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: []

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.

What is a Stack?

In Java, Stacks are a LIFO (Last In, First Out) Data structure for objects.

Stack API

Java contains a Stack API with the following methods

Stack()            //Creates an empty Stack
isEmpty()          //Is the Stack Empty?             Return Type: Boolean
push(Item item)    //push an item onto the stack
pop()              //removes item from top of stack  Return Type: Item
size()             //returns # of items in stack     Return Type: Int

Example

import java.util.*;

public class StackExample {

   public static void main(String args[]) {
      Stack st = new Stack();
      System.out.println("stack: " + st);
      
      st.push(10);
      System.out.println("10 was pushed to the stack");
      System.out.println("stack: " + st);
      
      st.push(15);
      System.out.println("15 was pushed to the stack");
      System.out.println("stack: " + st);
      
      st.push(80);
      System.out.println("80 was pushed to the stack");
      System.out.println("stack: " + st);
      
      st.pop();
      System.out.println("80 was popped from the stack");
      System.out.println("stack: " + st);
      
      st.pop();
      System.out.println("15 was popped from the stack");
      System.out.println("stack: " + st);
      
      st.pop();
      System.out.println("10 was popped from the stack");
      System.out.println("stack: " + st);
      
     if(st.isEmpty())
      {
         System.out.println("empty stack");
      }
   }
}

This Returns:

stack: []
10 was pushed to the stack
stack: [10]
15 was pushed to the stack
stack: [10, 15]
80 was pushed to the stack
stack: [10, 15, 80]
80 was popped from the stack
stack: [10, 15]
15 was popped from the stack
stack: [10]
10 was popped from the stack
stack: []
empty stack

A BlockingQueue is an interface, which is a queue that blocks when you try to dequeue from it and the queue is empty, or if you try to enqueue items to it and the queue is already full. A thread trying to dequeue from an empty queue is blocked until some other thread inserts an item into the queue. A thread trying to enqueue an item in a full queue is blocked until some other thread makes space in the queue, either by dequeuing one or more items or clearing the queue completely.

BlockingQueue methods come in four forms, with different ways of handling operations that cannot be satisfied immediately, but may be satisfied at some point in the future: one throws an exception, the second returns a special value (either null or false, depending on the operation), the third blocks the current thread indefinitely until the operation can succeed, and the fourth blocks for only a given maximum time limit before giving up.

Operation	Throws Exception	Special Value	Blocks	Times out
Insert	add()	offer(e)	put(e)	offer(e, time, unit)
Remove	remove()	poll()	take()	poll(time, unit)
Examine	element()	peek()	N/A	N/A

---

A BlockingQueue can be bounded or unbounded. A bounded BlockingQueue is one which is initialized with initial capacity.

BlockingQueue<String> bQueue = new ArrayBlockingQueue<String>(2);

Any calls to a put() method will be blocked if the size of the queue is equal to the initial capacity defined.

An unbounded Queue is one which is initialized without capacity, actually by default it initialized with Integer.MAX_VALUE.

---

Some common implementations of BlockingQueue are:

1. ArrayBlockingQueue
2. LinkedBlockingQueue
3. PriorityBlockingQueue

---

Now let's look at an example of ArrayBlockingQueue:

BlockingQueue<String> bQueue = new ArrayBlockingQueue<>(2);
bQueue.put("This is entry 1");
System.out.println("Entry one done");
bQueue.put("This is entry 2");
System.out.println("Entry two done");
bQueue.put("This is entry 3");
System.out.println("Entry three done");

This will print:

Entry one done
Entry two done

And the thread will be blocked after the second output.

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.

Type of operation	Throws exception	Returns special value
Insert	add(e)	offer(e)
Remove	remove()	poll()
Examine	element()	peek()

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:

Method	Brief description
getFirst()	Gets the first item of the head of the queue without removing it.
getLast()	Gets the first item of the tail of the queue without removing it.
addFirst(E e)	Adds an item to the head of the queue
addLast(E e)	Adds an item to the tail of the queue
removeFirst()	Removes the first item at the head of the queue
removeLast()	Removes the first item at the tail of the queue

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();