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Showing content from https://www.geeksforgeeks.org/java/java-util-linkedlist-peek-peekfirst-peeklast-java/ below:

Java.util.LinkedList.peek() , peekfirst(), peeklast() in Java

Last Updated : 10 Dec, 2018

Linked list class offers the functionality to "

look into

" the first and last elements of the list and hence can be useful in cases where

only the retrieval is required

and not necessarily the deletion is required. Three functionalities are present and all are discussed in this article.

1. peek() :

This method

retrieves, but does not remove

, the head (first element) of this list.

Declaration : 
   public E peek()
Return Value : 
  This method returns the head of this list, or null if this list is empty.

Java


 // Java code to demonstrate the working
// of peek() in LinkedList
import java.util.*;
public class LinkedPeek1 {

public static void main(String[] args)
    {
        // declaring a LinkedList
        LinkedList list = new LinkedList();

        // adding  elements
        list.add("Geeks");
        list.add(4);
        list.add("Geeks");
        list.add("8");

        // printing the list
        System.out.println("The initial list is :" + list);

        // peek at the head of the list
        // Prints 1st element, "Geeks"
        System.out.println("Head of the list : " + list.peek());
    }
}

Output:

The initial list is :[Geeks, 4, Geeks, 8]
Head of the list : Geeks

2. peekFirst() :

This method

retrieves, but does not remove

, the first element of this list, or returns null if this list is empty. This works similar to peek().

Declaration : 
   public E peekFirst()
Return Value : 
    This method returns the first element of this list, or null if this list is empty

Java


 // Java code to demonstrate the working
// of peekFirst() in LinkedList
import java.util.*;
public class LinkedPeek2 {

public static void main(String[] args)
    {
        // declaring a LinkedList
        LinkedList list = new LinkedList();

        // adding  elements
        list.add("Geeks");
        list.add(4);
        list.add("Geeks");
        list.add("8");

        // printing the list
        System.out.println("The initial list is :" + list);

        // peek at the first element of the list
        // Prints 1st element, "Geeks"
        System.out.println("First element of the list is : " + list.peekFirst());
    }
}

Output:

The initial list is :[Geeks, 4, Geeks, 8]
First element of the list is : Geeks

3. peekLast() :

This method

retrieves

, but does not remove, the

last element

of this list, or returns null if this list is empty.

Declaration
  public E peekLast()
Return Value
   This method returns the last element of this list, or null if this list is empty

Java


 // Java code to demonstrate the working
// of peekLast() in LinkedList
import java.util.*;
public class LinkedPeek3 {

public static void main(String[] args)
    {
        // declaring a LinkedList
        LinkedList list = new LinkedList();

        // adding  elements
        list.add("Geeks");
        list.add(4);
        list.add("Geeks");
        list.add(8);

        // printing the list
        System.out.println("The initial list is :" + list);

        // peek at the last element of the list
        // Prints last element, 8
        System.out.println("Last element of the list is : " + list.peekLast());
    }
}

Output:

The initial list is :[Geeks, 4, Geeks, 8]
Last element of the list is : 8

Practical Application :

The practical application that can be thought of is that this can be used in potentially the game of cards where the

individuals can peek the first or last element

of the deck on asking which element they want to see. Code below explains the working.

Java


 // Java code to demonstrate the application
// of peek()
import java.util.*;
public class LinkedPeekApp {

public static void main(String[] args)
    {

        // declaring a LinkedList
        LinkedList list = new LinkedList();

        // adding  elements in deck
        list.add(5);
        list.add(4);
        list.add("Jack");
        list.add(8);
        list.add("King");

        // printing the list
        System.out.println("The initial deck is :" + list);

        String d = "upper";

        System.out.println("The element chosen to peek is : " + d);

        if (d == "upper")
            System.out.println("The Upper element is : " + list.peekFirst());
        else
            System.out.println("The Lower element is : " + list.peekLast());
    }
}

Output :

The initial deck is :[5, 4, Jack, 8, King]
The element chosen to peek is : upper
The Upper element is : 5

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