Sets in Java

HashSet vs TreeSet in Java

In Java, both HashSet and TreeSet implement the Set interface, which represents a collection of unique elements (no duplicates). However, they differ in ordering, performance, and internal implementation.

🔹 1. HashSet

✅ Characteristics:

• Backed by a hash table
• Does not maintain any order of elements
• Allows null elements (only one)

📌 Example:

import java.util.HashSet;

public class Main {
    public static void main(String[] args) {
        HashSet<String> fruits = new HashSet<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Apple");  // Duplicate - ignored
        System.out.println(fruits);  // Order is not guaranteed
    }
}

🚀 Performance:

Operation	Time Complexity
add()	O(1) average
remove()	O(1) average
contains()	O(1) average

✅ Use Case:

• When fast lookups, insertions, and deletions are needed
• Ordering is not important

🔹 2. TreeSet

✅ Characteristics:

• Backed by a Red-Black Tree (self-balancing binary search tree)
• Sorted in natural order (or by a custom comparator)
• Does not allow null elements (throws NullPointerException)

📌 Example:

import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        TreeSet<String> fruits = new TreeSet<>();
        fruits.add("Banana");
        fruits.add("Apple");
        fruits.add("Cherry");
        System.out.println(fruits);  // Output: [Apple, Banana, Cherry]
    }
}

🚀 Performance:

Operation	Time Complexity
add()	O(log n)
remove()	O(log n)
contains()	O(log n)

✅ Use Case:

• When you need sorted data or need to perform range queries
• Order is important

✅ Summary

Feature	HashSet	TreeSet
Underlying Structure	Hash table	Red-Black Tree
Order	No order	Sorted (natural or custom)
Null Elements	Allowed (one)	Not allowed
Performance	Faster (O(1) avg ops)	Slower (O(log n))
Allows Duplicates	❌	❌
Thread-safe	❌ (use Collections.synchronizedSet())	❌