How to Configure a Java Priority Queue to Ignore Duplicates?

Question

What are the best practices to configure a Java Priority Queue for ignoring duplicates?

// Code snippet to demonstrate ignoring duplicates in a PriorityQueue
import java.util.PriorityQueue;
import java.util.HashSet;

public class UniquePriorityQueue {
    public static void main(String[] args) {
        PriorityQueue<Integer> priorityQueue = new PriorityQueue<>();
        HashSet<Integer> set = new HashSet<>();

        // Adding elements to the queue while ignoring duplicates
        int[] elements = {5, 3, 8, 5, 1, 8};
        for (int element : elements) {
            if (set.add(element)) { // Only add if it's not a duplicate
                priorityQueue.offer(element);
            }
        }

        System.out.println("PriorityQueue without duplicates:");
        while (!priorityQueue.isEmpty()) {
            System.out.println(priorityQueue.poll());
        }
    }
}

Answer

A Java PriorityQueue does not inherently prevent duplicates. However, you can implement a mechanism to ignore duplicates by using a secondary collection, such as a HashSet, to track the elements that have already been added. This ensures that only unique elements are inserted into the PriorityQueue, while maintaining the priority order.

import java.util.PriorityQueue;
import java.util.HashSet;

public class UniquePriorityQueue {
    public static void main(String[] args) {
        PriorityQueue<Integer> priorityQueue = new PriorityQueue<>();
        HashSet<Integer> set = new HashSet<>();

        // Adding elements to the queue while ignoring duplicates
        int[] elements = {5, 3, 8, 5, 1, 8};
        for (int element : elements) {
            if (set.add(element)) { // Only add if it's not a duplicate
                priorityQueue.offer(element);
            }
        }

        System.out.println("PriorityQueue without duplicates:");
        while (!priorityQueue.isEmpty()) {
            System.out.println(priorityQueue.poll());
        }
    }
}

Causes

The Java PriorityQueue implementation allows duplicate entries by default.
Duplicates can lead to unintended behavior in applications where unique elements are expected.

Solutions

Utilize a HashSet to track elements added to the PriorityQueue.
Before inserting an element into the queue, check if it already exists in the HashSet.
If the element does not exist in the HashSet, add it both to the HashSet and the PriorityQueue.

Common Mistakes

Mistake: Not using a HashSet, which leads to duplicates in the PriorityQueue.

Solution: Always use a HashSet to track added elements before attempting to insert them into the PriorityQueue.

Mistake: Assuming the PriorityQueue will handle duplicates automatically.

Solution: Remember that the PriorityQueue does not filter duplicates; this must be managed manually.

Helpers

Java PriorityQueue
ignore duplicates Java PriorityQueue
unique elements in PriorityQueue
configuring PriorityQueue
Java data structures