LinkedList in Java

Here’s a simple example of adding elements to a LinkedList in Java.

// Java program to add elements to a LinkedList
import java.util.LinkedList;

public class AddElements {

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

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

        // Printing the LinkedList
        System.out.println(list);
    }
}

[One, Two, Three, Four, Five]

Linked List is a part of the Collection framework present in java.util package. This class is an implementation of the LinkedList data structure which is a linear data structure where the elements are not stored in contiguous locations and every element is a separate object with a data part and address part. The elements are linked using pointers and addresses. Each element is known as a node. 

Due to the dynamicity and ease of insertions and deletions, they are preferred over the arrays. It also has a few disadvantages like the nodes cannot be accessed directly instead we need to start from the head and follow through the link to reach a node we wish to access.

How Does LinkedList work Internally?

Since a LinkedList acts as a dynamic array and we do not have to specify the size while creating it, the size of the list automatically increases when we dynamically add and remove items. And also, the elements are not stored in a continuous fashion. Therefore, there is no need to increase the size. Internally, the LinkedList is implemented using the doubly linked list data structure. 

The main difference between a normal linked list and a doubly LinkedList is that a doubly linked list contains an extra pointer, typically called the previous pointer, together with the next pointer and data which are there in the singly linked list. 

Constructors in the 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 ll, then, it can be created as: 

LinkedList ll = 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 ll, then, it can be created as: 

LinkedList ll = new LinkedList(C);

Methods for Java LinkedList

Below is the implementation of the above operations:

// Java Program to Demonstrate
// Implementation of LinkedList
// class

// Importing required classes
import java.util.*;

// Main class
public class GFG {

    // Driver code
    public static void main(String args[])
    {
        // Creating object of the
        // class linked list
        LinkedList<String> ll = new LinkedList<String>();

        // Adding elements to the linked list
        ll.add("A");
        ll.add("B");
        ll.addLast("C");
        ll.addFirst("D");
        ll.add(2, "E");

        System.out.println(ll);

        ll.remove("B");
        ll.remove(3);
        ll.removeFirst();
        ll.removeLast();

        System.out.println(ll);
    }
}

[D, A, E, B, C]
[A]

In the above illustration, AbstractList, CopyOnWriteArrayList, and AbstractSequentialList are the classes that implement the list interface. A separate functionality is implemented in each of the mentioned classes. They are:

• AbstractList: This class is used to implement an unmodifiable list, for which one needs to only extend this AbstractList Class and implement only the get() and the size() methods.
• CopyOnWriteArrayList: This class implements the list interface. It is an enhanced version of ArrayList in which all the modifications(add, set, remove, etc.) are implemented by making a fresh copy of the list.

Performing Various Operations on LinkedList

• Adding elements
• Updating elements
• Removing elements
• Iterating over elements
• To Array();
•  Size();
• remove First();
• remove last();

Operation 1: Adding Elements

In order to add an element to an ArrayList, we can use the add() method. This method is overloaded to 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.

Below is the implementation of the above operation:

// Java program to add elements
// to a LinkedList

import java.util.*;

public class GFG {

    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]

Operation 2: Changing Elements

After adding the elements, if we wish to change the element, it can be done using the set() method. Since a LinkedList is indexed, the element which we wish to change is referenced by the index of the element. Therefore, this method takes an index and the updated element which needs to be inserted at that index.

Below is the implementation of the above operation:

// Java program to change elements
// in a LinkedList

import java.util.*;

public class GFG {

    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]

Operation 3: Removing Elements

In order to remove an element from a LinkedList, we can use the remove() method. This method is overloaded to perform multiple operations based on different parameters. They are: 

• remove(Object): This method is used to simply remove an object from the LinkedList. If there are multiple such objects, then the first occurrence of the object is removed.
• remove(int index): Since a LinkedList is indexed, this method takes an integer value which simply removes the element present at that specific index in the LinkedList. After removing the element and the indices of elements are updated so do the object of LinkedList is updated giving a new List after the deletion of element/s.

Below is the implementation of the above operation:

// Java program to remove elements
// in a LinkedList

import java.util.*;

public class GFG {

    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]

Operation 4: Iterating the 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.

