Queue Implementation in Java

A queue is a particular kind of abstract data type or collection in which the entities in the collection are kept in order and the principal (or only) operations on the collection are the addition of entities to the rear terminal position, known as enqueue, and removal of entities from the front terminal position, known as dequeue. This makes the queue a First-In-First-Out (FIFO) data structure. In a FIFO data structure, the first element added to the queue will be the first one to be removed. This is equivalent to the requirement that once a new element is added, all elements that were added before have to be removed before the new element can be removed.

The simplest example of a queue is the typical line that we all participate in from time to time. We wait in a line for a movie, we wait in the check-out line at a grocery store, and we wait in the cafeteria line (so that we can pop the tray stack). Well-behaved lines, or queues, are very restrictive in that they have only one way in and only one way out. There is no jumping in the middle and no leaving before you have waited the necessary amount of time to get to the front.

Basic Queue Operations

Following code represents the implementation of a generic Queue.

 public class Queue<T> {  
      private class Node {  
           private T data;  
           private Node next;  
      }  
      private Node first;  
      private Node last;  
      private int size;  
      public Queue() {  
           this.first = null;  
           this.last = null;  
           this.size = 0;  
      }  
 }  

Enqueue

      public Queue<T> enqueue(T data) {  
           if (data == null) {  
                return this;  
           }  
           Node current = last;  
           last = new Node();  
           last.data = data;  
           if (size == 0) {  
                first = last;  
           } else {  
                current.next = last;  
           }  
           size++;  
           return this;  
      }  

Dequeue

      public T dequeue() {  
           if (size == 0) {  
                return null;  
           }  
           T value = first.data;  
           first = first.next;  
           if (--size == 0) {  
                last = null;  
           }  
           return value;  
      }  
      public boolean isEmpty() {  
           return this.size == 0;  
      }  

Source Code

 public class Queue<T> {  
      private class Node {  
           private T data;  
           private Node next;  
      }  
      private Node first;  
      private Node last;  
      private int size;  
      public Queue() {  
           this.first = null;  
           this.last = null;  
           this.size = 0;  
      }  
      public Queue<T> enqueue(T data) {  
           if (data == null) {  
                return this;  
           }  
           Node current = last;  
           last = new Node();  
           last.data = data;  
           if (size == 0) {  
                first = last;  
           } else {  
                current.next = last;  
           }  
           size++;  
           return this;  
      }  
      public T dequeue() {  
           if (size == 0) {  
                return null;  
           }  
           T value = first.data;  
           first = first.next;  
           if (--size == 0) {  
                last = null;  
           }  
           return value;  
      }  
      public boolean isEmpty() {  
           return this.size == 0;  
      }  
      @Override  
      public String toString() {  
           StringBuilder sb = new StringBuilder();  
           Node tmp = first;  
           while (tmp != null) {  
                sb.append(tmp.data).append(", ");  
                tmp = tmp.next;  
           }  
           return sb.toString();  
      }  
 }  

JUnit Test

 import static org.junit.Assert.assertFalse;  
 import static org.junit.Assert.assertTrue;  
 import org.junit.Test;  
 public class QueueAPITest {  
      @Test  
      public void testEnqueue() {  
           Queue<String> queueAPI = new Queue<String>();  
           queueAPI.enqueue("n1");  
           queueAPI.enqueue("n2");  
           queueAPI.enqueue("n3");  
           queueAPI.enqueue("n4");  
           queueAPI.enqueue("n5");  
           System.out.println(queueAPI.toString());  
      }  
      @Test  
      public void testDequeue() {  
           Queue<String> queueAPI = new Queue<String>();  
           queueAPI.enqueue("n1");  
           queueAPI.enqueue("n2");  
           queueAPI.enqueue("n3");  
           queueAPI.enqueue("n4");  
           queueAPI.enqueue("n5");  
           assertTrue("n1".equals(queueAPI.dequeue()));  
           assertTrue("n2".equals(queueAPI.dequeue()));  
           assertFalse("n6".equals(queueAPI.dequeue()));  
      }  
 }  

References

Queue on Wikipedia
Queue on Interactive Python