How To Purpose Priorityqueue Inwards Java? An Example

PriorityQueue is roughly other information construction from Java Collection framework, added inwards Java SE 5. This degree implements Queue interface in addition to provides a sorted chemical cistron from the caput of the queue. Though it provides sorting, it's picayune dissimilar amongst other Sorted collections e.g. TreeSet or TreeMap, which too allows you lot to iterate over all elements, inwards priority queue at that spot is no guarantee on iteration. The exclusively guaranteed PriorityQueue gives is that lowest or highest priority chemical cistron volition live on on the caput of the queue. So when you lot telephone phone remove() or poll() method, you lot volition acquire this chemical cistron in addition to side yesteryear side on priority volition acquire the caput spot. Like other collection classes which render sorting, PriorityQueue also uses Comparable in addition to Comparator interface for priority.

When you lot add together or take elements from PriorityQueue, other elements are compared to each other to position highest/lowest priority chemical cistron at the caput of the queue.  priority queue information structure is internally implemented using binary heap information structure, which allows constant fourth dimension access to the maximum in addition to minimum chemical cistron inwards a heap yesteryear implementing max heap in addition to 2nd heap information structure.

In this information construction origin of the binary tree ever comprise either maximum value (max heap) or minimum value (min heap), since you lot convey constant fourth dimension access to root, you lot tin give the sack acquire max or minimum value inwards constant time. Once this chemical cistron is removed from the root, side yesteryear side maximum/minimum is promoted to root.

So, if you lot desire to procedure elements inwards their relative priority order, you lot tin give the sack role PriorityQueue inwards Java. It provides constant fourth dimension access to highest or lowest priority element.

Good noesis of Java Collection framework is absolutely necessary to acquire an skillful Java developer in addition to if you lot desire to acquire one, I advise you lot to read Java Generics in addition to Collection yesteryear Maurice Naftalin. One of the best mass which extensively encompass this topic in addition to explains virtually almost all the collection classes available inwards Java till Java SE 6.

Java PriorityQueue Example

Here is our sample plan to demonstrate how to role PriorityQueue inwards Java. As I said you lot tin give the sack role PriorityQueue to shop objects which required processing inwards a for certain social club decided yesteryear their priority e.g. processing social club amongst the highest value offset or lowest value first. In this example, you lot volition larn how to add together elements into PriorityQueue, how to take elements from PriorityQueue, how to poll in addition to how to peek elements from PriorityQueue inwards Java.

We offset create a priority queue of integer values amongst a capacity of xvi elements. Then nosotros add together numbers using the add() method of the queue. Whenever you lot add together a number, caput of the queue is updated if a novel value is less than or greater than caput value.

By default, elements are sorted inwards increasing social club in addition to caput contains the smallest elements. You tin give the sack retrieve caput of priority queue without removing it yesteryear using peek() method.

You tin give the sack too role the poll() method of PriorityQueue to acquire caput value but this volition too take the chemical cistron from the queue. It is truly goodness if you lot are consuming elements instead of merely viewing it.

If you lot remember, Queue interface provides 2 sets of methods for like undertaking e.g. add() in addition to offer() for adding elements, poll() in addition to remove() for removing a caput chemical cistron from PriorityQueue in addition to peek() in addition to element() for retrieving the caput of the queue without removing.

The departure betwixt these 2 sets of the method is that 1 throws an exception if failed spell other render exceptional value e.g. false or null if failed. This is too the argue why PriorityQueue doesn't allow adding zip elements, because in addition to then you lot cannot differentiate betwixt the normal object in addition to exceptional value.

You tin give the sack too read  Core Java Volume 1 - Fundamentals yesteryear Cay S. Horstmann to larn to a greater extent than virtually PriorityQueue in addition to how to role it correctly inwards your Java application.

In this example, you lot volition too larn how to iterate over all elements of PriorityQueue inwards Java. You tin give the sack practice that yesteryear using Iterator interface in addition to truly its same every bit how to iterate over ArrayList inwards Java. Just retrieve that PriorityQueue doesn't buy the farm along all elements inwards roughly order, it exclusively makes endeavor to buy the farm along the lowest priority of chemical cistron at the caput of the queue.

