LinkedList in Java

LinkedList is a part of the Java Collections Framework and is present in the java.util package. It implements a doubly linked list, where each element is stored in a node containing the element and references to the previous and next nodes.

• The size of a LinkedList can grow or shrink dynamically.
• It maintains the insertion order of elements.
• It allows duplicate elements.

import java.util.LinkedList;

public class Geeks {

    public static void main(String[] args){
        
        // Creating a LinkedList
        LinkedList<String> l = new LinkedList<String>();

        // Adding elements to the LinkedList using add() method
        l.add("One");
        l.add("Two");
        l.add("Three");
        l.add("Four");
        l.add("Five");

        System.out.println(l);
    }
}

[One, Two, Three, Four, Five]

Explanation: The program creates an empty LinkedList of String elements and adds five elements using add(). The elements are printed in the same order in which they were inserted.

Note: LinkedList nodes cannot be accessed directly by index; elements must be accessed by traversing from the head.

Syntax

LinkedList<Type> list = new LinkedList<>();

Hierarchy of LinkedList

It implements the List and Deque interfaces, both of which are sub-interfaces of the Collection Interface.

Constructors of LinkedList

In order to create a LinkedList, we need to create an object of the LinkedList class. The LinkedList class consists of various constructors that allow the possible creation of the list. The following are the constructors available in this class:

1. LinkedList()

This constructor is used to create an empty linked list. If we wish to create an empty LinkedList with the name list, then it can be created as: 

LinkedList list = new LinkedList();  

2. LinkedList(Collection C)

This constructor is used to create an ordered list that contains all the elements of a specified collection, as returned by the collection's iterator. If we wish to create a LinkedList with the name list, then, it can be created as: 

LinkedList list = new LinkedList(C);

Performing Different Operations on LinkedList

1. Adding Elements

With the help of the add() method, we can add elements to a LinkedList This method can perform multiple operations based on different parameters. They are:  

• add(Object): This method is used to add an element at the end of the LinkedList.
• add(int index, Object): This method is used to add an element at a specific index in the LinkedList.

import java.util.*;

public class Geeks {

    public static void main(String args[])
    {
        LinkedList<String> ll = new LinkedList<>();

        ll.add("Geeks");
        ll.add("Geeks");
        ll.add(1, "For");

        System.out.println(ll);
    }
}

[Geeks, For, Geeks]

Explanation: The program adds two "Geeks" elements and inserts "For" at index 1. Therefore, the final list is [Geeks, For, Geeks].

2. Update Elements

With the help of the set() method, we can update an element in a LinkedList. This method takes an index and the updated element which needs to be inserted at that index.

import java.util.*;

public class Geeks {

    public static void main(String args[])
    {
        LinkedList<String> ll = new LinkedList<>();

        ll.add("Geeks");
        ll.add("Geeks");
        ll.add(1, "Geeks");

        System.out.println("Initial LinkedList " + ll);

        ll.set(1, "For");

        System.out.println("Updated LinkedList " + ll);
    }
}

Initial LinkedList [Geeks, Geeks, Geeks]
Updated LinkedList [Geeks, For, Geeks]

Explanation: The program first creates a list containing three "Geeks" elements. set(1, "For") replaces the element at index 1 with "For"

3. Removing Elements

Removes the first occurrence of the specified element from the list, if it exists. They are: 

• remove(Object): Removes the first occurrence of the specified object from the LinkedList.
• remove(int index): Removes the element at the given index and shifts subsequent elements.

import java.util.*;

public class Geeks {

    public static void main(String args[])
    {
        LinkedList<String> ll = new LinkedList<>();

        ll.add("Geeks");
        ll.add("Geeks");
        ll.add(1, "For");

        System.out.println("Initial LinkedList " + ll);

          // Function call
        ll.remove(1);

        System.out.println("After the Index Removal " + ll);

        ll.remove("Geeks");

        System.out.println("After the Object Removal "
                           + ll);
    }
}

Initial LinkedList [Geeks, For, Geeks]
After the Index Removal [Geeks, Geeks]
After the Object Removal [Geeks]

Explanation: remove(1) removes the element at index 1, while remove("Geeks") removes the first occurrence of "Geeks".

4. Iterating a LinkedList

There are multiple ways to iterate through LinkedList. The most famous ways are by using the basic for loop in combination with a get() method to get the element at a specific index and the advanced for-loop.

import java.util.*; 
  
public class Geeks { 
  
    public static void main(String args[]) 
    { 
        LinkedList<String> ll 
            = new LinkedList<>(); 
  
        ll.add("Geeks"); 
        ll.add("Geeks"); 
        ll.add(1, "For"); 
  
        // Using the Get method and the for loop 
        //Using get(i) in a loop is O(n^2) for LinkedList! Bad practice for large lists
        for (int i = 0; i < ll.size(); i++) { 
  
            System.out.print(ll.get(i) + " "); 
        } 
  
        System.out.println(); 
  
        // Using the for each loop OR Using iterator is recommended practise
        for (String str : ll) 
            System.out.print(str + " "); 
    } 
} 

Geeks For Geeks 
Geeks For Geeks 

Explanation: The enhanced for loop traverses the LinkedList from the first element to the last and prints each element.

Real-World Usage of LinkedList

• Implementing queues and deques.
• Managing data where frequent insertion and removal at the ends are required.
• Maintaining browser history or similar sequential navigation data.
• Implementing undo/redo-style structures.

Methods for Java LinkedList

ArrayList vs LinkedList