# HashSet (Condensed Guide)

## What Is HashSet?

`HashSet` is a `Set` implementation that stores **unique elements using hashing**.

Key characteristics:
- No duplicate elements
- No guaranteed ordering
- Allows **one null element**
- Fast lookup, insertion, and removal

```java
import java.util.Set;
import java.util.HashSet;

Set<Integer> numbers = new HashSet<>();

Iterable
 ↓
Collection
 ↓
Set
 ↓
HashSet

HashSet is one of the most commonly used implementations of the Set interface.

HashSet uses a hash table internally.

Steps when adding an element:

1. The element’s hashCode() is calculated
2. The hash determines a storage location
3. The element is placed into a bucket

Because of hashing:

• Searching is very fast
• Order of elements is not predictable

Example:

add(10)
add(20)
add(10)

Result:

10, 20

Duplicate values are ignored.

Worst case (many collisions) → O(n)

set.add(10);          // add element
set.remove(10);       // remove element
set.contains(10);     // check element
set.size();           // number of elements
set.isEmpty();        // check if empty

for (Integer num : set) {
    System.out.println(num);
}

Because HashSet does not guarantee ordering, elements may appear in any order.

Use HashSet when you need:

• Unique elements
• Fast membership checking
• Removing duplicates

Examples:

• Unique usernames
• Removing duplicates from a list
• Tracking visited nodes in algorithms
• Fast lookup tables

Set<String> fruits = new HashSet<>();

fruits.add("Apple");
fruits.add("Banana");
fruits.add("Apple");

System.out.println(fruits);

Output:

[Apple, Banana]

The duplicate "Apple" is not stored.

• Remove duplicates from a list
• Count unique numbers in an array
• Track visited nodes in graph traversal
• Compare HashSet with TreeSet ordering

HashSet is a fast implementation of the Set interface that stores unique elements using hashing.

Key characteristics:

• No duplicates
• No guaranteed ordering
• Very fast lookup and insertion