HashSet and the Set Family Architectures

The Set interface defines a collection of unique elements. Most Java Set implementations are not built from scratch; they are Wrappers around existing Map implementations.

1. HashSet: The Wrapper Pattern

A HashSet is essentially a HashMap where:

Keys = Your set elements.
Values = A shared static constant dummy object called PRESENT.

public boolean add(E e) {
    return map.put(e, PRESENT) == null; // Uniqueness is enforced by map key uniqueness
}

2. Uniqueness Enforcement: Equals & HashCode

To successfully deduplicate elements, your class must override hashCode() and equals():

hashCode(): The bucket index. If this is different, the elements are distinct.
equals(): If hashes collide, the set checks if they are equal.

3. The Set Family Comparison

Implementation	Backing Map	Ordering	Performance
`HashSet`	`HashMap`	None	$O(1)$
`LinkedHashSet`	`LinkedHashMap`	Insertion Order	$O(1)$ (higher memory)
`TreeSet`	`TreeMap`	Sorted Order	$O(\log n)$

4. Selection Guide

Use HashSet by default for maximum speed.
Use LinkedHashSet if you need to maintain the order in which elements were added (e.g., maintaining a history of unique visit tags).
Use TreeSet only when you need continuous sorted access to the elements (e.g., finding all values between "A" and "D").