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Overview
In computer science, Queue is a first in first out (FIFO) data structure. That means elements are added at the end of a queue and removed from the beginning.
The Queue interface in Java provides methods to add and remove elements. Details as below:
| Method | Function |
offer(element) | Insert element to the queue |
poll() | Get and remove an element at the head of the queue, returns null if the queue is empty |
remove() | Same as poll but throws an exception if the queue is empty |
peek() | Get but not remove an element from the beginning of the queue. Returns null if the queue list is empty |
element() | Same as peek() but will throw an exception if the queue is empty |
In short, the Queue interface has only one method to add elements but 2 sets of methods to get elements out of it.
Here is the diagram of the Queue interface in Java:
Queue in Action
Let’s consider the following code to understand Java’s Queue’s operation
package com.datmt.java_core.collection.queue;import java.util.LinkedList;import java.util.NoSuchElementException;import java.util.Queue;public class QueueOperation { public static void main(String[] args) { Queue<String> people = new LinkedList<>(); Queue<String> emptyQueue = new LinkedList<>(); //1. Adding elements to queue people.offer("Anna"); people.add("Bob"); //2. Get element from queue, not removing System.out.println(people.peek()); System.out.println(people.element()); System.out.printf("Now the queue contains %s people", people.size()); System.out.println(); //3. Get element and also remove System.out.println(people.remove()); System.out.println(people.poll()); System.out.printf("Now the queue contains %s people", people.size()); //4. Get element from an empty queue System.out.println("the following method are safe on empty queue"); System.out.println(emptyQueue.poll()); System.out.println(emptyQueue.peek()); System.out.println("the following method calls would throw exceptions"); try { System.out.println(emptyQueue.remove()); } catch (NoSuchElementException ex) { System.out.println("element does not exist"); } try { System.out.println(emptyQueue.element()); } catch (NoSuchElementException ex) { System.out.println("element does not exist"); } }}This is the output as you may expect:
Conclusion
As you can see, the Queue interface offers some convenient methods to deal with queues. However, there are a lot of features offered by Java regarding working with queues. We will discover such features in the next posts.
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