Java Queue Tutorial with Examples

Queue
Queue methods
Examples

1. Queue

Queue is a subinterface of Collection, so it has all the features of Collection. It has methods for you to access or remove the first element, and methods to insert an element into the queue. If you want to access a certain element, you must remove all elements that precede it from the queue.

Queue is a Collection that allows duplicate elements but not null elements.

A common example is a queue at a bar. At a time only the first person on the queue is served, the newcomer will be inserted somewhere in the queue. It may not be the last position, depending on the type of queue and the priority of newcomer.

public interface Queue<E> extends Collection<E>

Hierarchy of interfaces in Java Collection Framework:

Hierarchy of classes in Queue group:

Relationship between interfaces and classes in Queue group:

LinkedList
ArrayDeque
PriorityQueue
LinkedBlockingDeque
ConcurrentLinkedQueue
LinkedTransferQueue
ArrayBlockingQueue
ConcurrentLinkedDeque
DelayQueue
LinkedBlockingQueue
PriorityBlockingQueue
SynchronousQueue

LinkedList is a special class, which belongs to both List group and Queue group:

Java LinkedList

2. Queue methods

In addition to the methods inherited from Collection, Queue also has its own methods:

boolean add(E e);
boolean offer(E e);

E remove();
E poll();

E element();
E peek();

	Throws exception	Returns special value
Insert	boolean add(E)	boolean offer(E)
Remove	E remove()	E poll()
Examine	E element()	E peek()

boolean add(E)* / boolean offer(E)

boolean add(E)	Inserts an element into Queue. If there is no more space to insert, this method will throw an exception. The method returns true if the insertion is successful.
boolean offer(E)	Inserts an element into Queue. If Queue has no more spaces or fails, the method returns false.

When an element is inserted into Queue, its position is determined by Queue type and the priority of the element. You cannot specify its position.

Depending on the type of Queue you are using, it can limit the number of elements, or increase the size automatically.

E remove()* / E poll()

E remove()	Returns the first element of Queue and removes it from *Queue. This method throws an exception if Queue* has no elements.
E poll()	Returns the first element of Queue and removes it from *Queue. This method returns null* if *Queue* has no elements.

E element() * / E peek()

E element()	Returns the first element of Queue but does not remove it from *Queue. This method throws an exception if Queue* has no elements.
E peek()	Returns the first element of *Queue* but does not remove it from *Queue. This method returns null* if *Queue* has no elements.

3. Examples

LinkedList(Queue)

LinkedList represents a traditional queue. add/offer methods add an element to the tail of this queue.

LinkedListEx1.java

package org.o7planning.queue.ex;

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

public class LinkedListEx1 {

    public static void main(String[] args) {
       // Create Queue     
       Queue<String> queue = new LinkedList<String>();
       
       queue.offer("One");
       queue.offer("Two");
       queue.offer("Three");
       queue.offer("Four");
       queue.offer("Five");
       
       String current;
       
       while((current = queue.poll())!= null) {
           System.out.println(current);
       }
    }
}

Output:

One
Two
Three
Four
Five

PriorityQueue

PriorityQueue is a Queue that can automatically sort the elements by their priority order or by a given Comparator. This means that an element inserted into PriorityQueue may not be in the last position.

For example, a PriorityQueue<String> sorts the elements alphabetically.

PriorityQueueEx1.java

package org.o7planning.queue.ex;

import java.util.PriorityQueue;
import java.util.Queue;

public class PriorityQueueEx1 {

    public static void main(String[] args) {
       // Create Queue     
       Queue<String> queue = new PriorityQueue<String>();
       
       queue.offer("One");
       queue.offer("Two");
       queue.offer("Three");
       queue.offer("Four");
       queue.offer("Five");
       
       String current;
       
       while((current = queue.poll())!= null) {
           System.out.println(current);
       }
    }
}