Difference between HashMap and TreeMap

In Java, both HashMap and TreeMap are used for key-value pair storage. However, there are some differences between them:

1. Ordering: HashMap does not maintain any order for the elements, while TreeMap maintains the natural order of the keys or an order specified by a Comparator.
2. Performance: HashMap has better performance in terms of adding and searching elements, while TreeMap has better performance when iterating over all elements in order.
3. Memory usage: TreeMap uses more memory than HashMap due to the need to maintain the order of the keys.
4. Concurrency: HashMap is not thread-safe, while TreeMap can be made thread-safe by using Collections.synchronizedMap().
5. Usage: HashMap is suitable for applications that require frequent updates and searching, while TreeMap is suitable for applications that require frequent iteration in order or for storing keys that implement the Comparable interface.

In summary, HashMap is more suitable when we need better performance in adding and searching elements, while TreeMap is more suitable when we need to maintain order or perform operations like range queries over a set of keys.