Explain the differences between HashMap and HashSet data structures.

HashMap and HashSet are two different data structures in Java based on hashing, but serving different purposes:

• HashMap is a collection of key-value pairs. It stores objects as a key and its associated value. It allows quick retrieval of a value by its key.

• HashSet is a collection of unique elements without keys. In fact, HashSet is implemented based on HashMap, where elements act as keys, and the values are dummy.

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");

Summary:

• HashMap stores key-value pairs.
• HashSet stores only unique keys (elements).
• HashSet does not store values, only unique objects.
• HashMap allows retrieving a value by key, HashSet checks for the presence of an element.