What is PriorityQueue or priority queue data structure in Java with Example - Tutorial

PriorityQueue is an unbounded Queue implementation in Java, which is based on priority heap. PriorityQueue allows you to keep elements in a particular order, according to there natural order or custom order defined by Comparator interface in Java. Head of priority queue data structure will always contain least element with respect to specified ordering. For example, in this post, we will create a PriorityQueue of Items, which are ordered based upon there price, this will allow us to process Items, starting from lowest price. Priority queue is also very useful in implementing Dijkstra algorithm in Java. You can use to PriorityQueue to keep unsettled nodes for processing. One of the key thing to remember about PriorityQueue in Java is that it's Iterator doesn't guarantee any order, if you want to traverse in ordered fashion, better use Arrays.sort(pq.toArray()) method. PriorityQueue is also not synchronized, which means can not be shared safely between multiple threads, instead it's concurrent counterpart PriorityBlockingQueue is thread-safe and should be used in multithreaded environment. Priority queue provides O(log(n)) time performance for common enqueing and dequeing methods e.g. offer(), poll() and add(), but constant time for retrieval methods e.g. peek() and element().

1) PriorityQueue is not synchronized, if thread-safety is requirement use BlockingPriorityQueue.

2) Queue retrieval methods like poll(), peek() and element() access head of Queue, which keeps least element according to specified ordering.

3) Iterator returned by PriorityQueue doesn't offer any ordering traversal guarantee.

4) PriorityQueue doesn't allow null elements, if you try to add null, it will throw java.lang.NullPointerException.

That's all on what is PriorityQueue in Java and How to use them. It's an special class, which can be used in special scenarios, where you need to consume Orders in a particular order, remember BlockingQueue maintains insertion order, but if want to maintain a custom order, PriorityQueue or BlockingPriorityQueue is right collection to use. TreeSet also provides similar functionality, but doesn't have one short retrieval cum removal method e.g. poll().

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