Can you explain how the HashSet data structure works?

HashSet in Java is a data structure that stores unique elements without a specific order. It is based on HashMap, where elements act as keys, and values are placeholders.

Main features of HashSet:

• Fast access, addition, and removal of elements — operations are performed in approximately O(1).
• Does not allow duplicates.
• The order of elements is not guaranteed.

Example of usage:

import java.util.HashSet;

public class Example {
    public static void main(String[] args) {
        HashSet<String> set = new HashSet<>();
        set.add("apple");
        set.add("banana");
        set.add("apple"); // will not be added again, as it already exists

        for (String fruit : set) {
            System.out.println(fruit);
        }
    }
}

In this example, "apple" will be added only once, even if you try to add it twice.