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In Java, the PriorityQueue data structure offers an efficient way to manage tasks based on their assigned priorities. Elements with higher priority are dequeued before elements with lower priority. It uses a heap data structure internally to maintain the ordering of elements.

In this article, we will learn how to use a PriorityQueue to implement a priority-based task scheduler in Java.

Step-by-Step Implementation to Use a PriorityQueue to Implement a Priority-based Task Scheduler

Below are the steps to implement a Priority-based Task Scheduler.

Step 1: Define the Task Class

public class Task {
    private String description;
    private int priority; // Lower values mean higher priority
    private Object data; // Any data needed for execution
    // Getters, setters, and a constructor
}

Step 2: Create the PriorityQueue

PriorityQueue<Task> queue = new PriorityQueue<>(10, new TaskComparator());

Step 3: Implement the TaskComparator (if necessary)

public class TaskComparator implements Comparator<Task> {
    @Override
    public int compare(Task t1, Task t2) {
        return Integer.compare(t1.getPriority(), t2.getPriority());
    }
}

Step 4: Schedule Tasks

Task task1 = new Task("Download files", 1, ...);
Task task2 = new Task("Send report", 2, ...);
queue.add(task1);
queue.add(task2);

Step 5: Execute Tasks

while (!queue.isEmpty()) {
    Task currentTask = queue.poll(); // Removes and returns the highest-priority task
    // Execute the task using currentTask.data or other relevant information
    System.out.println("Executing task: " + currentTask.getDescription());
}

Program to Implement a Priority-based Task Scheduler in Java

Java

import java.util.PriorityQueue;

// Task class representing a task with a priority
class Task implements Comparable<Task> {
    private String name;
    private int priority;

    // Constructor to initialize task name and priority
    public Task(String name, int priority) {
        this.name = name;
        this.priority = priority;
    }

    // Getter method for task name
    public String getName() {
        return name;
    }

    // Getter method for task priority
    public int getPriority() {
        return priority;
    }

    // Implementation of compareTo method from Comparable interface
    // Allows tasks to be compared based on their priority
    @Override
    public int compareTo(Task other) {
        return Integer.compare(this.priority, other.priority);
    }

    // Override toString method for better task representation
    @Override
    public String toString() {
        return "Task{" +
                "name='" + name + '\'' +
                ", priority=" + priority +
                '}';
    }
}

// TaskScheduler class implementing priority-based task scheduling
class TaskScheduler 
{
    private PriorityQueue<Task> taskQueue;

    // Constructor to initialize PriorityQueue for task scheduling
    public TaskScheduler() 
    {
        // Initialize the priority queue with a comparator for task priority
        taskQueue = new PriorityQueue<>();
    }

    // Method to add a new task to the scheduler
    public void addTask(Task task) {
        taskQueue.offer(task); // Adds task to the priority queue
    }

    // Method to retrieve and execute the highest priority task
    public void executeNextTask() {
        Task task = taskQueue.poll(); // retrieves and removes the highest priority task
        if (task != null) {
            System.out.println("Executing task: " + task.getName()); 
        } else {
            System.out.println("No tasks left to execute."); 
        }
    }
}

// main class demonstrating the usage of TaskScheduler
public class PriorityTaskSchedulerExample {
    public static void main(String[] args) {
        TaskScheduler scheduler = new TaskScheduler(); // creates an instance of TaskScheduler

        // adding tasks with different priorities
        scheduler.addTask(new Task("Task 1", 3));
        scheduler.addTask(new Task("Task 2", 1));
        scheduler.addTask(new Task("Task 3", 2));

        // executing tasks in priority order
        scheduler.executeNextTask(); // Task 2 (highest priority)
        scheduler.executeNextTask(); // Task 3
        scheduler.executeNextTask(); // Task 1
        scheduler.executeNextTask(); // No tasks left
    }
}

Output

Executing task: Task 2
Executing task: Task 3
Executing task: Task 1
No tasks left to execute.

Explanation of the above code:

1. Task Class:

Represents a task with a name and priority (lower value means higher priority).
Implements Comparable<Task> for comparison based on priority using compareTo.
Defines toString for a clear string representation of the task.

2. TaskScheduler Class:

Holds a PriorityQueue<Task> to manage tasks based on their priority.
Provides methods to:
- Add tasks using addTask(Task task).
- Execute the highest priority task using executeNextTask(). This removes the task from the queue and prints its name.

3. PriorityTaskSchedulerExample (Main Class):

Creates a TaskScheduler instance.
Adds three tasks with different priorities.
Executes tasks repeatedly using executeNextTask() until the queue is empty.

How to Use a PriorityQueue to Implement a Priority-Based Task Scheduler in Java?