How are HashMap and HashSet data structures related in Java?

HashSet in Java is implemented based on HashMap. Inside HashSet, each element is stored as a key in the HashMap, and the value for all keys is the same placeholder object (usually a static final object). This allows HashSet to efficiently ensure element uniqueness and quick access, leveraging the capabilities of HashMap.

Example of a simplified HashSet implementation:

public class SimpleHashSet<E> {
    private static final Object PRESENT = new Object();
    private HashMap<E, Object> map = new HashMap<>();

    public boolean add(E e) {
        return map.put(e, PRESENT) == null;
    }

    public boolean contains(E e) {
        return map.containsKey(e);
    }

    public boolean remove(E e) {
        return map.remove(e) == PRESENT;
    }
}