Java Collection Framework Linked list

Java LinkedList class provides implementation of linked-list data structure. It used doubly linked list to store the elements. It implements List, Deque and Queue interface and extends AbstractSequentialList class. below is the declaration of LinkedList class.

LinkedList class Declaration

public class LinkedList<E> extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, Serializable

1. LinkedList class extends AbstractSequentialList and implements List,Deque and Queue inteface.
2. It can be used as List, stack or Queue as it implements all the related interfaces.
3. It allows null entry.
4. It is dynamic in nature i.e it allocates memory when required. Therefore insertion and deletion operations can be easily implemented.
5. It can contain duplicate elements and it is not synchronized.
6. Reverse Traversing is difficult in linked list.
7. In LinkedList, manipulation is fast because no shifting needs to be occurred.

LinkedList Constructors

LinkedList class has two constructors. First is used to create empty LinkedList and second for creating from existing collection.

  
LinkedList() // It creates an empty LinkedList
LinkedList( Collection c) // It creates a LinkedList that is initialized with elements of the Collection c
  

LinkedList class Example

Lets take an example to create a linked-list, no element is inserted so it creates an empty LinkedList. See the below example.

  
import java.util.*;
class Demo
{
  public static void main(String[] args)
  {
    // Creating LinkedList
    LinkedList< String> linkedList = new LinkedList< String>();
    // Displaying LinkedLIst
    System.out.println(linkedList);
  }
}
  

[]

LinkedList Methods

The below table contains methods of LinkedList. We can use them to manipulate its elements.

Add Elements to LinkedList

To add elements into LinkedList, we are using add() method. It adds elements into the list in the insertion order.

  
import java.util.*;
class Demo
{
  public static void main(String[] args)
  {
    // Creating LinkedList
    LinkedList< String> linkedList = new LinkedList< String>();
    linkedList.add("Delhi");
    linkedList.add("NewYork");
    linkedList.add("Moscow");
    linkedList.add("Dubai");
    // Displaying LinkedList
    System.out.println(linkedList);
  }
}
  

[Delhi, NewYork, Moscow, Dubai]

Removing Elements

To remove elements from the list, we are using remove method that remove the specified elements. We can also pass index value to remove the elements of it.

  
import java.util.*;
class Demo
{
  public static void main(String[] args)
  {
    // Creating LinkedList
    LinkedList< String> linkedList = new LinkedList< String>();
    linkedList.add("Delhi");
    linkedList.add("NewYork");
    linkedList.add("Moscow");
    linkedList.add("Dubai");
    // Displaying LinkedList
    System.out.println(linkedList);
    // Removing Elements
    linkedList.remove("Moscow");
    System.out.println("After Deleting Elements: \n"+linkedList);
    // Removing Second Element
    linkedList.remove(1);
    System.out.println("After Deleting Elements: \n"+linkedList);
  }
}
  

[Delhi, NewYork, Moscow, Dubai] After Deleting Elements: [Delhi, NewYork, Dubai] After Deleting Elements: [Delhi, Dubai]

Traversing Elements of LinkedList

Since LinkedList is a collection then we can use loop to iterate its elements. In this example we are traversing elements. See the below example.

  
import java.util.*;
class Demo
{
  public static void main(String[] args)
  {
    // Creating LinkedList
    LinkedList< String> linkedList = new LinkedList< String>();
    linkedList.add("Delhi");
    linkedList.add("NewYork");
    linkedList.add("Moscow");
    linkedList.add("Dubai");
    // Traversing ArrayList
    for(String element : linkedList) {
      System.out.println(element);    
    }
  }
}
  

Delhi NewYork Moscow Dubai

Get size of LinkedList

Sometimes we want to know number of elements an LinkedList holds. In that case we use size() then returns size of LinkedList which is equal to number of elements present in the list.

  
import java.util.*;
class Demo
{
  public static void main(String[] args)
  {
    // Creating LinkedList
    LinkedList< String> linkedList = new LinkedList< String>();
    linkedList.add("Delhi");
    linkedList.add("NewYork");
    linkedList.add("Moscow");
    linkedList.add("Dubai");
    // Traversing ArrayList
    for(String element : linkedList) {
      System.out.println(element);    
    }
System.out.println("Total Elements: "+linkedList.size());
  }
}
  

Delhi NewYork Moscow Dubai Total Elements: 4

Sorting LinkedList Elements

To sort elements of an LinkedList, Java provides a class Collections that includes a static method sort(). In this example, we are using sort method to sort the elements.

  
import java.util.*;
class Demo
{
  public static void main(String[] args)
  {
    // Creating LinkedList
    LinkedList< String> linkedList = new LinkedList< String>();
    linkedList.add("Delhi");
    linkedList.add("NewYork");
    linkedList.add("Moscow");
    linkedList.add("Dubai");
    // Sorting elements
    Collections.sort(linkedList);
    // Traversing ArrayList
    for(String element : linkedList) {
      System.out.println(element);    
    }
  }
}
  

Delhi Dubai Moscow NewYork