Set Interface (HashSet, TreeSet)

🧺 Set Interface (HashSet, TreeSet)

The Set interface in Java is a part of the Collections Framework and represents a collection that does not allow duplicate elements. Unlike List, sets are unordered (in most implementations) and are mainly used to store unique values.

🧩 Key Features of Set Interface:

• No duplicates allowed

• At most one null element (depends on implementation)

• Does not guarantee order (except in some implementations like TreeSet)

• Backed by different data structures for different performance trade-offs

🔧 Implementations of Set

1. HashSet – Unordered, Fast Access

• Based on a hash table

• Does not maintain order

• Allows one null element

• Best for performance (O(1) for add, remove, contains)

✅ Example: HashSet

import java.util.HashSet;

public class HashSetExample {
    public static void main(String[] args) {
        HashSet<String> set = new HashSet<>();

        set.add("Apple");
        set.add("Banana");
        set.add("Mango");
        set.add("Banana");  // Duplicate, ignored

        System.out.println(set);
    }
}

[Mango, Banana, Apple]

Note: The output order may vary, because HashSet does not maintain insertion order.

2. TreeSet – Sorted Set

• Based on a Red-Black Tree

• Maintains elements in sorted (ascending) order

• Does not allow null

• Slower than HashSet (O(log n) for operations)

✅ Example: TreeSet

import java.util.TreeSet;

public class TreeSetExample {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();

        numbers.add(40);
        numbers.add(10);
        numbers.add(20);
        numbers.add(10);  // Duplicate, ignored

        System.out.println(numbers);
    }
}

[10, 20, 40]

The output is always sorted in ascending order.

⚖️ HashSet vs TreeSet

🧠 Common Methods in Set

✅ Real-world Use Cases

• Storing unique usernames

• Checking for duplicate entries

• Keeping track of visited web pages

• Implementing set operations (union, intersection)