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What is the difference between TreeSet and HashSet?

Anubhav Sharma 642 18 Jul 2024

What is the difference between TreeSet and HashSet?

java javajavac
Updated 19 Jul 2024
Anubhav Sharma
Anubhav Sharma

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1 Answers

Ravi Vishwakarma
Ravi Vishwakarma 19 Jul 2024
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TreeSet and HashSet are both implementations of the Set interface in Java, but they have significant differences in terms of their internal workings, performance, and behavior. 

TreeSet

TreeSet is a class in Java that implements the Set interface and is part of the Java Collections Framework. It uses a red-black tree internally to store elements in a sorted order. The elements in a TreeSet are sorted either according to their natural ordering or according to a Comparator provided at set creation time.

Key Characteristics:

  • Maintains elements in ascending order.
  • Provides guaranteed log(n) time cost for the basic operations (add, remove, and contains).
  • Does not allow null elements.
  • Implements the NavigableSet interface, providing methods for navigating the set.
  • Useful for scenarios where sorted order and fast range operations are required.

Example:

import java.util.Set;
import java.util.TreeSet;

public class TreeSetExample {
    public static void main(String[] args) {
        Set<String> treeSet = new TreeSet<>();
        treeSet.add("Apple");
        treeSet.add("Banana");
        treeSet.add("Orange");

        // Elements are in sorted order
        for (String element : treeSet) {
            System.out.println(element);
        }
    }
}

Output:

Apple
Banana
Orange

HashSet

HashSet is a class in Java that implements the Set interface and is part of the Java Collections Framework. It uses a hash table (internally backed by a HashMap) to store elements, providing constant-time performance for basic operations (add, remove, and contains), assuming the hash function disperses the elements properly among the buckets.

Key Characteristics:

  • Does not maintain any order of elements.
  • Allows one null element.
  • Provides O(1) time complexity for basic operations in the average case.
  • Not synchronized, but can be synchronized externally if needed.
  • Suitable for applications that require a fast, unordered collection of unique elements.

Example:

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

public class HashSetExample {
    public static void main(String[] args) {
        Set<String> hashSet = new HashSet<>();
        hashSet.add("Apple");
        hashSet.add("Banana");
        hashSet.add("Orange");

        // Elements are not in a specific order
        for (String element : hashSet) {
            System.out.println(element);
        }
    }
}

Output:

Apple
Orange
Banana

Note

  • TreeSet: A sorted set backed by a red-black tree, providing log(n) time complexity for basic operations and maintaining elements in ascending order.
  • HashSet: An unordered set backed by a hash table, providing constant-time performance for basic operations and not maintaining any specific order.

Summary of Differences

Feature HashSet TreeSet
Internal Structure Hash table Red-black tree
Ordering No order Sorted order
Performance O(1) for basic operations O(log n) for basic operations
Null Handling Allows one null element Does not allow null elements
Use Cases Fast access, insertion, deletion Sorted elements, range operations

The choice between HashSet and TreeSet depends on the specific requirements of your application. If you need a fast, unordered collection, HashSet is appropriate. If you need a sorted collection and can tolerate slightly slower performance, TreeSet is the better option.

Read more 

How to get the highest (last) element in trees when trees are sorted with another attribute

What is the difference between HashSet and TreeSet?

What are the Similarities between ArrayList and HashSet?

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