Below is the implementation of the above operation:

// Java program to iterate the elements 
// in an LinkedList
  
import java.util.*; 
  
public class GFG { 
  
    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 
        for (int i = 0; i < ll.size(); i++) { 
  
            System.out.print(ll.get(i) + " "); 
        } 
  
        System.out.println(); 
  
        // Using the for each loop 
        for (String str : ll) 
            System.out.print(str + " "); 
    } 
} 

Geeks For Geeks 
Geeks For Geeks 

Operation 4:  Linked list to To Array by using  toArray();

import java.util.*;
public class GFG2 {
    public static void main(String[] args)
    {
        LinkedList<Integer> list= new LinkedList<Integer>();
        list.add(123);
        list.add(12);
        list.add(11);
        list.add(1134);
        System.out.println("LinkedList: "+ list);
        Object[] a = list.toArray();
        System.out.print("After converted LinkedList to Array: ");
        for(Object element : a)
        System.out.print(element+" ");
    }
}

LinkedList: [123, 12, 11, 1134]
After converted LinkedList to Array: 123 12 11 1134 

Operation 5-size();

import java.io.*;
import java.util.LinkedList;
public class GFG2 {
    public static void main(String args[]) {
        LinkedList<String> list = new LinkedList<String>();
        list.add("Geeks for Geeks ");
        list.add("is best ");
        // Displaying the size of the list
        System.out.println("The size of the linked list is: " + list.size());
    }
}

The size of the linked list is: 2

Operation 7 – removeFirst();

import java.io.*;
import java.util.LinkedList;
public class GFG2 {
    public static void main(String args[]) {

        LinkedList<Integer> list = new LinkedList<Integer>();
        list.add(10);
        list.add(20);
        list.add(30);
        System.out.println("LinkedList:" + list);
        System.out.println("The  remove first element is: " + list.removeFirst());
        // Displaying the final list
        System.out.println("Final LinkedList:" + list);
    }
}

LinkedList:[10, 20, 30]
The  remove first element is: 10
Final LinkedList:[20, 30]

Operation 8- removelast();

import java.io.*;
import java.util.LinkedList;
public class GFG2 {
    public static void main(String args[])
    {
        
        LinkedList<Integer> list = new LinkedList<Integer>();
        list.add(10);
        list.add(20);
        list.add(30);
        System.out.println("LinkedList:" + list);
        // Remove the tail using removeLast()
        System.out.println("The last element is removed: " + list.removeLast());
        // Displaying the final list
        System.out.println("Final LinkedList:" + list);
        // Remove the tail using removeLast()
        System.out.println("The last element is removed: " + list.removeLast());
        // Displaying the final list
        System.out.println("Final LinkedList:" + list);
    }
}

LinkedList:[10, 20, 30]
The last element is removed: 30
Final LinkedList:[10, 20]
The last element is removed: 20
Final LinkedList:[10]

The LinkedList class in Java is a part of the Java Collections Framework and provides a linked list implementation of the List interface. It allows for the storage and retrieval of elements in a doubly-linked list data structure, where each element is linked to its predecessor and successor elements.

Here is a simple example that demonstrates how to use a LinkedList in Java:

import java.util.LinkedList;

public class LinkedListExample {
    public static void main(String[] args) {
        // Create a new linked list
        LinkedList<Integer> linkedList = new LinkedList<>();

        // Add elements to the linked list
        linkedList.add(1);
        linkedList.add(2);
        linkedList.add(3);

        // Add an element to the beginning of the linked list
        linkedList.addFirst(0);

        // Add an element to the end of the linked list
        linkedList.addLast(4);

        // Print the elements of the linked list
        for (int i : linkedList) {
            System.out.println(i);
        }
    }
}

0
1
2
3
4

Advantages of using LinkedList in Java

• Dynamic size: As with Vector, the size of a LinkedList can grow or shrink dynamically, so you don’t have to worry about setting an initial size.
• Efficient Insertions and Deletions: LinkedList is an efficient data structure for inserting or deleting elements in the middle of the list because you only need to change the links between elements, rather than shifting all elements after the insertion or deletion point.
• Flexible Iteration: With a linked list, you can efficiently iterate through the list in either direction, since each element has a reference to both its predecessor and successor elements.

Disadvantages of using LinkedList in Java

• Performance: LinkedList has a slower performance than ArrayList when it comes to accessing individual elements. This is because you need to traverse the list to reach the desired element, whereas with ArrayList, you can simply access the desired element using an index.
• Memory overhead: LinkedList requires more memory than ArrayList because each element requires additional memory for the links to its predecessor and successor elements.

Reference Book: 

A good reference book for learning about the Java Collections Framework and LinkedList is “Java Collections” by Naftalin and Wadler. This book provides a comprehensive look at the Java collections framework, including LinkedList, and includes many examples and exercises to help you understand how to use these classes effectively.

GeeksforGeeks

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