexity of HashMap is O(1), while the complexity of TreeMap is O(log n).