Introduction

• Queue is the child interface of the Collection interface.
• It is present in the java.util package.
• It was introduced in JDK 1.5 version.

• Hierarchy of Queue Interface:
  • 

• Queue is used to hold elements prior to processing. It follows the FIFO (First-In-First-Out) order, where the element inserted first is removed first.

• Syntax :-

  package java.util;
  
  public interface Queue<E> extends Collection
  {
      // Methods (abstract)
  }

Methods of Queue Interface:

• Below are the methods defined specifically in the Queue interface:

Program :

• In the below program, we are using Queue implemented class i.e. LinkedList.

• import java.util.LinkedList;
  import java.util.Queue;
  
  public class QueueDemo
  {
      public static void main(String[] args)
      {
          Queue<String> queue = new LinkedList<>();
  
          // Adding elements
          queue.add("Amit");
          queue.add("Deepak");
          queue.add("Kamal");
          queue.add("Rahul");
          queue.add("Ravi");
  
          // Displaying the queue
          System.out.println("Initial Queue: " + queue);
  
          System.out.println("-------------------------");
  
          // Accessing the head element
          System.out.println("Head element (peek): " + queue.peek());
  
          System.out.println("-------------------------");
  
          // Removing elements
          System.out.println("Removed element (poll): " + queue.poll());
          System.out.println("Queue after poll: " + queue);
  
          System.out.println("-------------------------");
  
          // Adding another element using offer()
          queue.offer("Mohit");
          System.out.println("Queue after offer: " + queue);
  
          System.out.println("-------------------------");
  
          // Iterating through the queue
          System.out.println("Iterating the Queue elements:");
          for (String name : queue)
          {
              System.out.println(name);
          }
      }
  }

  Output:

  Initial Queue: [Amit, Deepak, Kamal, Rahul, Ravi]
  -------------------------
  Head element (peek): Amit
  -------------------------
  Removed element (poll): Amit
  Queue after poll: [Deepak, Kamal, Rahul, Ravi]
  -------------------------
  Queue after offer: [Deepak, Kamal, Rahul, Ravi, Mohit]
  -------------------------
  Iterating the Queue elements:
  Deepak
  Kamal
  Rahul
  Ravi
  Mohit

Properties of Queue Interface:

1. Queue is an ordered data structure that stores elements in a sequence for processing.
2. It generally follows the FIFO (First-In-First-Out) principle — the element inserted first is removed first.
3. Queue does not provide random access like List; elements are accessed only from the head or tail.
4. Queue can store heterogeneous elements if used without generics, but with generics it stores only homogeneous elements of the specified type.
5. Queue allows duplicate elements.
6. Queue allows null values only in certain implementations (like LinkedList), but some implementations (like PriorityQueue) do not allow null elements.
7. Queue does not maintain insertion order strictly in all cases — for example, PriorityQueue orders elements according to their priority instead of insertion sequence.