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How to use Java PriorityQueue with Examples

#java #priorityqueue #example

https://grokonez.com/java/how-to-use-java-priorityqueue-with-examples

[no_toc]In the tutorial, Grokonez introduces how to work with Java PriorityQueue by examples:

  • How does Java PriorityQueue work?
  • Explore Java PriorityQueue Hierarchy
  • Create Java PriorityQueue with Integer, String, and Custom Object Class

Related posts:

Java PriorityQueue

  • Java PriorityQueue is an unbounded priority queue.
  • The elements of the priority queue are ordered according to their natural ordering, or by a Comparator.
    • The front of the priority queue contains the least element.
    • And the rear of the priority queue contains the greatest element.

grokonez-howto-java-priority-queue-examples

  • PriorityQueue does not permit null elements.
  • PriorityQueue does not permit insertion of non-comparable objects. ClassCastException will be throwed.
  • The head of this queue is the least element. The queue operations poll, remove, peek, and element access the element at the head of the queue.
  • Java PriorityQueue is not synchronized. Multiple threads should not access the same PriorityQueue concurrently if having any of the threads modifies the queue.

    Java PriorityQueue Hierarchy

grokonez-java-priority-queue-hierarchy-examples

Java PriorityQueue with Integer Example

In the follwing example, We create an Integer Java PriorityQueue, then do a list operations on the queue:

  • Insert an element to PriorityQueue with offer and add method.
  • Retrieve a head value of the Queue, but NOT remove it by peek() method.
  • Removes a single instance of the specified element from this queue by remove(Object o) method.
  • Retrieves and removes the head of this queue by poll() method.

Example:

 import java.util.PriorityQueue; import java.util.Queue; public class JavaPriorityQueueExamples { public static void main(String[] args) { Queue intQueue = new PriorityQueue(); // Inserts the specified element into this priority queue. intQueue.offer(10); intQueue.offer(1); intQueue.offer(5); intQueue.offer(9); // E poll(): Retrieves and removes the head of this queue System.out.println(intQueue.remove()); /* * 1 */ // Retrieves, but does not remove, the head of this queue int head = intQueue.peek(); System.out.println("headValue = " + head); // headValue = 5 // Inserts the specified element into this priority queue. intQueue.add(7); intQueue.add(2); head = intQueue.element(); System.out.println("headValue = " + head); // headValue = 2 // Removes a single instance of the specified element from this queue, if it is present. boolean containOrNot = intQueue.remove(9); System.out.println(containOrNot); // true System.out.println("---"); while(!intQueue.isEmpty()) { System.out.println(intQueue.poll()); } /* * 2 * 5 * 7 * 10 */ } }

  • In above example, we use PriorityQueue() Constructor to create a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering.

  • add() and offer() method, both can insert an element to a PriorityQueue, But what is the difference?

  • The offer method inserts an element if possible, otherwise returning false.

  • This differs from the Collection.add method, which can fail to add an element only by throwing an unchecked exception.

  • The remove() and poll() methods remove and return the head of the queue.
    -> BUT when the queue is empty, the remove() method throws an exception, while the poll() method returns null.

  • The element() and peek() retrive the head's value of the queue and NOT remove. When the queue is empty:

  • element() throws NoSuchElementException.

  • peek() returns null.

Now we do an example to iterate all elements of a Java PriorityQueue:

  • using iterator(): to return an iterator over the elements in the queue.
  • using toArray(): to return an array containing all the elements in the queue.
 import java.util.Iterator; import java.util.PriorityQueue; import java.util.Queue; public class JavaPriorityQueueExamples{ public static void main(String[] args) { Queue<Integer> intQueue = new PriorityQueue<Integer>(); // Inserts the specified element into this priority queue. intQueue.offer(10); intQueue.offer(1); intQueue.offer(5); intQueue.offer(9); // Returns an iterator over the elements in this queue. Iterator<Integer> intIterator = intQueue.iterator(); while (intIterator.hasNext()) System.out.print(intIterator.next() + " "); /* * 1 9 5 10 */ System.out.println(); // Returns an array containing all of the elements in this queue. Object[] intArr = intQueue.toArray(); for(int i=0; i<intArr.length; i++) { System.out.print(intArr[i] + " "); } /* * 1 9 5 10 */ } }

Java PriorityQueue with String Example

  • Example: Create Java String PriorityQueue using Natural Ordering of String
 import java.util.PriorityQueue; import java.util.Queue; public class JavaPriorityQueueExamples { public static void main(String[] args) { Queue strQueue = new PriorityQueue(); // Inserts the specified element into this priority queue. strQueue.offer("Jack"); strQueue.offer("Charlie"); strQueue.offer("Joe"); strQueue.offer("George"); while(!strQueue.isEmpty()) { System.out.print(strQueue.poll() + " "); } /* * -> Charlie George Jack Joe */ } }

Create Java PriorityQueue with a custom Comparator

  • Create a Java PriorityQueue using a Custom Comparator that compares two Strings by their length:
 import java.util.Comparator; import java.util.PriorityQueue; import java.util.Queue; public class JavaPriorityQueueExamples { public static void main(String[] args) { Queue strQueue = new PriorityQueue((Comparator super String>) new Comparator() { public int compare(String s1, String s2) { return s1.length() - s2.length(); } }); // Inserts the specified element into this priority queue. strQueue.offer("Jack"); strQueue.offer("Charlie"); strQueue.offer("Joe"); strQueue.offer("George"); while(!strQueue.isEmpty()) { System.out.print(strQueue.poll() + " "); } } }

-> Output:

 Joe Jack George Charlie

Java PriorityQueue with Custom Object Example

  • Create an Java Developer class:
 class Developer{ Integer id; String name; Double salary; public Developer(Integer id, String name, Double salary){ this.id = id; this.name = name; this.salary = salary; } public int getId() { return this.id; } public String getName() { return this.name; } public Double getSalary() { return this.salary; } public String toString() { return String.format("[id = %d, name = %s, salary = %.2f]", id, name, salary); } }
  • Implement Comparable interface to compare its elements and order them accordingly as below:
 class Developer implements Comparable{ ... @Override public int compareTo(Developer d) { if(this.getSalary() > d.getSalary()) { return 1; } else if (this.getSalary() < d.getSalary()) { return -1; } else { return 0; } } }
  • Create a Java PriorityQueue Example with Developer class:
 import java.util.PriorityQueue; import java.util.Queue; public class JavaPriorityQueueExamples { public static void main(String[] args) { Queue devQueue = new PriorityQueue(); // Inserts the specified element into this priority queue. devQueue.offer(new Developer(1, "Jack", 6000.0)); devQueue.offer(new Developer(4, "Joe", 7000.0)); devQueue.offer(new Developer(10, "Davis", 6500.0)); devQueue.offer(new Developer(6, "Mary", 5700.0)); while(!devQueue.isEmpty()) { System.out.println(devQueue.poll()); } } }

-> Output:

 /* * [id = 6, name = Mary, salary = 5700,00] * [id = 1, name = Jack, salary = 6000,00] * [id = 10, name = Davis, salary = 6500,00] * [id = 4, name = Joe, salary = 7000,00] */

-> The Developer with smallest salary is removed first

Conclusion

We had learned how to work with Java PriorityQueue by examples:

  • Create Java PriorityQueue with Natural Ordering of Elements.
  • Create Java PriorityQueue with Custom Comparator.
  • Create Java PriorityQueue with Custom Object that implements Comparable interface.
  • Do a list operations of Java PriorityQueue: offer, peek, poll, remove...

Happy Learning! See you later!

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