Difference Between PriorityQueue and TreeSet

PriorityQueue and TreeSet are classes from the Java Collections Framework, but they serve different purposes. PriorityQueue is designed for processing elements based on priority, while TreeSet stores unique elements in a sorted order. This article explains their differences with clear examples and a comparison table.

PriorityQueue

A PriorityQueue is used when elements need to be processed according to priority rather than insertion order. It implements the Queue interface and internally uses a heap-based data structure.

• Allows duplicate elements.
• Orders elements using natural ordering or a provided Comparator.
• Only the head element is guaranteed to be the smallest (or highest priority).

When to use PriorityQueue

• When elements must be processed based on priority.
• When frequent access to the smallest or largest element is required.
• When duplicate elements are allowed.Hey Geek !!
  Thanks for pointing out, we have updated the article and the changes will be reflected soon.
• When full sorting of all elements is not required.

Example:

import java.util.*;
class PriorityQueueDemo {
    public static void main(String args[])
    {
        PriorityQueue<String> pQueue = new PriorityQueue<>();
        pQueue.add("Geeks");
        pQueue.add("For");
        pQueue.add("Geeks");
        System.out.println(pQueue.peek());
        System.out.println(pQueue.poll());
        System.out.println(pQueue.peek());
    }
}

For
For
Geeks

Explanation:

• Elements are arranged based on natural ordering.
• peek() returns the highest-priority element without removing it.
• poll() removes and returns the highest-priority element.

TreeSet

TreeSet is an implementation of the SortedSet interface that stores unique elements in sorted order. It is backed by a Red-Black Tree and also implements NavigableSet.

• Does not allow duplicate elements.
• Maintains sorted order at all times.
• Ordering must be consistent with equals().

When to use TreeSet

• When elements must remain sorted at all times.
• When duplicate elements are not allowed.
• When range-based or navigational operations are needed.
• When ordered iteration is required.

Example:

import java.util.*;
class TreeSetDemo {
    public static void main(String[] args)
    {
        TreeSet<String> ts = new TreeSet<String>();
        ts.add("Geek");
        ts.add("For");
        ts.add("Geeks");
        System.out.println("Tree Set is " + ts);
        String check = "Geeks";
        System.out.println("Contains " + check + " "
                           + ts.contains(check));
        System.out.println("First Value " + ts.first());

        System.out.println("Last Value " + ts.last());
        String val = "Geek";
        System.out.println("Higher " + ts.higher(val));
        System.out.println("Lower " + ts.lower(val));
    }
}

Tree Set is [For, Geek, Geeks]
Contains Geeks true
First Value For
Last Value Geeks
Higher Geeks
Lower For

Explanation:

• Elements are stored in sorted order automatically.
• first() and last() return boundary elements.
• higher() and lower() perform navigational operations.

PriorityQueue Vs TreeSet

Understanding how these two collections differ helps in choosing the right data structure for priority-based or sorted data processing.