Queue Interface In Java

The Queue interface is part of the java.util package and extends the Collection interface. It represents a data structure where elements are processed based on a specific order.

• Elements follow FIFO (First-In-First-Out) in LinkedList and priority order in PriorityQueue
• Elements cannot be accessed directly using an index
• Allows storing duplicate elements

Declaration

public interface Queue<E> extends Collection<E>

We cannot instantiate a Queue directly as it is an interface. Instead, we use classes like PriorityQueue, LinkedList, or ArrayDeque that implement the Queue interface.

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

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

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

Hierarchy of Queue Interface

It extends the Collection interface and has implementations like LinkedList, ArrayDeque, PriorityQueue.

Common Implementations of Queue Interface

• LinkedList: Implements List and Deque interfaces, allows null elements, and can be used as a FIFO queue when used through the Queue interface.
• 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

Different operations on the Queue interface using PriorityQueue demonstrate how elements can be added, removed, and accessed while maintaining a priority-based ordering instead of simple FIFO.

1. Adding Elements

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

[For, Geeks, Geeks]

2. Removing Elements

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

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

3. Accessing Elements

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

Head using peek(): For
Head using element(): For
Queue 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 the for loop. The queue has also an inbuilt iterator which can be used to iterate through the queue. 

For Geeks Geeks 

Methods of Queue Interface