What is Set in Java?

The Set interface is a part of the Java Collections Framework and is available in the java.util package. It represents a collection that stores unique elements, meaning duplicate values are not allowed.

Key Characteristics of Set

• A Set does not allow duplicate elements. If an attempt is made to add a duplicate element, it is ignored.
• Most Set implementations allow at most one null value. However, TreeSet does not permit null elements because it stores elements in sorted order.
• Set provides efficient operations for searching, inserting, and deleting elements.
• It does not support index-based access like the List interface.
• The order of elements depends on the implementation:
  • HashSet → No guaranteed order
  • LinkedHashSet → Maintains insertion order
  • TreeSet → Maintains sorted order

Implementations of Set Interface

1. HashSet
HashSet is an implementation of the Set interface that stores unique elements using a hash table.

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

Characteristics

• Does not allow duplicates
• Allows one null value
• Does not maintain insertion order
• Fastest among Set implementations

Example

import java.util.HashSet;

public class Main {
    public static void main(String[] args) {

        HashSet<String> set = new HashSet<>();

        set.add("Java");
        set.add("React");
        set.add("SQL");
        set.add("Java");

        System.out.println(set);
    }
}

Output
[React, Java, SQL]
(Order may vary.)

Internal Data Structure
Hash Table

When to Use

• Duplicates are not allowed
• Order is not important
• Fast performance is required

2. LinkedHashSet
LinkedHashSet is a HashSet that maintains insertion order.

Set<String> set = new LinkedHashSet<>();

Characteristics

• No duplicates
• Maintains insertion order
• Allows one null
• Slightly slower than HashSet

Example

import java.util.LinkedHashSet;

public class Main {
    public static void main(String[] args) {

        LinkedHashSet<String> set =
                new LinkedHashSet<>();

        set.add("Java");
        set.add("React");
        set.add("SQL");

        System.out.println(set);
    }
}

Output
[Java, React, SQL]
Order remains same as insertion order.

Internal Data Structure
Hash Table + Doubly Linked List

When to Use

• Duplicates are not allowed
• Insertion order must be preserved

3. TreeSet

TreeSet stores unique elements in sorted order.

Set<Integer> set = new TreeSet<>();

Characteristics

• No duplicates
• Automatically sorts elements
• Does not allow null
• Slower than HashSet

Example

import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {

        TreeSet<Integer> set =
                new TreeSet<>();

        set.add(50);
        set.add(10);
        set.add(30);

        System.out.println(set);
    }
}

Output
[10, 30, 50]
Automatically sorted in ascending order.

Internal Data Structure
Red-Black Tree
(A self-balancing binary search tree.)