The poll() method of Queue Interface returns and removes the element at the front end of the container. It deletes the element in the container. The method does not throws an exception when the Queue is empty, it returns null instead.
Syntax:
E poll()
Returns: This method returns the element at the front of the container or the head of the Queue. It returns null when the Queue is empty.
Below programs illustrate poll() method of Queue:
Program 1: With the help of LinkedList.
// Java Program Demonstrate poll()// method of Queue import java.util.*; public class GFG { public static void main(String[] args) throws IllegalStateException { // create object of Queue Queue<Integer> Q = new LinkedList<Integer>(); // Add numbers to end of Queue Q.add(7855642); Q.add(35658786); Q.add(5278367); Q.add(74381793); // print queue System.out.println("Queue: " + Q); // print head and deletes the head System.out.println("Queue's head: " + Q.poll()); // print head and deleted the head System.out.println("Queue's head: " + Q.poll()); }} |
Output:
Queue: [7855642, 35658786, 5278367, 74381793] Queue's head: 7855642 Queue's head: 35658786
Program 2: To Demonstrate poll() method of Queue when the Queue becomes empty
// Java Program Demonstrate poll()// method of Queue when the Queue becomes empty import java.util.*; public class GFG { public static void main(String[] args) throws IllegalStateException { // create object of Queue Queue<Integer> Q = new LinkedList<Integer>(); // Add numbers to end of Queue Q.add(423); Q.add(3432); // print queue System.out.println("Queue: " + Q); // print head and deletes the head System.out.println("Queue's head: " + Q.poll()); // print head and deleted the head System.out.println("Queue's head: " + Q.poll()); // print queue System.out.println("Queue: " + Q); // print null as Queue is empty now System.out.println("Queue's head: " + Q.poll()); }} |
Output:
Queue: [423, 3432] Queue's head: 423 Queue's head: 3432 Queue: [] Queue's head: null
Program 3: With the help of ArrayDeque.
// Java Program Demonstrate poll()// method of Queue import java.util.*; public class GFG { public static void main(String[] args) throws IllegalStateException { // create object of Queue Queue<Integer> Q = new ArrayDeque<Integer>(); // Add numbers to end of Queue Q.add(7855642); Q.add(35658786); Q.add(5278367); Q.add(74381793); // print queue System.out.println("Queue: " + Q); // print head and deletes the head System.out.println("Queue's head: " + Q.poll()); // print head and deleted the head System.out.println("Queue's head: " + Q.poll()); }} |
Output:
Queue: [7855642, 35658786, 5278367, 74381793] Queue's head: 7855642 Queue's head: 35658786
Program 4: With the help of ConcurrentLinkedDeque.
// Java Program Demonstrate poll()// method of Queue import java.util.*;import java.util.concurrent.ConcurrentLinkedDeque; public class GFG { public static void main(String[] args) throws IllegalStateException { // create object of Queue Queue<Integer> Q = new ConcurrentLinkedDeque<Integer>(); // Add numbers to end of Queue Q.add(7855642); Q.add(35658786); Q.add(5278367); Q.add(74381793); // print queue System.out.println("Queue: " + Q); // print head and deletes the head System.out.println("Queue's head: " + Q.poll()); // print head and deleted the head System.out.println("Queue's head: " + Q.poll()); }} |
Output:
Queue: [7855642, 35658786, 5278367, 74381793] Queue's head: 7855642 Queue's head: 35658786
Program 5: With the help of LinkedBlockingDeque.
// Java Program Demonstrate poll()// method of Queue import java.util.*;import java.util.concurrent.LinkedBlockingDeque; public class GFG { public static void main(String[] args) throws IllegalStateException { // create object of Queue Queue<Integer> Q = new LinkedBlockingDeque<Integer>(); // Add numbers to end of Queue Q.add(7855642); Q.add(35658786); Q.add(5278367); Q.add(74381793); // print queue System.out.println("Queue: " + Q); // print head and deletes the head System.out.println("Queue's head: " + Q.poll()); // print head and deleted the head System.out.println("Queue's head: " + Q.poll()); }} |
Output:
Queue: [7855642, 35658786, 5278367, 74381793] Queue's head: 7855642 Queue's head: 35658786
Reference: https://docs.oracle.com/javase/8/docs/api/java/util/Queue.html#poll–