Can you explain the main differences between HashMap and HashSet data structures and situations for their use?.

HashMap and HashSet are data structures from the Java Collections Framework based on hashing, but they serve different purposes:

• HashMap is a collection of key-value pairs. It stores unique keys and their associated values. It allows for fast searching, adding, and removing elements by key.

• HashSet is a collection of unique elements without keys and values. Essentially, it is a HashMap where the values are not important, only the keys (elements) are stored.

When to use:

• HashMap — when you need to store and quickly retrieve data by key, such as a dictionary or cache.
• HashSet — when the uniqueness of elements and fast search are important, such as checking if an element has been encountered before.

Example:

HashMap<String, Integer> map = new HashMap<>();
map.put("apple", 3);
map.put("banana", 5);

HashSet<String> set = new HashSet<>();
set.add("apple");
set.add("banana");