What are the differences between HashMap and Set data structures in the context of their usage?

HashMap and Set are different data structures with different purposes and usage features:

• HashMap is a collection of key-value pairs. It allows storing and quickly retrieving a value by its key. Keys are unique, values can repeat.

• Set is a collection of unique elements without keys. It is designed to store a set of unique objects.

Main differences:

• In HashMap, each element is a pair (key, value), while in Set, it is just an element.
• HashMap allows quick lookup of a value by key, Set checks for the presence of an element.
• Set is often implemented based on HashMap, where elements act as keys, and values are dummy.

Example usage:

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

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

In conclusion, choose HashMap if you need to store key-value pairs, and Set if you need to store unique elements without values.