Linked List Implementation in Java

LinkedList.java

//Implementing Singly Linked List in Java

class LinkedList{
  Node head; //Reference to the Node class to create head node of the list.

  //Node class to create nodes. here we make static class so that main method
  //can recoginze it.
  static class Node{
    int data;
    Node next;

    //Constructor
    Node(int d){
      data = d;
      next = null;
    }
  }

  //To print the data of all the nodes of the list.
  public void printList(){
    Node tempNode = head;
    while (tempNode!= null) {
      System.out.print(tempNode.data + " " );
      tempNode = tempNode.next;
    }
  }

  public static void main(String[] args) {
    LinkedList list = new LinkedList();
    list.head = new Node(5);
    Node secondNode = new Node(6);
    Node thirdNode = new Node(7);

    //Connecting nodes of the list by adding Reference to the next node.
    list.head.next = secondNode;
    secondNode.next = thirdNode;

    //Printing data of all the nodes of the list.
    list.printList();
  }
}

LinkedList_Insertion.java

//Insertion operation in a Linked List.
//There are three possibilities for Insertion in a Linked List.
//1- Insertion at the start of the list.
//2- Insertion at the end of the list.
//3- Insertion after a specific node.

//Insertion at the start of the list.

public void insertionAtStart(int new_node_data){
  Node new_node = new Node(new_node_data);
  //Creating a new node and adding data into it.
  //Adding reference of head node into new node.
  new_node.next = head;
  //And now new node becomes the head of the list.
  head = new_node;
  }
}

//Insertion at the end of the list.

public void insertionAtEnd(int new_node_data){
  //Creating a new node and adding data into it.
  Node new_node = new Node(new_node_data);

  //Check if there the list is empty.
  if(head == null){
    head = new_node;
  }
  //else traverse through the whole list, untill we reach the last node.
  else{
    Node last = head;
    while(last.next != null){
      last = last.next;
    }
    last.next = new_node;
  }
}

//Insertion after a specific node.

//Here we pass data of new node as well as previous node - after which the new
//node is to be inserted.
public void insertionAfterSpecificNode(Node prev_node, int new_node_data){
  if(prev_node == null){
    System.out.println("Error -- Given Previous Node  is Null.");
    return ;
  }
  Node new_node = new Node(new_node_data);
  new_node.next = prev_node.next;
  prev_node.next = new_node;
}

LinkedList_with_All_Insertion.java

//Implementing Linked List with all three insertion possibilities in Java

class LinkedList{
  Node head; //Reference to the Node class to create head node of the list.

  //Node class to create nodes. here we make static class so that main method
  //can recoginze it.
  static class Node{
    int data;
    Node next;

    //Constructor
    Node(int d){
      data = d;
      next = null;
    }
  }
  
  //Insertion at the start of the list.

  public void insertionAtStart(int new_node_data){
    Node new_node = new Node(new_node_data);
    //Creating a new node and adding data into it.
    //Adding reference of head node into new node.
    new_node.next = head;
    //And now new node becomes the head of the list.
    head = new_node;
    }

  //Insertion at the end of the list.

  public void insertionAtEnd(int new_node_data){
    //Creating a new node and adding data into it.
    Node new_node = new Node(new_node_data);

    //Check if there the list is empty.
    if(head == null){
      head = new_node;
    }
    //else traverse through the whole list, untill we reach the last node.
    else{
      Node last = head;
      while(last.next != null){
        last = last.next;
      }
      last.next = new_node;
    }
  }

  //Insertion after a specific node.

  //Here we pass data of new node as well as previous node - after which the new
  //node is to be inserted.
  public void insertionAfterSpecificNode(Node prev_node, int new_node_data){
    if(prev_node == null){
      System.out.println("Error -- Given Previous Node  is Null.");
      return ;
    }
    Node new_node = new Node(new_node_data);
    new_node.next = prev_node.next;
    prev_node.next = new_node;
  }

  //To print the data of all the nodes of the list.
  public void printList(){
    Node tempNode = head;
    while (tempNode!= null) {
      System.out.print(tempNode.data + " " );
      tempNode = tempNode.next;
    }
  }

  public static void main(String[] args) {
    LinkedList list = new LinkedList();
    //Creating a head node for the list.
    list.head = new Node(5);
    //Inserting a new node at the end of the list.
    list.insertionAtEnd(10);
    //Insertinf a new node at the start of the list.
    list.insertionAtStart(15);
    //inserting a new node after second node in the list.
    list.insertionAfterSpecificNode(list.head.next,7);

    //Printing data of all the nodes of the list.
    list.printList();
  }

}