DelayQueue poll() method in Java with Examples

The poll() method of DelayQueue is used to retrieve head of the DelayQueue. This method also removes the head from the DelayQueue. Thus when poll method is called, DelayQueue size is reduced by one.
Syntax:

public E poll ()

Parameters:
This method does not accept any parameter.
Returns:
This method returns head of the DelayQueue and also removes it from this DelayQueue.
Exception:
Null Pointer Exception: if head is not present, this function will return null.
Below program to illustrate DelayQueue poll() in Java:
Example: 

Java

// Java Program Demonstrate DelayQueue poll() method

import java.util.concurrent.*;
import java.util.*;

// The DelayObject for DelayQueue
// It must implement Delayed and
// its getDelay() and compareTo() method
class DelayObject implements Delayed {

    private String name;
    private long time;

    // Constructor of DelayObject
    public DelayObject(String name, long delayTime)
    {
        this.name = name;
        this.time = System.currentTimeMillis()
                    + delayTime;
    }

    // Implementing getDelay() method of Delayed
    @Override
    public long getDelay(TimeUnit unit)
    {
        long diff = time - System.currentTimeMillis();
        return unit.convert(diff, TimeUnit.MILLISECONDS);
    }

    // Implementing compareTo() method of Delayed
    @Override
    public int compareTo(Delayed obj)
    {
        if (this.time < ((DelayObject)obj).time) {
            return -1;
        }
        if (this.time > ((DelayObject)obj).time) {
            return 1;
        }
        return 0;
    }

    // Implementing toString() method of Delayed
    @Override
    public String toString()
    {
        return "\n{"
            + " " + name + ", time=" + time + "}";
    }
}

// Driver Class
public class GFG {
    public static void main(String[] args) throws InterruptedException
    {

        // create object of DelayQueue
        // using DelayQueue() constructor
        BlockingQueue<DelayObject> DQ
            = new DelayQueue<DelayObject>();

        // Add numbers to end of DelayQueue
        // using add() method
        DQ.add(new DelayObject("A", 1));
        DQ.add(new DelayObject("B", 2));
        DQ.add(new DelayObject("C", 3));
        DQ.add(new DelayObject("D", 4));

        // Print delayqueue
        System.out.println("Original DelayQueue: "
                           + DQ + "\n");
        // Print size of original DelayQueue
        System.out.println("Original DelayQueue size: "
                           + DQ.size() + "\n");

        // poll() method for returning head of the DelayQueue
        System.out.println("Head of the DelayQueue: "
                           + DQ.poll());
        // print DelayQueue size after poll method
        System.out.println("\nAfter poll() method, DelayQueue size: "
                           + DQ.size());
    }
}

Output:

Original DelayQueue: [
{ A, time=1545817370442}, 
{ B, time=1545817370443}, 
{ C, time=1545817370444}, 
{ D, time=1545817370445}]

Original DelayQueue size: 4

Head of the DelayQueue: 
{ A, time=1545817370442}

After poll() method, DelayQueue size: 3

Example: 

Java

// Java Program Demonstrate DelayQueue poll() method

import java.util.concurrent.*;
import java.util.*;

// The DelayObject for DelayQueue
// It must implement Delayed and
// its getDelay() and compareTo() method
class DelayObject implements Delayed {

    private String name;
    private long time;

    // Constructor of DelayObject
    public DelayObject(String name, long delayTime)
    {
        this.name = name;
        this.time = System.currentTimeMillis()
                    + delayTime;
    }

    // Implementing getDelay() method of Delayed
    @Override
    public long getDelay(TimeUnit unit)
    {
        long diff = time - System.currentTimeMillis();
        return unit.convert(diff, TimeUnit.MILLISECONDS);
    }

    // Implementing compareTo() method of Delayed
    @Override
    public int compareTo(Delayed obj)
    {
        if (this.time < ((DelayObject)obj).time) {
            return -1;
        }
        if (this.time > ((DelayObject)obj).time) {
            return 1;
        }
        return 0;
    }

    // Implementing toString() method of Delayed
    @Override
    public String toString()
    {
        return "\n{"
            + " " + name + ", time=" + time + "}";
    }
}

// Driver Class
public class GFG {
    public static void main(String[] args) throws InterruptedException
    {

        // create object of DelayQueue
        // using DelayQueue() constructor
        BlockingQueue<DelayObject> DQ
            = new DelayQueue<DelayObject>();

        // Print size of original DelayQueue
        System.out.println("Original DelayQueue size: "
                           + DQ.size() + "\n");

        // poll() method for returning head of the DelayQueue
        System.out.println("Head of the DelayQueue: "
                           + DQ.poll());
    }
}

Output:

Original DelayQueue size: 0

Head of the DelayQueue: null