PriorityBlockingQueue Class in Java

The PriorityBlockingQueue class is part of the java.util.concurrent package and implements a thread-safe, priority-based blocking queue. It is similar to the PriorityQueue, but it supports operations for blocking threads, such as take() and put() which are not available in PriorityQueue.

• Thread-safe and supports concurrent access
• Elements are ordered by priority, not insertion order
• Can block threads when the queue is empty or full
• Allows custom prioritization via a comparator or natural ordering

Example: This example demonstrates adding elements to a PriorityBlockingQueue and printing the queue, which displays the elements in their internal order, not necessarily the order they were added.

PriorityBlockingQueue: [1, 5, 20, 10]

PriorityBlockingQueue Hierarchy

The below diagram demonstrates the class and interface hierarchy of PriorityBlockingQueue, showing its relationships with Queue, BlockingQueue, AbstractQueue and AbstractCollection.

Declaration of PriorityBlockingQueue

In Java, the declaration of PriorityBlockingQueue can be done as:

PriorityBlockingQueue<Type> queue = new PriorityBlockingQueue<>();

Note: Here, "Type" represents the type of elements the queue will hold(e.g. Integer, String, or any custom object)

Constructors

Example 1: This example demonstrates the creation of a PriorityBlockingQueue and adds integers to it, printing the queue's contents.

PriorityBlockingQueue:[1, 2, 3, 4, 5]

Example 2: This example demonstrates the creation of a PriorityBlockingQueue using a Collection vector to initialize the queue with elements.

PriorityBlockingQueue:[1, 2, 3, 4, 5]

Example 3: This example demonstrates the creation of a PriorityBlockingQueue with a specified initial capacity and adds elements to it.

PriorityBlockingQueue:[1, 2, 3]

Example 4: This example demonstrates the creation of a PriorityBlockingQueue with a specified initial capacity and a custom comparator(Comparator.reverseOrder()) to order elements in descending order.

PriorityBlockingQueue:[3, 1, 2]

Performing Various Operations on PriorityBlockingQueue

1. Adding Elements: We can use the add() method to insert elements to the PriorityBlockingQueue.

Example: This example demonstrates adding elements to a PriorityBlockingQueue with a specified initial capacity.

PriorityBlockingQueue:[1, 2, 3]

2. Removing Elements: We can use the remove() method to remove the element from the queue. This method removes a single instance of the element passed as the parameter if it is present. 

Example: This example demonstrates adding, removing a specific element, and clearing all elements from a PriorityBlockingQueue.

PriorityBlockingQueue:[1, 2, 3]
PriorityBlockingQueue:[1, 3]
PriorityBlockingQueue:[]

3. Accessing Elements: We can use peek() method to access the elements. It returns the element at the head of the PriorityBlockingQueue. If the PriorityBlockingQueue does not contain any element, then this method returns null.

Example: This example demonstrates adding elements to a PriorityBlockingQueue and accessing the head (highest priority element) using the peek() method.

PriorityQueue: [10, 20]
Head of Queue: 10

4. Iterating Elements: We can use the iterator() method to iterate over the element of PriorityBlockingQueue. The elements returned from this method do not follow any order.

Example: This example demonstrates iterating over the elements of a PriorityBlockingQueue using the iterator() method.

Elements in PriorityBlockingQueue are:
C++ Java Python Js 

5. Comparator Example: The comparator() method of PriorityBlockingQueue returns the comparator that can be used to order the elements in a PriorityBlockingQueue. The method returns a null value if the queue follows the natural ordering pattern of the elements

Example: This example demonstrates using a custom Comparator to order elements in a PriorityBlockingQueue and retrieving the comparator with the comparator() method.

Comparator value: Geeks$1@1b28cdfa
PriorityBlockingQueue follows : Geeks$1@1b28cdfa

Methods

Methods Declared in Class java.util.AbstractQueue

Methods Declared in Class java.util.AbstractCollection

Methods Declared in Interface java.util.concurrent.BlockingQueue

Methods Declared in Interface java.util.Collection

Methods Declared in Interface java.util.Queue