PriorityBlockingQueue in Java

In our last tutorial we have see about PriorityQueue and how its used in Java with simple example. In this tutorial we will see about PriorityBlockingQueue and it works and also the difference between PriorityQueue and PriorityBlockingQueue. 

• PriorityBlockingQueue uses same unbounded blocking queue that uses the same ordering rules as PriorityQueue and supplies blocking retrieval operations. 
• PriorityBlockingQueue does not allow null values to be added.
• PriorityBlockingQueue also uses natural sorting order and will not permit non-comparable objects to be insert which will result in ClassCastException exception.
• PriorityBlockingQueue uses methods as same as PriorityQueue class except drainTo() and remainingCapacity() methods. 
• drainTo(Collection<? super E> c) method used remove all available elements from the queue and adds them to the given collection.
• drainTo(Collection<? super E> c, int maxElements) method used to remove at most the given number of available elements from the queue and adds them to the given collection.
• remainingCapacity() method used to return Integer.MAX_VALUE because a PriorityBlockingQueue is not capacity constrained.
• Method iterator() is not guaranteed to traverse the elements of PriorityBlockingQueue in any particular order. We can see this in below example. 

Difference Between PriorityBlockingQueue and PriorityQueue:

• PriorityQueue is not thread safe, where as PriorityBlockingQueue is thread safe. 
• PriorityBlockingQueue introduced in JDK 5 which added with concurrent package. 

Now lets see simple example for PriorityBlockingQueue and how its working under multi-threading.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.PriorityBlockingQueue;

public class PriorityBlockingQueueTest {
 
 PriorityBlockingQueue<String> priBlockQ = new PriorityBlockingQueue<String>();
 
 public PriorityBlockingQueueTest(){
  priBlockQ.add("300");
  priBlockQ.add("400");
  priBlockQ.add("100");
  priBlockQ.add("500");
  priBlockQ.add("200");  
 }
 
 public void performTask() {
  // traversing order not guaranteed
  System.out.println("\nQueue iterator() : ");
  Iterator<String> itr = priBlockQ.iterator();
  while (itr.hasNext()) {
   String string = (String) itr.next();
   System.out.print(string+", ");
  }
  
  System.out.println("\n\nList iterator() : ");
  List<String> list = new ArrayList<String>();
  priBlockQ.drainTo(list);
  itr = list.iterator();
  while (itr.hasNext()) {
   String string = (String) itr.next();
   System.out.print(string+", ");
  }
  
  System.out.println("\n\ncontains() : "+priBlockQ.contains("400"));
  System.out.println("\nremainingCapacity() : "+priBlockQ.remainingCapacity());
 }
 
 public static void main(String[] args) {
  new PriorityBlockingQueueTest().performTask();
 }
}

OUTPUT:

Queue iterator() : 
100, 200, 300, 500, 400, 

List iterator() : 
100, 200, 300, 400, 500, 

contains() : false

remainingCapacity() : 2147483647