Queue Interface In Java

08 Jul, 2022
The Queue interface is present in java.util package and extends the Collection interface is used to hold the elements about to be processed in FIFO(First In First Out) order. It is an ordered list of objects with its use limited to inserting elements at the end of the list and deleting elements from the start of the list, (i.e.), it follows the FIFO or the First-In-First-Out principle.
Being an interface the queue needs a concrete class for the declaration and the most common classes are the PriorityQueue and LinkedList in Java. Note that neither of these implementations is thread-safe. PriorityBlockingQueue is one alternative implementation if the thread-safe implementation is needed.
Declaration: The Queue interface is declared as:
public interface Queue extends Collection  
Creating Queue Objects: Since Queue is an interface, objects cannot be created of the type queue. We always need a class which extends this list in order to create an object. And also, after the introduction of Generics in Java 1.5, it is possible to restrict the type of object that can be stored in the Queue. This type-safe queue can be defined as:
// Obj is the type of the object to be stored in Queue 
Queue<Obj> queue = new PriorityQueue<Obj> ();  
Example: Queue
Java
// Java program to demonstrate a Queue  import java.util.LinkedList;import java.util.Queue;  public class QueueExample {      public static void main(String[] args)    {        Queue<Integer> q            = new LinkedList<>();          // Adds elements {0, 1, 2, 3, 4} to        // the queue        for (int i = 0; i < 5; i++)            q.add(i);          // Display contents of the queue.        System.out.println("Elements of queue "                           + q);          // To remove the head of queue.        int removedele = q.remove();        System.out.println("removed element-"                           + removedele);          System.out.println(q);          // To view the head of queue        int head = q.peek();        System.out.println("head of queue-"                           + head);          // Rest all methods of collection        // interface like size and contains        // can be used with this        // implementation.        int size = q.size();        System.out.println("Size of queue-"                           + size);    }}
Elements of queue [0, 1, 2, 3, 4]
removed element-0
[1, 2, 3, 4]
head of queue-1
Size of queue-4
 
Let’s see how to perform a few frequently used operations on the queue using the Priority Queue class.
1. Adding Elements: In order 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. 
Example
Java
// Java program to add elements// to a Queue  import java.util.*;  public class GFG {      public static void main(String args[])    {        Queue<String> pq = new PriorityQueue<>();          pq.add("Geeks");        pq.add("For");        pq.add("Geeks");          System.out.println(pq);    }}
[For, Geeks, Geeks]
 
2. Removing Elements: In order to remove an element from a queue, we can use the remove() method. If there are multiple such objects, then the first occurrence of the object is removed. Apart from that, poll() method is also used to remove the head and return it. 
Example
Java
// Java program to remove elements// from a Queue  import java.util.*;  public class GFG {      public static void main(String args[])    {        Queue<String> pq = new PriorityQueue<>();          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 For
Final Queue [Geeks]
 
3. 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. However, the queue also has an inbuilt iterator which can be used to iterate through the queue. 
Example
Java
// Java program to iterate elements// to a Queue  import java.util.*;  public class GFG {      public static void main(String args[])    {        Queue<String> pq = new PriorityQueue<>();          pq.add("Geeks");        pq.add("For");        pq.add("Geeks");          Iterator iterator = pq.iterator();          while (iterator.hasNext()) {            System.out.print(iterator.next() + " ");        }    }}
For Geeks Geeks
 
Characteristics of a Queue: The following are the characteristics of the queue:
The Queue is used to insert elements at the end of the queue and removes from the beginning of the queue. It follows FIFO concept.
The Java Queue supports all methods of Collection interface including insertion, deletion, etc.
LinkedList, ArrayBlockingQueue and PriorityQueue are the most frequently used implementations.
If any null operation is performed on BlockingQueues, NullPointerException is thrown.
The Queues which are available in java.util package are Unbounded Queues.
The Queues which are available in java.util.concurrent package are the Bounded Queues.
All Queues except the Deques supports insertion and removal at the tail and head of the queue respectively. The Deques support element insertion and removal at both ends. 
1. PriorityQueue: PriorityQueue class which is implemented in the collection framework provides us a way to process the objects based on the priority. It is known that a queue follows the First-In-First-Out algorithm, but sometimes the elements of the queue are needed to be processed according to the priority, that’s when the PriorityQueue comes into play. Let’s see how to create a queue object using this class.
Example
Java
// 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<Integer> pQueue            = new PriorityQueue<Integer>();          // 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());    }}
10
10
15
 
2. LinkedList: LinkedList is a class which is implemented in the collection framework which inherently implements the linked list data structure. It is a linear data structure where the elements are not stored in contiguous locations and every element is a separate object with a data part and address part. The elements are linked using pointers and addresses. Each element is known as a node. Due to the dynamicity and ease of insertions and deletions, they are preferred over the arrays or queues. Let’s see how to create a queue object using this class.
Example
Java
// 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<Integer> ll            = new LinkedList<Integer>();          // 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());    }}
10
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20
 
3. PriorityBlockingQueue: It is to be noted that both the implementations, the PriorityQueue and LinkedList are not thread-safe. PriorityBlockingQueue is one alternative implementation if thread-safe implementation is needed. PriorityBlockingQueue is an unbounded blocking queue that uses the same ordering rules as class PriorityQueue and supplies blocking retrieval operations. Since it is unbounded, adding elements may sometimes fail due to resource exhaustion resulting in OutOfMemoryError. Let’s see how to create a queue object using this class.
Example
Java
// Java program to demonstrate the// creation of queue object using the// PriorityBlockingQueue class  import java.util.concurrent.PriorityBlockingQueue;import java.util.*;  class GfG {    public static void main(String args[])    {        // Creating empty priority        // blocking queue        Queue<Integer> pbq            = new PriorityBlockingQueue<Integer>();          // Adding items to the pbq        // using add()        pbq.add(10);        pbq.add(20);        pbq.add(15);          // Printing the top element of        // the PriorityBlockingQueue        System.out.println(pbq.peek());          // Printing the top element and        // removing it from the        // PriorityBlockingQueue        System.out.println(pbq.poll());          // Printing the top element again        System.out.println(pbq.peek());    }}
10
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The queue interface inherits all the methods present in the collections interface while implementing the following methods:
Method
Description
Chinmoy Lenka
BharatGupta3
codekaust
KaashyapMSK
siddharth chandran.e
maxfuller5
adhavrushikesh6
nandinigujral
hardikkoriintern
Java-Collections
java-queue
Java
Queue
Java
Queue
Java-Collections
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