PriorityBlockingQueue Class in Java (original) (raw)

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.

Java `

// Adding elements to priorityBlockingQueue import java.util.concurrent.PriorityBlockingQueue;

public class Geeks { public static void main(String[] args) {

    // Create a PriorityBlockingQueue
    PriorityBlockingQueue<Integer> q 
    = new PriorityBlockingQueue<>();

    // Add elements to the q
    q.add(10);
    q.add(5);
    q.add(20);
    q.add(1);

    System.out.println("PriorityBlockingQueue: " + q);
}

}

`

Output

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 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.

Java `

// Java program to demonstrate // PriorityBlockingQueue() constructor import java.util.concurrent.PriorityBlockingQueue;

public class Geeks {

public static void main(String[] args)
{
    // create object of PriorityBlockingQueue
    // using PriorityBlockingQueue() constructor
    PriorityBlockingQueue<Integer> pbq
            = new PriorityBlockingQueue<Integer>();

    // add numbers
    pbq.add(1);
    pbq.add(2);
    pbq.add(3);
    pbq.add(4);
    pbq.add(5);

    System.out.println("PriorityBlockingQueue:" + pbq);
}

}

`

Output

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.

Java `

// Java program to demonstrate // PriorityBlockingQueue(Collection c) constructor import java.util.concurrent.PriorityBlockingQueue; import java.util.*;

public class Geeks {

public static void main(String[] args)
{
    // Creating a Collection
    Vector<Integer> v = new Vector<Integer>();
    v.addElement(1);
    v.addElement(2);
    v.addElement(3);
    v.addElement(4);
    v.addElement(5);

    // create object of PriorityBlockingQueue
    // using PriorityBlockingQueue(Collection c)
    // constructor
    PriorityBlockingQueue<Integer> pbq
            = new PriorityBlockingQueue<Integer>(v);

    System.out.println("PriorityBlockingQueue:" + pbq);
}

}

`

Output

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.

Java `

// Java program to demonstrate // PriorityBlockingQueue(int initialCapacity) // constructor import java.util.concurrent.PriorityBlockingQueue;

public class Geeks {

public static void main(String[] args)
{
    // define capacity of PriorityBlockingQueue
    int c = 15;

    // create object of PriorityBlockingQueue
    // using PriorityBlockingQueue(int initialCapacity)
    // constructor
    PriorityBlockingQueue<Integer> pbq
            = new PriorityBlockingQueue<Integer>(c);

    // add numbers
    pbq.add(1);
    pbq.add(2);
    pbq.add(3);

    // print queue
    System.out.println("PriorityBlockingQueue:" + pbq);
}

}

`

Output

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.

Java `

// Java program to demonstrate // PriorityBlockingQueue(int initialCapacity, Comparator // comparator) constructor import java.util.concurrent.PriorityBlockingQueue; import java.util.*;

public class Geeks {

public static void main(String[] args)
{
    // define capacity of PriorityBlockingQueue
    int c = 15;

    // create object of PriorityBlockingQueue
    PriorityBlockingQueue<Integer> pbq
            = new PriorityBlockingQueue<Integer>(
            c, Comparator.reverseOrder());

    // add numbers
    pbq.add(1);
    pbq.add(2);
    pbq.add(3);

    System.out.println("PriorityBlockingQueue:" + pbq);
}

}

`

Output

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.

Java `

// Java program to demonstrate adding elements // to the PriorityBlockingQueue import java.util.concurrent.PriorityBlockingQueue;

public class Geeks {

public static void main(String[] args)
{
    // define capacity of PriorityBlockingQueue
    int c = 15;

    // create object of PriorityBlockingQueue
    PriorityBlockingQueue<Integer> pbq
        = new PriorityBlockingQueue<Integer>(c);

    // add numbers
    pbq.add(1);
    pbq.add(2);
    pbq.add(3);

    System.out.println("PriorityBlockingQueue:" + pbq);
}

}

`

Output

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.

Java `

// Java program to demonstrate removing // elements from the PriorityBlockingQueue import java.util.concurrent.PriorityBlockingQueue;

public class Geeks {

public static void main(String[] args)
{
    // define capacity of PriorityBlockingQueue
    int c = 15;

    // create object of PriorityBlockingQueue
    PriorityBlockingQueue<Integer> pbq
            = new PriorityBlockingQueue<Integer>(c);

    // add numbers
    pbq.add(1);
    pbq.add(2);
    pbq.add(3);

    // print queue
    System.out.println("PriorityBlockingQueue:" + pbq);

    // remove element 2
    pbq.remove(2);

    // print queue
    System.out.println("PriorityBlockingQueue:" + pbq);

    // remove all the elements
    pbq.clear();
    System.out.println("PriorityBlockingQueue:" + pbq);

}

}

`

Output

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.

Java `

// Java Program Demonstrate accessing // elements of PriorityBlockingQueue import java.util.concurrent.PriorityBlockingQueue;

public class Geeks { public static void main(String[] args) { // define capacity of PriorityBlockingQueue int c = 5;

    // create object of PriorityBlockingQueue
    PriorityBlockingQueue<Integer> Pbq
            = new PriorityBlockingQueue<Integer>(
            c);

    // Add elements to PriorityBlockingQueue
    Pbq.add(10);
    Pbq.add(20);

    // print PrioQueue
    System.out.println("PriorityQueue: " + Pbq);

    // get head of PriorityBlockingQueue
    int h = Pbq.peek();

    // print head of PriorityBlockingQueue
    System.out.println("Head of Queue: " + h);
}

}

`

Output

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.

Java `

// Java Program Demonstrate iterating // over PriorityBlockingQueue import java.util.concurrent.PriorityBlockingQueue; import java.util.*;

public class Geeks{ public static void main(String[] args) {

    // define capacity of PriorityBlockingQueue
    int c = 5;

    // create object of PriorityBlockingQueue
    PriorityBlockingQueue<String> pbq
            = new PriorityBlockingQueue<String>(
            c);

    // Adding elements
    pbq.add("Java");
    pbq.add("C++");
    pbq.add("Python");
    pbq.add("Js");


    // Call iterator() method of PriorityBlockingQueue
    Iterator i = pbq.iterator();

    // Print elements of iterator
    // created from PriorityBlockingQueue
    System.out.println("Elements in PriorityBlockingQueue are:");

    while (i.hasNext()) {
        System.out.print(i.next() + " ");
    }
}

}

`

Output

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.

Java `

// Java Program Demonstrate comparator() // method and passing Comparator to PriorityBlockingQueue import java.util.concurrent.PriorityBlockingQueue; import java.util.*;

public class Geeks { public static void main(String[] args) throws InterruptedException {

    // create object of PriorityBlockingQueue
    PriorityBlockingQueue<Integer> pbq
            = new PriorityBlockingQueue<Integer>(
            10, new Comparator<Integer>() {
        public int compare(Integer a, Integer b)
        {
            return a - b;
        }
    });

    // Add numbers to PriorityBlockingQueue
    pbq.put(45815616);
    pbq.put(4981561);
    pbq.put(4594591);
    pbq.put(9459156);

    // get String representation of
    // PriorityBlockingQueue
    String str = pbq.toString();

    // Creating a comparator using comparator()
    Comparator c = pbq.comparator();

    // Displaying the comparator values
    System.out.println("Comparator value: " + c);

    if (c == null)
        System.out.println(
                "PriorityBlockingQueue follows natural ordering");
    else
        System.out.println(
                "PriorityBlockingQueue follows : " + c);
}

}

`

Output

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