The usage of the PriorityQueue

PriorityQueue is a data structure. Like SortedSet, PriorityQueue sorts also its elements based on their priorities. The elements, which have a higher priority, comes first. The type of the PriorityQueue should implement comparable or comparator interface, whose methods decides the priorities of the elements of the data structure.

//The type of the PriorityQueue is Integer.
PriorityQueue<Integer> queue = new PriorityQueue<Integer>();

//The elements are added to the PriorityQueue
queue.addAll( Arrays.asList( 9, 2, 3, 1, 3, 8 ) );

//The PriorityQueue sorts the elements by using compareTo method of the Integer Class
//The head of this queue is the least element with respect to the specified ordering
System.out.println( queue );  //The Output: [1, 2, 3, 9, 3, 8]
queue.remove();
System.out.println( queue );  //The Output: [2, 3, 3, 9, 8]
queue.remove();
System.out.println( queue );  //The Output: [3, 8, 3, 9]
queue.remove();
System.out.println( queue );  //The Output: [3, 8, 9]
queue.remove();
System.out.println( queue );  //The Output: [8, 9]
queue.remove();
System.out.println( queue );  //The Output: [9]
queue.remove();
System.out.println( queue );  //The Output: []

LinkedList as a FIFO Queue

The java.util.LinkedList class, while implementing java.util.List is a general-purpose implementation of java.util.Queue interface too operating on a FIFO (First In, First Out) principle.

In the example below, with offer() method, the elements are inserted into the LinkedList. This insertion operation is called enqueue. In the while loop below, the elements are removed from the Queue based on FIFO. This operation is called dequeue.

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

queue.offer( "first element" );
queue.offer( "second element" );
queue.offer( "third element" );
queue.offer( "fourth. element" );
queue.offer( "fifth. element" );

while ( !queue.isEmpty() ) {
  System.out.println( queue.poll() );
}

The output of this code is

first element
second element
third element
fourth element
fifth element

As seen in the output, the first inserted element “first element” is removed firstly, “second element” is removed in the second place etc.

Stacks

BlockingQueue

Queue Interface

Basics

A Queue is a collection for holding elements prior to processing. Queues typically, but not necessarily, order elements in a FIFO (first-in-first-out) manner.

Head of the queue is the element that would be removed by a call to remove or poll. In a FIFO queue, all new elements are inserted at the tail of the queue.

The Queue Interface

public interface Queue<E> extends Collection<E> {
    boolean add(E e);

    boolean offer(E e);

    E remove();

    E poll();

    E element();

    E peek();
}

Each Queue method exists in two forms:

• one throws an exception if the operation fails;
• other returns a special value if the operation fails (either null or false depending on the operation.

Deque

A Deque is a “double ended queue” which means that a elements can be added at the front or the tail of the queue. The queue only can add elements to the tail of a queue.

The Deque inherits the Queue interface which means the regular methods remain, however the Deque interface offers additional methods to be more flexible with a queue. The additional methods really speak for them self if you know how a queue works, since those methods are intended to add more flexibility:

Of course the same options for offer, poll and peek are available, however they do not work with exceptions but rather with special values. There is no point in showing what they do here.

Adding and Accessing Elements

To add elements to the tail of a Deque you call its add() method. You can also use the addFirst() and addLast() methods, which add elements to the head and tail of the deque.

Deque<String> dequeA = new LinkedList<>();

dequeA.add("element 1");      //add element at tail
dequeA.addFirst("element 2"); //add element at head
dequeA.addLast("element 3");  //add element at tail

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. You can also use the getFirst() and getLast() methods, which return the first and last element in the Deque. Here is how that looks:

String firstElement0 = dequeA.element();
String firstElement1 = dequeA.getFirst();
String lastElement = dequeA.getLast();

Removing Elements

To remove elements from a deque, you call the remove(), removeFirst() and removeLast() methods. Here are a few examples:

String firstElement = dequeA.remove();
String firstElement = dequeA.removeFirst();
String lastElement  = dequeA.removeLast();