Last Updated on June 28, 2024 by Abhishek Sharma
Priority queues are specialized data structures that manage a set of elements each with an associated priority. Unlike regular queues, where elements are dequeued in the order they were enqueued (FIFO – First In, First Out), priority queues dequeue elements based on their priority. In Java, priority queues are often implemented using the PriorityQueue class, which relies on natural ordering or a custom comparator to determine the order of elements. Implementing a priority queue with a custom comparator allows developers to define complex ordering criteria, providing flexibility to handle various use cases such as task scheduling, simulations, and pathfinding algorithms.
What are Priority Queue?
Priority queue is an abstract data type, It is a type of queue in which each element has a priority assigned to it. The priority of the element determines the order in which elements are removed from the priority queue. In the priority queue, all the elements are arranged either in ascending order or descending order.
Priority Queue Properties:
- In the priority queue, every element has priority assigned to it.
- The element with highest priority first.
- If two elements are having the same priority then they are served according to their order in the queue.
Priority Queue Comparator method:
Priority queue comparator function is used to return the order of the elements that are stored in the priority queue Comparator method returns the null value if the queue follows the same order of the elements.
Syntax:
Comp_set = (PriorityQueue)Priority_Queue.comparator()Comparator method does not take any parameters.
Implementation 1: Using natural order of the elements.
import java.util.*;
class Priority_Queue_Demo {
public static void main(String[] args)
{
PriorityQueue<Integer> queue = new PriorityQueue<Integer>();
queue.add(2);
queue.add(24);
queue.add(3);
queue.add(35);
queue.add(45);
queue.add(50);
System.out.println("Priority queue values are: " + queue);
Comparator comp = queue.comparator();
System.out.println("Since the Comparator value is: " + comp);
System.out.println("Because it follows natural ordering");
}
}
Implementation 2: Using the specific comparator.
import java.util.Comparator; import java.util.PriorityQueue; class The_Comparator implements Comparator{ public int compare(String str1, String str2) { String first_Str; String second_Str; first_Str = str1; second_Str = str2; return second_Str.compareTo(first_Str); } } class Priority_Queue_Demo { public static void main(String[] args) { PriorityQueue queue = new PriorityQueue (new The_Comparator()); queue.add("P"); queue.add("R"); queue.add("E"); queue.add("P"); queue.add("4"); System.out.println("The elements with the highest priority element at front of queue" + "order:"); while(!queue.isEmpty()){ System.out.print(" "+queue.poll()); } } }
While implementing the priority queue using a comparator function the elements of the priority queue are ordered according to their natural order or by using a comparator provided at the construction time.
And it depends which constructor we are using:
-
Public PriorityQueue(): It creates a priority queue with default capacity that orders its elements according to their order.
-
Public PriorityQueue(int initialCapacity, Comparator \ comparator): It creates a priority queue with the specified initial capacity in which elements are ordered in accordance to their specified comparator
-
Public PriorityQueue(SortedSet PQ): It Creates a priority queue containing all the elements in the sorted set.
-
PriorityQueue(Collection C): It creates a priority queue containing all the elements in a specified collection.
Implementation of priority queue using comparator method:
import java.util.*;
class Example {
public static void main(String[] args){
Scanner in = new Scanner(System.in);
PriorityQueue pq = new
PriorityQueue(5, new StudentComparator());
Student student1 = new Student("Ankit", 3.2);
pq.add(student1);
Student student2 = new Student("Amit", 3.6);
pq.add(student2);
Student student3 = new Student("Pal", 4.0);
pq.add(student3);
System.out.println("Students served in their priority order");
while (!pq.isEmpty()) {
System.out.println(pq.poll().getName());
}
}
}
class StudentComparator implements Comparator{
public int compare(Student s1, Student s2) {
if (s1.cgpa < s2.cgpa)
return 1;
else if (s1.cgpa > s2.cgpa)
return -1;
return 0;
}
}
class Student {
public String name;
public double cgpa;
public Student(String name, double cgpa) {
this.name = name;
this.cgpa = cgpa;
}
public String getName() {
return name;
}
}
Conclusion
Implementing a priority queue with a custom comparator in Java provides a powerful tool for managing elements based on complex priority rules. By defining a comparator, developers can customize the ordering of elements beyond the natural ordering, enabling efficient and flexible solutions for a wide range of applications. Mastering the use of comparators in priority queues enhances your ability to solve problems where ordering is a critical factor, leveraging Java’s robust collection framework to manage data effectively.
FAQs related to Priority Queue Comparator Java
Below are some FAQs related to Priority Queue Comparator Java:
1. What is a priority queue in Java?
A priority queue is a data structure that allows elements to be processed based on their priority. In Java, the PriorityQueue class is used to implement a priority queue where elements with higher priority are dequeued before those with lower priority.
2. How does the natural ordering work in a priority queue?
Natural ordering in a priority queue refers to the ordering of elements based on their natural comparison, defined by the Comparable interface. For instance, for integers, natural ordering means the smallest number has the highest priority.
3. What is a comparator in Java?
A comparator is an interface in Java (Comparator) used to define custom ordering of objects. It provides a method compare(T o1, T o2) that compares two objects and returns a negative integer, zero, or a positive integer if the first argument is less than, equal to, or greater than the second, respectively.
4. How do you implement a custom comparator for a priority queue in Java?
To implement a custom comparator, you need to create a class that implements the Comparator interface and override the compare method. Then, pass an instance of this comparator to the PriorityQueue constructor.
5. What are some common use cases for priority queues with custom comparators?
Common use cases include task scheduling (where tasks have different priorities), event simulation (events are processed based on priority), and implementing algorithms like Dijkstra’s shortest path or A* search where nodes are processed based on cost or heuristic values.
6. Can a priority queue be used for sorting?
Indirectly, yes. By inserting all elements into a priority queue and then removing them, you can retrieve the elements in sorted order according to the priority queue’s comparator. This approach is less efficient than traditional sorting algorithms for large datasets but can be useful for certain scenarios.