The Queue interface is part of thejava.util package. It extends theCollection interface.

• Elements are processed in the order determined by the queue implementation (First In First Out or FIFO for LinkedList, priority order for PriorityQueue).
• Elements cannot be accessed directly by index.
• A queue can store duplicate elements.

importjava.util.PriorityQueue;importjava.util.Queue;publicclassGeeks{publicstaticvoidmain(String[]args){// Create a PriorityQueue of IntegersQueue<Integer>pq=newPriorityQueue<>();// Adding elements to the PriorityQueuepq.add(50);pq.add(20);pq.add(40);pq.add(10);pq.add(30);// Display the PriorityQueue elementsSystem.out.println("PriorityQueue elements: "+pq);}}

PriorityQueue elements: [10, 20, 40, 50, 30]

Note:PriorityQueue arranges elements according to priority order (ascending by default), not insertion order.

Declaration of Java Queue Interface

public interface Queue extends Collection

We cannot instantiate a Queue directly as it is an interface. Here, we can use a PriorityQueue class that implements this interface.

Queue<Obj> queue = new LinkedList<Obj>();

Hierarchy of Queue Interface

It extends theCollection interface and has implementations likeLinkedList,ArrayDeque,PriorityQueue.

Common Implementations of Queue Interface

• LinkedList:A FIFO queue that allows null elements and implements both Queue and Deque interfaces.
• ArrayDeque: A resizable array-based queue that is faster than LinkedList and does not allow nulls.
• PriorityQueue: A queue where elements are processed according to their priority instead of insertion order.
• ConcurrentLinkedQueue: A thread-safe, non-blocking queue suitable for concurrent environments.
• BlockingQueue: A thread-safe queue that supports blocking operations for producer-consumer scenarios.

Different Operations on Queue Interface using PriorityQueue class

1. Adding Elements

To add an element in a queue, we can use theadd() method. The insertion order is not retained in the PriorityQueue. The elements are stored based on the priority order which is ascending by default. 

importjava.util.*;publicclassGeeks{publicstaticvoidmain(Stringargs[]){Queue<String>pq=newPriorityQueue<>();pq.add("Geeks");pq.add("For");pq.add("Geeks");System.out.println(pq);}}

[For, Geeks, Geeks]

2. Removing Elements

To remove an element from a queue, we can use theremove() method. If there are multiple objects, then the first occurrence of the object is removed. Thepoll() methodis also used to remove the head and return it. 

importjava.util.*;publicclassGeeks{publicstaticvoidmain(Stringargs[]){Queue<String>pq=newPriorityQueue<>();pq.add("Geeks");pq.add("For");pq.add("Geeks");System.out.println("Initial Queue: "+pq);pq.remove("Geeks");System.out.println("After Remove: "+pq);System.out.println("Poll Method: "+pq.poll());System.out.println("Final Queue: "+pq);}}

Initial Queue: [For, Geeks, Geeks]After Remove: [For, Geeks]Poll Method: ForFinal Queue: [Geeks]

3. Accessing Elements

We can access the head element without removing it usingpeek() or element().

importjava.util.PriorityQueue;importjava.util.Queue;publicclassGeeks{publicstaticvoidmain(String[]args){// Create a PriorityQueue of StringsQueue<String>pq=newPriorityQueue<>();// Adding elements to the queuepq.add("Geeks");pq.add("For");pq.add("Geeks");// Access the head element without removingSystem.out.println("Head using peek(): "+pq.peek());System.out.println("Head using element(): "+pq.element());// Display the queue to show elements are not removedSystem.out.println("Queue after accessing head: "+pq);}}

Head using peek(): ForHead using element(): ForQueue after accessing head: [For, Geeks, Geeks]

4. Iterating the Queue

There are multiple ways to iterate through the Queue. The most famous way is converting the queue to the array and traversing using thefor loop. The queue has also an inbuilt iterator which can be used to iterate through the queue. 

importjava.util.*;publicclassGeeks{publicstaticvoidmain(Stringargs[]){Queue<String>pq=newPriorityQueue<>();pq.add("Geeks");pq.add("For");pq.add("Geeks");Iteratoriterator=pq.iterator();while(iterator.hasNext()){System.out.print(iterator.next()+" ");}}}

For Geeks Geeks

Methods of Queue Interface

Queue in Java

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