package dto;  import java.util.Iterator; import java.util.PriorityQueue;  /**  * Simple Java Program to demonstrate how to role PriorityQueue  * inwards Java. PriorityQueue is used to procedure elements every bit  * per their priority defined yesteryear Comparator or Comparable.  *  * @author Javin Paul  */ public class PriorityQueueDemo {      public static void main(String[] args) {          // creating an illustration of PriorityQueue inwards Java         // Its ameliorate to define initial capacity because         // its backed yesteryear array         PriorityQueue<Integer> pq = new PriorityQueue<Integer>(16);                         // adding numbers into PriorityQueue inwards arbitrary order         pq.add(3);         pq.add(7);         pq.add(2);         pq.add(4);         pq.add(1);         pq.add(5);                 // Now, let's if PriorityQueue keeps the smallest         // chemical cistron inwards caput or not. Let's role peek  method         // to depository fiscal establishment jibe that, peek() returns the caput of the         // queue                 Integer caput = pq.peek();         System.out.println("size of PriorityQueue : " + pq.size());         System.out.println("head of the PriorityQueue : " + head); // 1                 // Now, let's take the caput in addition to run across what comes         // next, you lot tin give the sack role poll() method to remove         // chemical cistron from head                 caput = pq.poll(); // 1         caput = pq.peek();                 System.out.println("Current value of caput : " + head);         System.out.println("size of PriorityQueue : " + pq.size());                     // Iterating over PriorityQueue, iterator returned         // yesteryear PriorityQueue doesn't guarantee whatsoever order         Iterator<Integer> itr = pq.iterator();         System.out.println("Iterating over PriorityQueue");                 while(itr.hasNext()){             System.out.println(itr.next());         }     }  }  Output size of PriorityQueue : 6 caput of the PriorityQueue : 1 Current value of caput : 2 size of PriorityQueue : 5 Iterating over PriorityQueue 2 4 3 7 5

When to role PriorityQueue inwards Java?

Use when you lot desire to procedure elements on roughly priority e.g. processing the highest priority chemical cistron offset or lowest priority chemical cistron first. You tin give the sack determine relative priority comparing yesteryear using Comparator interface in addition to overriding compare() method. You tin give the sack role PriorityQueue to procedure a undertaking of highest priority first. If you lot are implementing a dashboard form of interface, you lot tin give the sack too role this degree to bubble upwards of import issues, alerts or notification at the top. Since you lot cannot shop object, which is non comparable, its useless for them. You too demand to pay roughly cost for keeping the caput every bit per priority thence if you lot are non required that characteristic ameliorate non to role this class.

Important points virtually PriorityQueue

1) PriorityQueue was added inwards JDK 1.5 in addition to this degree is written yesteryear Josh Bloch in addition to Doug Lea.

2) PriorityQueue is based upon priority heap where elements are ordered on their priority, according to their natural social club provided yesteryear Comparable or custom social club provided yesteryear Comparator.

3) Any elements which you lot desire to shop inwards PriorityQueue must implement either Comparator or Comparable interface. Priority of elements are represented yesteryear implementing compare() or compareTo() method of respective interface.

4) You cannot shop incompatible elements inwards PriorityQueue, which agency you lot cannot shop Integer in addition to String on a PriorityQueue of Object, this volition lawsuit inwards ClassCastException

5) PriorityQueue does non allow zip elements, adding nulls volition lawsuit inwards NullPointerException

6) If multiple elements are tied for to the lowest degree value in addition to then anyone of them tin give the sack occupy the caput of the queue, ties are broken arbitrarily.

7) PriorityQueue is unbounded, which agency you lot tin give the sack add together every bit many elements every bit you lot want, but it's backed yesteryear an array in addition to has an internal capacity to principle the size of an array.

8) Iterator returned yesteryear PriorityQueue doesn't guarantee whatsoever ordered traversal. If you lot demand ordered traversal reckon using Arrays.sort(pq.toArray());

9) The priority queue degree is non synchronized, thence you lot in addition to non percentage betwixt multiple threads if 1 of the threads modifies the queue.

10) Time complexity of enqueuing in addition to dequeuing elements is inwards social club of O(log(n))

11) Time complexity is liner for remove(object) in addition to contains(object)

11) PriorityQueue provides constant fourth dimension functioning for peek(), element() in addition to size() method, which agency you lot tin give the sack retrieve a maximum or minimum chemical cistron inwards constant fourth dimension from priority queue inwards Java.


That's all virtually how to role PriorityQueue inwards Java amongst an example. You convey too learned roughly of import things virtually PriorityQueue. Just retrieve that PriorityQueue inwards Java was added on JDK 1.5 in addition to available inwards Java SE 6, vii in addition to 8 but non available inwards JDK 1.4. Also, like to TreeSet or TreeMap you lot tin give the sack exclusively shop elements which implement Comparable or Comparator interface. If you lot are using Queue interface to access PriorityQueue object in addition to then I advise you lot to role the method defined their instead of Collection interface method e.g. prefer offer() over add(),  peek() over element() in addition to poll() over remove() method spell working amongst priority queue inwards Java.

Further Learning
Java In-Depth: Become a Complete Java Engineer
Java Fundamentals: Collections
Data Structures in addition to Algorithms: Deep Dive Using Java
Algorithms in addition to Data Structures - Part 1 in addition to 2
Data Structures inwards Java ix yesteryear Heinz Kabutz