What is a PriorityQueue in Java?

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Pragya Keshap answered on February 4, 2022 Popularity 10/10 Helpfulness 2/10

answer What is a PriorityQueue in Java?

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What is a PriorityQueue in Java?

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A PriorityQueue is data structure based on Queue. Unlike Queue,

the elements on PriorityQueue are not returned in FIFO order.

A PriorityQueue maintains the natural order of its elements or it

uses a Comparator provided at initialization.

It is an unbounded queue based on a priority heap.

PriorityQueue does not allow null values. We cannot add any object

that does not provide natural ordering to PriorityQueue.

PriorityQueue in Java is not thread-safe.

It gives O(log n) time for enqueing and dequeing operations.

public class PriorityQueueUserDefinedObjectExample {
    public static void main(String[] args) {
        /*
           The requirement for a PriorityQueue of user defined objects is that

           1. Either the class should implement the Comparable interface and provide
              the implementation for the compareTo() function.
           2. Or you should provide a custom Comparator while creating the PriorityQueue.
        */

        // Create a PriorityQueue
        PriorityQueue<Employee> employeePriorityQueue = new PriorityQueue<>();

        // Add items to the Priority Queue
        employeePriorityQueue.add(new Employee("Rajeev", 100000.00));
        employeePriorityQueue.add(new Employee("Chris", 145000.00));
        employeePriorityQueue.add(new Employee("Andrea", 115000.00));
        employeePriorityQueue.add(new Employee("Jack", 167000.00));

        /*
            The compareTo() method implemented in the Employee class is used to determine
            in what order the objects should be dequeued.
        */
        while (!employeePriorityQueue.isEmpty()) {
            System.out.println(employeePriorityQueue.remove());
        }
    }
}

https://www.callicoder.com/java-priority-queue/#:~:text=A%20priority%20queue%20in%20Java,at%20the%20time%20of%20creation.

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Tags: java priority-queue

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Contributed on Feb 04 2023

Pragya Keshap

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import java.util.Arrays;

public class PriorityQueue {
  private int[] items = new int[5];
  private int count;

  // O(n)
  public void add(int item) {
    if (isFull())
      throw new IllegalStateException();

    var i = shiftItemsToInsert(item);
    items[i] = item;
    count++;
  }

  public boolean isFull() {
    return count == items.length;
  }

  private int shiftItemsToInsert(int item) {
    int i;
    for (i = count - 1; i >= 0; i--) {
      if (items[i] > item)
        items[i + 1] = items[i];
      else
        break;
    }
    return i + 1;
  }

  // O(1)
  public int remove() {
    if (isEmpty())
      throw new IllegalStateException();

    return items[--count];
  }

  public boolean isEmpty() {
    return count == 0;
  }

  @Override
  public String toString() {
    return Arrays.toString(items);
  }
}

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Contributed on Feb 04 2022

PrashantUnity

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What is a PriorityQueue in Java?

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