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PriorityBlockingQueue poll() method in Java
  • Last Updated : 26 Nov, 2018

1. poll() Method

The poll() method of PriorityBlockingQueue retrieves and removes element from head of this PriorityBlockingQueue. This method returns the element it removes from PriorityBlockingQueue but when the queue is empty then method will return null.

Syntax:

public E poll()

Returns: This method returns the element from the head of this PriorityBlockingQueue, or null if this queue is empty.

Below programs illustrate poll() method of PriorityBlockingQueue:

Example 1: Program to demonstrate poll() method on PriorityBlockingQueue to remove a element from a list of numbers.






// Java Program Demonstrate poll()
// method of PriorityBlockingQueue
  
import java.util.concurrent.PriorityBlockingQueue;
  
public class GFG {
    public static void main(String[] args)
    {
  
        // define capacity of PriorityBlockingQueue
        int capacityOfQueue = 5;
  
        // create object of PriorityBlockingQueue
        PriorityBlockingQueue<Integer> PrioQueue
            = new PriorityBlockingQueue<Integer>(capacityOfQueue);
  
        // Add numbers to PriorityBlockingQueue
        PrioQueue.offer(35658786);
        PrioQueue.offer(5278367);
        PrioQueue.offer(74381793);
        PrioQueue.offer(87625142);
  
        // remove numbers from head using poll()
        // and print removed number
        int removedItem = PrioQueue.poll();
  
        // print details
        System.out.println("Removed Element: " + removedItem);
        System.out.println("Now Queue Contains:");
        System.out.println(PrioQueue.toString());
    }
}
Output:
Removed Element: 5278367
Now Queue Contains:
[35658786, 87625142, 74381793]

Example 2: Program to demonstrate poll() method to remove String from a list of String values and return null if the list is empty.




// Java Program Demonstrate poll()
// method of PriorityBlockingQueue
  
import java.util.concurrent.PriorityBlockingQueue;
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
    {
  
        // define capacity of PriorityBlockingQueue
        int capacityOfQueue = 5;
  
        // create object of PriorityBlockingQueue which contains
        // name of students
        PriorityBlockingQueue<String> names
            = new PriorityBlockingQueue<String>(capacityOfQueue);
  
        // Add names of students of girls college
        names.offer("Joyita");
        names.offer("Priyanka");
  
        // remove two names from list of names
        // and print removed name
        String removedName1 = names.poll();
        String removedName2 = names.poll();
  
        // print details
        System.out.println("Removed Name 1: " + removedName1);
        System.out.println("Removed Name 2: " + removedName2);
        System.out.println("Now Queue Contains:");
        System.out.println(names.toString());
  
        // try to remove from empty PriorityBlockingQueue
        String removedName3 = names.poll();
        System.out.println("Removed Name 3: " + removedName3);
    }
}
Output:
Removed Name 1: Joyita
Removed Name 2: Priyanka
Now Queue Contains:
[]
Removed Name 3: null

2. poll(long timeout, TimeUnit unit) Method

The poll(long timeout, TimeUnit unit) method of PriorityBlockingQueue retrieves and removes element from head of this PriorityBlockingQueue.If the PriorityBlockingQueue is empty then it will, wait till a specified time for an element to become available.Waiting time and unit of time is given as parameters to method.

Syntax:

public E poll(long timeout, TimeUnit unit) throws InterruptedException

Parameter:
This method accepts two parameters:

  1. timeout(long) : how long to wait before giving up, in units of unit.
  2. unit(TimeUnit) : a TimeUnit determining how to interpret the timeout parameter.

Returns: This method returns the element from the head of this PriorityBlockingQueue, or null if the specified waiting time elapses before an element is available.

Exception: This method throws only one Exception InterruptedException – if interrupted while waiting

Below programs illustrate poll(long timeout, TimeUnit unit) method of PriorityBlockingQueue:

Example 1: Program to demonstrate poll(long timeout, TimeUnit unit) method on PriorityBlockingQueue to remove a element from a list of numbers.




// Java Program Demonstrate poll(long timeout, TimeUnit unit)
// method of PriorityBlockingQueue
  
import java.util.concurrent.PriorityBlockingQueue;
import java.util.concurrent.TimeUnit;
  
public class GFG {
    public static void main(String[] args)
        throws InterruptedException
    {
  
        // define capacity of PriorityBlockingQueue
        int capacityOfQueue = 5;
  
        // create object of PriorityBlockingQueue
        PriorityBlockingQueue<Integer> PrioQueue
            = new PriorityBlockingQueue<Integer>(capacityOfQueue);
  
        // Add numbers to PriorityBlockingQueue
        PrioQueue.offer(35658786);
        PrioQueue.offer(5278367);
  
        // Try to poll  elements from PriorityBlockingQueue
        System.out.println("Removed Number: "
                           + PrioQueue.poll(10, TimeUnit.SECONDS));
        System.out.println("List Contains" + PrioQueue);
  
        System.out.println("Removed Number: "
                           + PrioQueue.poll(10, TimeUnit.SECONDS));
        System.out.println("List Contains" + PrioQueue);
  
        System.out.println("Removed Number: "
                           + PrioQueue.poll(10, TimeUnit.SECONDS));
        System.out.println("List Contains" + PrioQueue);
    }
}
Output:
Removed Number: 5278367
List Contains[35658786]
Removed Number: 35658786
List Contains[]
Removed Number: null
List Contains[]

Reference:

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