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Queue in Java

A queue is an object that symbolizes a data structure that is intended to have elements added to the end and deleted from the beginning. There are several components in the java queue before the procedure. In Java, the queue's items are arranged in FIFO (first-in, first-out) order. It offers extra actions, including insertion, examination, and deletion. In this guide we will be deep diving into details of what are queue in java, its importance, key operations and much more. So, let’s get started.

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What are Queues in Java

In Java, a Queue interface is a part of the Java Collections Framework and defines a collection that holds elements in a specific order for processing. It follows the First-In-First-Out (FIFO) principle, which means that the first element added to the java queue will be the first one to be removed. The Queue interface extends the java.util. Collection interface and adds methods specific to queues in java.

Importance of Understanding Java Queue Operations

Below are the importance of java queue operations in detail:

  • Flexibility: The Queue is a subtype of the Collection interface, which implies that, depending on the needs of the application, it can be used with a wide range of alternative data structures and algorithms. 
  • Performance: The Queue interface is a helpful tool for managing collections of components in performance-critical applications since it offers efficient implementations for adding, removing, and examining members. 
  • Order Preservation: Using the first-in, first-out (FIFO) concept, the Queue interface offers a mechanism to store and retrieve information in a certain order. 
  • Thread-Safety: Some Queue interface implementations, like the Java.util.concurrent. The ConcurrentLinkedQueue class allow several threads to access them concurrently without encountering conflicts. 

java queue

Read about: Instance Variable in Java

Common Java Queue Operations

Below are some of the key java queue operations in detail:

Enqueue: Adding Elements to the Queue

The add() method can be used to add an element to a queue. The PriorityQueue does not keep track of the insertion order. The elements are kept in increasing order according to their priority. 

// Java to add elements
// to a Queue

import Java.util.*;

public class GFG {

  public static void main(String args[])
  {
    Queue pq = new PriorityQueue<>();

    pq.add("HeroVired");
    pq.add("For");
    pq.add("HeroVired");

    System.out.println(pq);
  }
}

[For, HeroVired, HeroVired]

Dequeue: Removing Elements from the Java Queue

The remove() method can be used to delete an element from a queue. The first instance is eliminated if there are numerous instances of the same item. In addition, the head is removed and returned using the poll() method. 

// Java to remove elements
// from a Queue

import Java.util.*;

public class GFG {

  public static void main(String args[])
  {
    Queue pq = new PriorityQueue<>();

    pq.add("HeroVired");
    pq.add("For");
    pq.add("HeroVired");

    System.out.println("Initial Queue " + pq);

    pq.remove("HeroVired");

    System.out.println("After Remove " + pq);

    System.out.println("Poll Method " + pq.poll());

    System.out.println("Final Queue " + pq);
  }
}

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

Peek: Retrieving the Element at the Front of the Java Queue

The queue of the methods is an element () or peek (), without removing the element from the queue. 

If the task is successful, Element (); Element returns the head of the queue; otherwise, if the queue is empty, it throws an exception. 

Peek (): If the task is successful, Peek returns the head of the queue; otherwise, if the queue is empty, it returns null. 

// peek the elements from Queue
public static void main(String[] arg) {
// declaring variable 
// Queue is a interface it has two methods to add elements
Queue queueOne = new LinkedList<>();
queueOne.add(6); // add method to use insert element
queueOne.add(1);
queueOne.add(8);
queueOne.add(4);
queueOne.add(7);
System.out.println("The queue is: " + queueOne);
// peek method returns the first element from the Queue
int positionpeek = queueOne.peek();
// Element method returns the first element from the Queue
int positionElement = queueOne.element();
System.out.println("using Peek method first value from Queue : "+positionpeek);
System.out.println("using Element method first value from Queue : "+positionElement);
}

Output:
The queue is: [6, 1, 8, 4, 7]
using Peek method first value from Queue: 6
using Element method first value from Queue: 6

Different Implementations of Queue in Java

We are unable to provide the direct implementation of the java Queue because it is an interface. We must utilize classes that implement Queue in order to use its features: 

LinkedList as a java Queue Implementation

Linked list, also known as an array-like data structure, is where every node is connected to the subsequent one via a memory address connection. 

