Difference between a HashMap and a HashSet in java.

HashMap and HashSet are both part of the Java Collections Framework, but they are designed to serve different purposes.

HashMap is a key-value data structure that allows you to store and retrieve values based on keys. Each key in a HashMap is associated with a single value. HashMap is implemented using hash tables, which provide efficient O(1) average-case performance for put and get operations.

Here's an example of using HashMap in Java:

HashSet, on the other hand, is a collection that stores only unique elements. It does not allow duplicates. HashSet is implemented using a hash table, similar to HashMap, but it stores only keys and not key-value pairs.

Here's an example of using HashSet in Java:

Here are some key differences between HashMap and HashSet:

1. Use case: HashMap is used when you need to store key-value pairs and retrieve the value associated with a key. HashSet is used when you need to store a collection of unique elements and check for the presence of an element in the set.

2. Duplicate elements: HashMap can have duplicate values, but not duplicate keys. HashSet does not allow duplicates at all.

3. Retrieval of elements: In a HashMap, you can retrieve a value by its corresponding key. In a HashSet, you can only check for the presence of an element.

4. Performance: HashMap offers faster performance than HashSet for retrieving and updating key-value pairs. HashSet, however, offers faster performance for checking the presence of an element in the set.

5. Iteration: HashMap offers more flexibility when it comes to iterating over its elements, as it allows you to iterate over both keys and values. HashSet only allows iteration over its elements.

In summary, HashMap and HashSet are both useful data structures in Java, but they serve different purposes. If you need to store key-value pairs, use HashMap. If you need to store a collection of unique elements, use HashSet.