What is the difference between a hashset and a hashmap?

HashSet and HashMap are both commonly used collection classes in Java, but there are some important differences between them.

1. Data structures: HashSet is implemented based on a hash table, using a hash function to determine the storage location of elements, allowing for fast insertion and retrieval. Similarly, HashMap is also based on a hash table, but stores mappings of key-value pairs.
2. Storage in the elements: HashSet stores unique elements, not allowing repeated values. HashMap stores key-value pairs, where each key is unique but values can be repeated.
3. Accessing elements: HashSet does not provide a direct method to access elements, you can only iterate through the collection using iterators or the enhanced for loop. HashMap allows you to directly access the corresponding value using keys.
4. Iterating order: The elements in a HashSet do not have a specific order, so their storage and traversal order is unpredictable. Similarly, HashMap stores key-value pairs based on the hash value of the keys, resulting in an unpredictable order as well.
5. Thread safety: HashSet and HashMap are not thread-safe, meaning if multiple threads concurrently modify the collection, it may lead to concurrency issues. To achieve thread safety, you can use the synchronizedSet or synchronizedMap methods from the Collections class.

In summary, HashSet is suitable for storing unique elements and scenarios where elements do not need to be accessed by keys; whereas HashMap is suitable for storing key-value pairs and scenarios where values need to be quickly accessed through keys.