// Java program to demonstrate the
// creation of queue object using the
// LinkedList class

import Java.util.*;

class GfG {

  public static void main(String args[])
  {
    // Creating empty LinkedList
    Queue ll
      = new LinkedList();

    // Adding items to the ll
    // using add()
    ll.add(10);
    ll.add(20);
    ll.add(15);

    // Printing the top element of
    // the LinkedList
    System.out.println(ll.peek());

    // Printing the top element and removing it
    // from the LinkedList container
    System.out.println(ll.poll());

    // Printing the top element again
    System.out.println(ll.peek());
  }
}

Output:
10
10
20

ArrayDeque as a java Queue Implementation

The collection framework's PriorityQueue class gives us the means to process things according to their priority. The First-In-First-Out method is known to govern queues, but occasionally it is necessary to process queue items in accordance with priority; this is when the PriorityQueue is useful. Let's look at how to use this class to generate a java queue object. 

// Java program to demonstrate the
// creation of queue object using the
// PriorityQueue class

import Java.util.*;

class GfG {

  public static void main(String args[])
  {
    // Creating empty priority queue
    Queue pQueue
      = new PriorityQueue();

    // Adding items to the pQueue
    // using add()
    pQueue.add(10);
    pQueue.add(20);
    pQueue.add(15);

    // Printing the top element of
    // the PriorityQueue
    System.out.println(pQueue.peek());

    // Printing the top element and removing it
    // from the PriorityQueue container
    System.out.println(pQueue.poll());

    // Printing the top element again
    System.out.println(pQueue.peek());
  }
}

Output:
10
10
15

java queue

Features of a Queue in Java

  • The FIFO principle is utilized to add and remove elements from a queue in java. 
  • The Java Queue supports each and every method of the Collection interface. This includes deletion, insertion, and more. 
  • The most popular java queue implementations are LinkedList, ArrayBlockingQueue, and PriorityQueue. 
  • Any null operation on the blocking queues results in the NullPointerException being thrown. 
  • Unbounded Queues are those Queues that are included in the util package. 
  • Bounded Queues are the ones coming in addition to the util.concurrent package.
  • All queues make it simpler and faster to get out and in at the back and front of the line, respectively. Here, Deques is the only exception. 
  • Deques actually allow for element removal and insertion at both ends.  

Read more about: Hierarchical Inheritance in Java and Thread in Java

Conclusion

In this guide we have learned all about queue in java. In Java, a queue is a linear data structure that allows you to manage a set of ordered elements. Adding elements from one end and removing them from the other follow the FIFO principle. You are already familiar with add, offer, poll, and remove techniques. 

Applying Queue is advised when there is no need for synchronous data transfers. The LinkedList class implements the unbounded (not size-restricted) Queue Interface. You can experiment with Priority Queue, Double-Ended Queue, and Circular Queue in addition to Queue in Java. 

Your knowledge of Java Data Forms will increase thanks to Hero Vired’s DevOps & Cloud Engineering Course, which will enable you to broaden your horizons.

FAQ's

Java Queue interface is a part of the Java Collections Framework and defines a collection that holds elements in a specific order for processing. It follows the First-In-First-Out (FIFO) principle, which means that the first element added to the java queue will be the first one to be removed.
Numerous Java classes, such as LinkedList, ArrayDeque, and PriorityQueue, implement the Queue interface. Each of these classes offers various queue interface implementations with unique performance traits and capabilities.
To initiate a queue in Java, you must use objects of first-class containers like list and deque.
The methods of a queue in Java are as follows:
add() Inserts/adds the specified element into the queue
element() Returns the head of the queue
offer() Inserts the specified element into the queue
peek() Returns the head of the queue
remove() Returns and removes the queue head
The applications of queue include the following: CPU scheduling, spooling in printers, semaphores, memory management, FCFS scheduling, and buffer for devices.

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