Java PriorityQueue

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Sriparno08
Published May 10, 2022Updated May 11, 2025
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A PriorityQueue implements the Queue interface. It stores elements in a collection prioritized by natural ordering or by a custom Comparator. The elements are prioritized with the least value element at the head of the queue, and the Queue methods .peek() and .poll() operate on that element.

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Creating a PriorityQueue

Using natural ordering:

import java.util.PriorityQueue;


PriorityQueue<DataType> q = new PriorityQueue<DataType>();

Using a custom Comparator:

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


PriorityQueue<DataType> q = new PriorityQueue<DataType>(new CustomComparator());

In this syntax:

  • q: A PriorityQueue created with the new keyword.
  • DataType: The data type to be stored in q.
  • CustomComparator: A custom Comparator for DataType or a parent class of DataType.

Adding Items to PriorityQueue

The .add() method adds new items to a PriorityQueue.

Syntax

priorityqueue.add(item)

Parameters:

  • item: The item to be added to the priorityqueue. It should match the data type of priorityqueue.

Return value:

Returns true if item is successfully added, and false otherwise.

Example

import java.util.PriorityQueue;


public class Main {
  public static PriorityQueue<String> SetupPriorityQueue() {
    PriorityQueue<String> result = new PriorityQueue<String>();


    result.add("Cabbage");
    result.add("Pizza");
    result.add("Sausage");
    result.add("Potatoes");
    result.add("Salad");


    return result;
  }


  public static void main(String[] args) {
    PriorityQueue<String> food = SetupPriorityQueue();


    System.out.println(food);
  }
}

Output:

[Cabbage, Pizza, Sausage, Potatoes, Salad]

Removing Items from PriorityQueue

The .remove() method removes specific items from a PriorityQueue.

Syntax

priorityqueue.remove(item)

Parameters:

  • item: The item to be removed from priorityqueue. If item is not found, the method has no effect.

Return value:

Returns true if item is successfully removed, and false otherwise.

Example

import java.util.PriorityQueue;


public class Main {
  public static PriorityQueue<String> SetupPriorityQueue() {
    PriorityQueue<String> food = new PriorityQueue<String>();


    food.add("Cabbage");
    food.add("Pizza");
    food.add("Sausage");
    food.add("Potatoes");
    food.add("Salad");


    return food;
  }


  public static void main(String[] args) {
    PriorityQueue<String> food = SetupPriorityQueue();


    food.remove("Pizza");


    while (food.size() > 0) {
      System.out.println(food.poll());
    }
  }
}

Output:

Cabbage
Potatoes
Salad
Sausage

Searching for an Item in PriorityQueue

The .contains() method checks whether a specific item exists in a PriorityQueue.

Syntax

priorityqueue.contains(item);

Parameters:

  • item: The item to search for in priorityqueue.

Return value:

Returns true if item is found, and false otherwise.

Example

import java.util.PriorityQueue;


public class Main {
  public static PriorityQueue<String> SetupPriorityQueue() {
    PriorityQueue<String> food = new PriorityQueue<String>();


    food.add("Cabbage");
    food.add("Pizza");
    food.add("Sausage");
    food.add("Potatoes");
    food.add("Salad");


    return food;
  }


  public static void main(String[] args) {
    PriorityQueue<String> food = SetupPriorityQueue();


    if (food.contains("Nuts")) {
      System.out.println("Allergen warning!");
    } else {
      System.out.println("Safe to eat.");
    }
  }
}

Output:

Safe to eat.

Returning the Size of a PriorityQueue

The .size() method returns the number of items in a PriorityQueue.

Syntax

priorityqueue.size()

Example

import java.util.PriorityQueue;


public class Main {
  public static PriorityQueue<String> SetupPriorityQueue() {
    PriorityQueue<String> food = new PriorityQueue<String>();


    food.add("Cabbage");
    food.add("Pizza");
    food.add("Sausage");
    food.add("Potatoes");
    food.add("Salad");


    return food;
  }


  public static void main(String[] args) {
    PriorityQueue<String> food = SetupPriorityQueue();


    System.out.println(food.size());
  }
}

Output:

5

Traversing a PriorityQueue

A PriorityQueue can be traversed with a for-each loop. Items will be returned in the order they were inserted, not in priority order.

Example

import java.util.PriorityQueue;


public class Main {
  public static PriorityQueue<String> SetupPriorityQueue() {
    PriorityQueue<String> food = new PriorityQueue<String>();


    food.add("Cabbage");
    food.add("Pizza");
    food.add("Sausage");
    food.add("Potatoes");
    food.add("Salad");


    return food;
  }


  public static void main(String[] args) {
    PriorityQueue<String> food = SetupPriorityQueue();


    for (String i : food) {
      System.out.println(i);
    }
  }
}

Output:

Cabbage
Pizza
Sausage
Potatoes
Salad

.peek() and .poll() Methods

The .peek() method retrieves the head of the queue without removing it, while .poll() retrieves and removes it.

Syntax

priorityqueue.peek();
priorityqueue.poll();

Example

import java.util.PriorityQueue;


public class Main {
  public static PriorityQueue<String> SetupPriorityQueue() {
    PriorityQueue<String> food = new PriorityQueue<String>();


    food.add("Cabbage");
    food.add("Pizza");
    food.add("Sausage");
    food.add("Potatoes");
    food.add("Salad");


    return food;
  }


  public static void main(String[] args) {
    PriorityQueue<String> food = SetupPriorityQueue();


    System.out.println(food.peek());


    while (food.size() > 0) {
      System.out.println(food.poll());
    }


    System.out.println(food.peek());
  }
}

Output:

Cabbage
Cabbage
Pizza
Potatoes
Salad
Sausage
null

Using a Custom Comparator

A custom Comparator can be used to define an alternative priority scheme.

Syntax

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


static class CustomComparator implements Comparator<DataType> {
  @Override
  public int compare(DataType item1, DataType item2) {
    // Return a positive value if item1 < item2, and a negative value if item1 > item2
  }
}


PriorityQueue<DataType> q = new PriorityQueue<DataType>(new CustomComparator());

Parameters:

  • DataType: The data type being compared.

Return value:

Returns a positive value if the first argument comes before the second and a negative value if it comes after.

Example

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


public class Main {
  static class CustomComparator implements Comparator<String> {
    @Override
    public int compare(String item1, String item2) {
      return item1.compareTo(item2) < 0 ? 1 : -1;
    }
  }


  public static void main(String[] args) {
    PriorityQueue<String> food = new PriorityQueue<String>(new CustomComparator());


    food.add("Cabbage");
    food.add("Pizza");
    food.add("Sausage");
    food.add("Potatoes");
    food.add("Salad");


    while (food.size() > 0) {
      System.out.println(food.poll());
    }
  }
}

Output:

Sausage
Salad
Potatoes
Pizza
Cabbage

Frequently Asked Questions

1. What is the default ordering of elements in a PriorityQueue?

By default, a PriorityQueue uses the natural ordering of its elements. This means:

  • For numeric types (e.g., Integer), lower numbers have higher priority.
  • For strings, lexicographical order is used (e.g., “apple” < “banana”).

2. Can a PriorityQueue contain duplicate elements?

Yes, a PriorityQueue allows duplicate elements. However, their relative ordering is determined by the priority rules, not insertion order.

3. Is PriorityQueue thread-safe?

No, PriorityQueue is not thread-safe. For concurrent access, use PriorityBlockingQueue from the java.util.concurrent package.

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