Linked List Program in Java

The LinkedList class of the Java collections framework provides the functionality of the linked list data structure (doubly linked list).

Each element in a linked list is known as a node. It consists of 3 fields:

Prev – stores an address of the previous element in the list. It is null for the first element
Next – stores an address of the next element in the list. It is null for the last element
Data – stores the actual data

Creating a Java LinkedList

Here is how we can create linked lists in Java:

LinkedList linkedList = new LinkedList<>();
Here, Type indicates the type of a linked list. For example,

// create Integer type linked list
LinkedList linkedList = new LinkedList<>();

// create String type linked list
LinkedList linkedList = new LinkedList<>();

Example: Create LinkedList in Java

import java.util.LinkedList;

class Main {
  public static void main(String[] args){

    // create linkedlist
    LinkedList<String> animals = new LinkedList<>();

    // Add elements to LinkedList
    animals.add("Dog");
    animals.add("Cat");
    animals.add("Cow");
    System.out.println("LinkedList: " + animals);
  }
}

Output

LinkedList: [Dog, Cat, Cow]

In the above example, we have created a LinkedList named animals.

Here, we have used the add() method to add elements to the LinkedList. We will learn more about the add() method later in this tutorial.

Working of a Java LinkedList

Elements in linked lists are not stored in sequence. Instead, they are scattered and connected through links (Prev and Next).

Here we have 3 elements in a linked list.

Dog – it is the first element that holds null as previous address and the address of Cat as the next address
Cat – it is the second element that holds an address of Dog as the previous address and the address of Cow as the next address
Cow – it is the last element that holds the address of Cat as the previous address and null as the next element

Methods of Java LinkedList

LinkedList provides various methods that allow us to perform different operations in linked lists. We will look at four commonly used LinkedList Operators in this tutorial:

  • Add elements
  • Access elements
  • Change elements
  • Remove elements

Add elements to a LinkedList

We can use the add() method to add an element (node) at the end of the LinkedList. For example,

import java.util.LinkedList;

class Main {
  public static void main(String[] args){
    // create linkedlist
    LinkedList<String> animals = new LinkedList<>();

    // add() method without the index parameter
    animals.add("Dog");
    animals.add("Cat");
    animals.add("Cow");
    System.out.println("LinkedList: " + animals);

    // add() method with the index parameter
    animals.add(1, "Horse");
    System.out.println("Updated LinkedList: " + animals);
  }
}

Output

LinkedList: [Dog, Cat, Cow]
Updated LinkedList: [Dog, Horse, Cat, Cow]

In the above example, we have created a LinkedList named animals. Here, we have used the add() method to add elements to animals.

Notice the statement,

animals.add(1, "Horse");
Here, we have used the index number parameter. It is an optional parameter that specifies the position where the new element is added.

Access LinkedList elements

The get() method of the LinkedList class is used to access an element from the LinkedList. For example,

import java.util.LinkedList;

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

    // add elements in the linked list
    languages.add("Python");
    languages.add("Java");
    languages.add("JavaScript");
    System.out.println("LinkedList: " + languages);

    // get the element from the linked list
    String str = languages.get(1);
    System.out.print("Element at index 1: " + str);
  }
}

Output

LinkedList: [Python, Java, JavaScript]
Element at index 1: Java

In the above example, we have used the get() method with parameter 1. Here, the method returns the element at index 1.

We can also access elements of the LinkedList using the iterator() and the listIterator() method.

Change Elements of a LinkedList

The set() method of LinkedList class is used to change elements of the LinkedList. For example,

import java.util.LinkedList;

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

    // add elements in the linked list
    languages.add("Java");
    languages.add("Python");
    languages.add("JavaScript");
    languages.add("Java");
    System.out.println("LinkedList: " + languages);

    // change elements at index 3
    languages.set(3, "Kotlin");
    System.out.println("Updated LinkedList: " + languages);
  }
}

Output

LinkedList: [Java, Python, JavaScript, Java]
Updated LinkedList: [Java, Python, JavaScript, Kotlin]

In the above example, we have created a LinkedList named languages. Notice the line,

languages.set(3, "Kotlin");
Here, the set() method changes the element at index 3 to Kotlin.

Remove element from a LinkedList

The remove() method of the LinkedList class is used to remove an element from the LinkedList. For example,

import java.util.LinkedList;

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

    // add elements in LinkedList
    languages.add("Java");
    languages.add("Python");
    languages.add("JavaScript");
    languages.add("Kotlin");
    System.out.println("LinkedList: " + languages);

    // remove elements from index 1
    String str = languages.remove(1);
    System.out.println("Removed Element: " + str);

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

Output

LinkedList: [Java, Python, JavaScript, Kotlin]
Removed Element: Python
New LinkedList: [Java, JavaScript, Kotlin]

Here, the remove() method takes the index number as the parameter. And, removes the element specified by the index number.

Other Methods

Methods Description
contains( ) checks if the linkedlist contains the element
indexof( ) returns the index of the first occurrence of the element
lastindexof( ) returns the index of the last occurrence of the elements
clear( ) removes all the elements of the linkedlist
iterator( ) returns an iterator to iterate over linkedlist

Iterating through LinkedList

We can use the Java for-each loop to iterate through LinkedList. For example,

import java.util.LinkedList;

class Main {
    public static void main(String[] args) {
        // Creating a linked list
        LinkedList<String> animals = new LinkedList<>();
        animals.add("Cow");
        animals.add("Cat");
        animals.add("Dog");
        System.out.println("LinkedList: " + animals);

        // Using forEach loop
        System.out.println("Accessing linked list elements:");
        for(String animal: animals) {
            System.out.print(animal);
            System.out.print(", ");
        }
    }
}

Output

LinkedList: [Cow, Cat, Dog]
Accessing linked list elements:
Cow, Cat, Dog, 

LinkedList Vs. ArrayList

Both the Java ArrayList and LinkedList implements the List interface of the Collections framework. However, there exists some difference between them.

LinkedList ArrayList
Implements List, Queue, and Deque interfaces. Implements List Interface.
Stores 3 values (previous address, data, and next address) in a single position. Stores a single value in a single position.
Provides the doubly-linked list implementation. Provides a resizable array implementation.
Whenever an element is added, prev and next addresses are changed. Whenever an element is added, all elements after that position are shifted.
To access an element, we need to iterate from the beginning to the element. Can randomly access elements using indexes.

Conclusion
In this article, we discussed linked lists in Java. Having knowledge about data structures like linked lists in Java plays an important role in the placement. Practice a lot for becoming an expert coder.

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