Java PriorityQueue With Comparator

1. Overview

In this tutorial, We'll learn how to use the comparator with the PriorityQueue class in java to implement our own custom priority field.

This post covers in older java version and with java 8 later also.

PriorityQueue is a normal queue but it has the special ability to process them based on its priority field. That means each object inside the PriorityQueue is associated with the "priority" field.

This field decides which object should be popped out from the queue. Whichever is having the highest priority will be coming out first and the lowest priority one comes last from the PriorityQueue.

Let us write simple examples to understand PriorityQueue with a custom comparator.

It is useful to understand how to sort multiple fields with a comparator in java 8 streams?

2. Java - PriorityQueue with Comparator

PriorityQueue class constructor comes with the comparator as an argument. We are now going to this one in the examples.

First, let us create a Customer class with the fields id, name and vip. Here, we add a few customer objects to the priority queue and then process the first vip customers and then the regular customers.

class Customer {

	private int id;
	private String name;
	private boolean vip;

	public Customer(int id, String name, boolean vip) {
		this.id = id;
		this.name = name;
		this.vip = vip;
	}

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public boolean isVip() {
		return vip;
	}

	public void setVip(boolean vip) {
		this.vip = vip;
	}

}

Now create the comparator implementation class with the vip logic. Here vip field is a boolean.

For boolean fields, we need to use Boolean.compareTo() method.

This VipComparator sorts based on the vip field and all vip customers are sent to the starting of the queue.

class VipComparator implements Comparator<Customer> {

	public int compare(Customer c1, Customer c2) {

		return Boolean.valueOf(c2.isVip()).compareTo(Boolean.valueOf(c1.isVip()));
	}
}

Next, create the main class with PriorityQueue and pass VipComparator object to the priority queue comparator.

And then add a few customers objects to it and iterated the priority queue through while loop.

package com.javaprogramto.java8.priorityqueue;

import java.util.Comparator;
import java.util.PriorityQueue;
import java.util.Queue;

public class PriorityQueueComparatorExample {

	public static void main(String[] args) {

		Queue<Customer> priorityVipQueue = new PriorityQueue<>(new VipComparator());

		priorityVipQueue.add(new Customer(1, "name 1", true));
		priorityVipQueue.add(new Customer(2, "Name 2", false));
		priorityVipQueue.add(new Customer(3, "Name 3", false));
		priorityVipQueue.add(new Customer(4, "Name 4", true));
		priorityVipQueue.add(new Customer(5, "Name 5", false));

		while (!priorityVipQueue.isEmpty()) {
			Customer c = priorityVipQueue.poll();
			System.out.println("Name - " + c.getName() + " - is Vip - " + c.isVip());
		}

	}
}

Output

Name - name 1 - is Vip - true
Name - Name 4 - is Vip - true
Name - Name 2 - is Vip - false
Name - Name 3 - is Vip - false
Name - Name 5 - is Vip - false

From the output, we can see that first all priority customers are processed first and at the last, all non-priority customers have been taken care of.

3. Java 8 - PriorityQueue with Comparator

Adding the custom comparator to PriorityQueue can be done in java 8 2 ways.

3.1 Using Lamba Expression

The same comparator can be created using Java 8 api. We do not need to create a new class for VipComparator now because we can use the lambda expressions from java 8.

public class PriorityQueueComparatorExample2 {

	public static void main(String[] args) {

		Queue<Customer> priorityVipQueue = new PriorityQueue<>(
				(c1, c2) -> Boolean.valueOf(c2.isVip()).compareTo(Boolean.valueOf(c1.isVip())));

		priorityVipQueue.add(new Customer(1, "name 1", true));
		priorityVipQueue.add(new Customer(2, "Name 2", false));
		priorityVipQueue.add(new Customer(3, "Name 3", false));
		priorityVipQueue.add(new Customer(4, "Name 4", true));
		priorityVipQueue.add(new Customer(5, "Name 5", false));

		while (!priorityVipQueue.isEmpty()) {
			Customer c = priorityVipQueue.poll();
			System.out.println("Name - " + c.getName() + " - is Vip - " + c.isVip());
		}

	}

}

We can see clearly, comparator code is reduced to a single line with the help of lambda's.

Let us execute the code and see the output is expected or not.

Output

Name - name 1 - is Vip - true
Name - Name 4 - is Vip - true
Name - Name 2 - is Vip - false
Name - Name 3 - is Vip - false
Name - Name 5 - is Vip - false

3.2 Using Method Reference

Method references can be used to create the comparator using Comparator.comparing() method.

Queue<Customer> priorityVipQueue = new PriorityQueue<>(Comparator.comparing(Customer::isVip).reversed());

This code also produces the same output but the comparator has to reverse using Comparator.reversed() method.

Comparator.reversed() example with arraylist

4. Conclusion

In this article, we've seen how to implement a comparator for PriorityQueue in java and new java 8 versions.

GitHub

PriorityQueue

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