Java - Collection Framework Queues

Introduction

Simple queues are represented by an instance of the Queue interface.

A queue always has an entry point and an exit point for its elements.

The exit point is called the head and the entry point is called the tail.

A queue lets you perform three basic operations:

  • Add an element to its tail
  • Remove an element from its head
  • Peek the element at its head

The Queue interface defines two methods for each of the three operations.

One method throws an exception if the operation is not possible; the other method returns a value (false or null) to indicate the failure.

The Queue interface adds six methods to provide the functionality of a FIFO queue.

Category
Method
Description
Adding an element to the queue

boolean add(E e)

Adds an element to the queue if it is possible.
Otherwise, it throws an exception.
Adding an element to the queue


boolean offer(E e)


Adds an element to the queue without throwing an
exception if the element cannot not be added. It
returns false on failure and true on success.
Removing an element from the queue

E remove()

Retrieves and removes the head of the queue.
It throws an exception if the queue is empty.
Removing an element from the queue


E poll()


Performs the same job as the remove() method.
However, it returns null if the queue is empty
instead of throwing an exception.
Peeking at the head of the queue


E element()


Retrieves the head of the queue without removing it
from the queue. It throws an exception if the queue
is empty.
Peeking at the head of the queue


E peek()


Performs the same job as the element() method.
However, it returns null if the queue is empty
instead of throwing an exception.

LinkedList and PriorityQueue are two implementation classes for the Queue interface.

The following code uses a LinkedList as a FIFO queue.

An instance of the LinkedList class can be used as a FIFO queue or a LIFO queue.

Demo

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

public class Main {
  public static void main(String[] args) {
    Queue<String> queue = new LinkedList<>();
    queue.add("XML");

    // offer() will work the same as add()
    queue.offer("Javascript");
    queue.offer("Json");
    queue.offer("Java");

    System.out.println("Queue: " + queue);

    // Let's remove elements until the queuee is empty
    while (queue.peek() != null) {
      System.out.println("Head Element: " + queue.peek());
      queue.remove();/* w ww  . ja va  2s  . c  o m*/
      System.out.println("Removed one element from Queue");
      System.out.println("Queue: " + queue);
    }

    // Now Queue is empty. Try calling the peek(),
    // element(), poll() and remove() methods
    System.out.println("queue.isEmpty(): " + queue.isEmpty());

    System.out.println("queue.peek(): " + queue.peek());
    System.out.println("queue.poll(): " + queue.poll());

    try {
      String str = queue.element();
      System.out.println("queue.element(): " + str);
    } catch (NoSuchElementException e) {
      System.out.println("queue.element(): Queue is empty.");
    }
    try {
      String str = queue.remove();
      System.out.println("queue.remove(): " + str);
    } catch (NoSuchElementException e) {
      System.out.println("queue.remove(): Queue is empty.");
    }
  }
}

Result