Queue

Queue Interface in Java

Overview

The Queue interface in Java represents a collection designed for holding elements prior to processing. It is part of the java.util package and extends the Collection interface. Elements are typically ordered in a FIFO (First-In-First-Out) manner.

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Characteristics of Queue Interface

Feature	Details
Package	java.util
Allows Duplicates	Yes
Ordering	FIFO (First-In-First-Out)
Subinterfaces	Deque, BlockingQueue, TransferQueue
Implementations	LinkedList, PriorityQueue, ArrayDeque

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Key Features of Queue Interface

• FIFO Ordering: Elements are processed in the order they are added, unless overridden by implementations like PriorityQueue.
• Dynamic Size: The size of a Queue can grow or shrink dynamically.
• Specialized Methods: Provides additional methods like offer, poll, and peek for queue operations.

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Initializing Different Types Using the Queue Interface

You can initialize a Queue using its implementations like LinkedList, PriorityQueue, and ArrayDeque. Here's how:

Queue<Type> priorityQueue = new PriorityQueue<>();
Queue<Type> deque = new ArrayDeque<>();
Queue<Type> queue = new LinkedList<>();

• Using PriorityQueue _{PriorityQueueDataStructure.java}

  Queue<String> priorityQueue = new PriorityQueue<>();
  Queue<Integer> priorityIntegerQueue = new PriorityQueue<>();

• Using ArrayDeque _{ArrayDequeDataStructure.java}

  Queue<String> arrayDeque = new ArrayDeque<>();
  Queue<Integer> arrayIntegerDeque = new ArrayDeque<>();

• Using LinkedList _{LinkedListDataStructure.java}

  Queue<String> linkedQueue = new LinkedList<>();
  Queue<Integer> linkedIntegerQueue = new LinkedList<>();

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Use Cases for Queue Implementations

Implementation	Use Case
LinkedList	General-purpose queue. Example: Implementing a FIFO scheduler.
PriorityQueue	Maintaining a priority-based processing order. Example: Task scheduling.
ArrayDeque	Faster than LinkedList for adding/removing elements at both ends.

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Commonly Used Methods

Queue<Object> queue = new QueueType<>();

Method	Description	Return Type	Example
add(E e)	Inserts the specified element into the queue, throwing an exception if the queue is full.	boolean	queue.add("Hello");
offer(E e)	Inserts the specified element into the queue, returning false if the queue is full.	boolean	queue.offer("Hello");
remove()	Retrieves and removes the head of the queue, throwing an exception if the queue is empty.	E	queue.remove();
poll()	Retrieves and removes the head of the queue, or returns null if the queue is empty.	E	queue.poll();
element()	Retrieves, but does not remove, the head of the queue, throwing an exception if it is empty.	E	queue.element();
peek()	Retrieves, but does not remove, the head of the queue, or returns null if it is empty.	E	queue.peek();
isEmpty()	Checks if the queue is empty.	boolean	queue.isEmpty();
size()	Returns the number of elements in the queue.	int	queue.size();
contains(Object o)	Checks if the queue contains the specified element.	boolean	queue.contains("Hello");
clear()	Removes all elements from the queue.	void	queue.clear();
iterator()	Returns an iterator over the elements in the queue.	Iterator	Iterator<String> iterator = queue.iterator();
toArray()	Converts the queue to an array.	Object[]	Object[] arr = queue.toArray();

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Example Code

import java.util.LinkedList;
import java.util.Queue;

public class QueueInterface {
    public static void main(String[] args) {
        // Create a queue
        Queue<String> queue = new LinkedList<>();

        // Add elements
        queue.offer("Apple");
        queue.offer("Banana");
        queue.offer("Cherry");

        // Check the head of the queue
        System.out.println("Head of queue: " + queue.peek());

        // Remove an element
        System.out.println("Removed: " + queue.poll());
        System.out.println("Queue after removal: " + queue);

        // Check size
        System.out.println("Size of queue: " + queue.size());

        // Iterate over elements
        System.out.println("Queue elements:");
        for (String item : queue) {
            System.out.println(item);
        }

        // Clear the queue
        queue.clear();
        System.out.println("Is queue empty? " + queue.isEmpty());
    }
}

For more: QueueInterface.java

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More to Queue Implementations :

• PriorityQueue _{PriorityQueueDataStructure.java}

• ArrayDeque _{ArrayDequeDataStructure.java}

• Using LinkedList _{LinkedListDataStructure.java}