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Java Collections - Queue

Jakob Jenkov
Last update: 2014-06-23

The java.util.Queue interface is a subtype of the java.util.Collection interface. It represents an ordered list of objects just like a List, but its intended use is slightly different. A queue is designed to have elements inserted at the end of the queue, and elements removed from the beginning of the queue. Just like a queue in a supermarket.

Here is a list of the topics covered in this text:

  1. Queue Implementations
  2. Adding and Accessing Elements
  3. Removing Elements
  4. Generic Queues
  5. More Details in the JavaDoc

Queue Implementations

Being a Collection subtype all methods in the Collection interface are also available in the Queue interface.

Since Queue is an interface you need to instantiate a concrete implementation of the interface in order to use it. You can choose between the following Queue implementations in the Java Collections API:

  • java.util.LinkedList
  • java.util.PriorityQueue

LinkedList is a pretty standard queue implementation.

PriorityQueue stores its elements internally according to their natural order (if they implement Comparable), or according to a Comparator passed to the PriorityQueue.

There are also Queue implementations in the java.util.concurrent package, but I will leave the concurrency utilities out of this tutorial.

Here are a few examples of how to create a Queue instance:

Queue queueA = new LinkedList();
Queue queueB = new PriorityQueue();

Adding and Accessing Elements

To add elements to a Queue you call its add() method. This method is inherited from the Collection interface. Here are a few examples:

Queue queueA = new LinkedList();

queueA.add("element 1");
queueA.add("element 2");
queueA.add("element 3");

The order in which the elements added to the Queue are stored internally, depends on the implementation. The same is true for the order in which elements are retrieved from the queue. You should consult the JavaDoc's for more information about the specific Queue implementations.

You can peek at the element at the head of the queue without taking the element out of the queue. This is done via the element() method. Here is how that looks:

Object firstElement = queueA.element();

To take the first element out of the queue, you use the remove() method which is described later.

You can also iterate all elements of a queue, instead of just processing one at a time. Here is how that looks:

Queue queueA = new LinkedList();

queueA.add("element 0");
queueA.add("element 1");
queueA.add("element 2");

//access via Iterator
Iterator iterator = queueA.iterator();
while(iterator.hasNext(){
  String element = (String) iterator.next();
}

//access via new for-loop
for(Object object : queueA) {
    String element = (String) object;
}

When iterating the queue via its Iterator or via the for-loop (which also uses the Iterator behind the scene, the sequence in which the elements are iterated depends on the queue implementation.


Removing Elements

To remove elements from a queue, you call the remove() method. This method removes the element at the head of the queue. In most Queue implementations the head and tail of the queue are at opposite ends. It is possible, however, to implement the Queue interface so that the head and tail of the queue is in the same end. In that case you would have a stack.

Here is a remove example();

Object firstElement = queueA.remove();

Generic Queue

By default you can put any Object into a Queue, but from Java 5, Java Generics makes it possible to limit the types of object you can insert into a Queue. Here is an example:

Queue<MyObject> queue = new LinkedList<MyObject>();

This Queue can now only have MyObject instances inserted into it. You can then access and iterate its elements without casting them. Here is how it looks:

MyObject myObject = queue.remove();

for(MyObject anObject : queue){
   //do someting to anObject...   
}

For more information about Java Generics, see the Java Generics Tutorial.


More Details in the JavaDoc

There is a lot more you can do with a Queue, but you will have to check out the JavaDoc for more details. This text focused on the two most common operations: Adding / removing elements, and iterating the elements.

Jakob Jenkov




Copyright  Jenkov Aps
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