Introduction :
Java provides a utility class called LinkedList to create linked lists. Unlike any other programming languages like C, we can create one Linked list easily using this class.
This class also provides a lot of public methods to operate on the linked list elements. poll(), pollFirst() and pollLast() are such methods to remove elements from a linked list. In this tutorial, we will check how these methods actually work.
poll() :
This method is used to remove the head or first element of a linked list. It returns the head of the list or if the list is empty, it returns null. For example :
import java.util.LinkedList;
public class Main {
public static void main(String[] args){
//1
LinkedList<string> firstList = new LinkedList<>();
LinkedList<string> secondList = new LinkedList<>();
//2
firstList.add("first");
firstList.add("second");
firstList.add("third");
firstList.add("fourth");
firstList.add("fifth");
//3
System.out.println("Before : "+firstList);
//4
System.out.println("Polling from the first list : "+firstList.poll());
System.out.println("Polling from the second list : "+secondList.poll());
//5
System.out.println("After : "+firstList);
}
}Explanation :
The commented numbers in the above program denote the step numbers below :
-
We have created two LinkedList variables at the start of the program: firstList and secondList.
-
Add five different string elements to the first linked list firstList. We are not adding anything to the other list secondList.
-
Print the values of firstList to the user.
-
Use poll on both of these lists. Note that secondList is null here.
-
Print out the firstList to the user.
Output :
Before : [first, second, third, fourth, fifth]
Polling from the first list : first
Polling from the second list : null
After : [second, third, fourth, fifth]
As you can see here that the item is removed from the list and it was returned by the poll() method. null is returned for the empty list.
pollFirst() :
This method retrieves and removes the first element of the list and returns null for an empty list.
For example :
import java.util.LinkedList;
public class Main {
public static void main(String[] args){
LinkedList<string> firstList = new LinkedList<>();
LinkedList<string> secondList = new LinkedList<>();
firstList.add("first");
firstList.add("second");
firstList.add("third");
firstList.add("fourth");
firstList.add("fifth");
System.out.println("Before : "+firstList);
System.out.println("Polling from the first list : "+firstList.pollFirst());
System.out.println("Polling from the second list : "+secondList.pollFirst());
System.out.println("After : "+firstList);
}
}Output :
Before : [first, second, third, fourth, fifth]
Polling from the first list : first
Polling from the second list : null
After : [second, third, fourth, fifth]
As you can see here, the first element is removed from the linked list.
pollLast() :
pollLast() is used for removing the last element from a linked list :
import java.util.LinkedList;
public class Main {
public static void main(String[] args){
LinkedList<string> firstList = new LinkedList<>();
LinkedList<string> secondList = new LinkedList<>();
firstList.add("first");
firstList.add("second");
firstList.add("third");
firstList.add("fourth");
firstList.add("fifth");
System.out.println("Before : "+firstList);
System.out.println("Polling from the first list : "+firstList.pollLast());
System.out.println("Polling from the second list : "+secondList.pollLast());
System.out.println("After : "+firstList);
}
}Output :
Before : [first, second, third, fourth, fifth]
Polling from the first list : fifth
Polling from the second list : null
After : [first, second, third, fourth]