LinkedList - Add, Remove, Modify and Display Nodes

LinkedList.java

import java.io.IOException;
import java.util.Scanner;

public class LinkedList {

    private final Scanner input = new Scanner(System.in);
    private Node head;

    private class Node {
        String data;
        int intKey;
        Node next;

        public Node(String data, int intKey) {
            this.data = data;
            this.intKey = intKey;
            this.next = null;
        }

        @Override
        public String toString() {
            return "Node{" + "data=" + data + ", intKey=" + intKey + ", next=" + next + '}';
        }
    }

    public void addNodeAtLast(Node newNode) {
        Node first = this.head;
        
        if (first == null) {
            this.head = newNode;
        } else {
            while (first.next != null) {
                first = first.next;
            }

            first.next = newNode;
        }

        System.out.println("Successful!");
        this.printNodes();
    }

    public void addNodeAtFirst(Node newNode) {
        Node first = this.head;
        
        if (first == null) {
            this.head = newNode;
        } else {
            newNode.next = first;
            this.head = newNode;
        }

        System.out.println("Successful!");
        this.printNodes();
    }

    public void addNodeAtKey(int intKey, Node newNode) {
        Node first = this.head;
        boolean flag = false;

        if (first != null) {
            if (first.intKey == intKey) {
                newNode.next = first;
                this.head = newNode;
                flag = true;
            } else {
                while (first.next != null) {
                    if (first.next.intKey == intKey) {
                        newNode.next = first.next;
                        first.next = newNode;
                        flag = true;
                        break;
                    }

                    first = first.next;
                }
            }

            System.out.println("Successful!");
        }

        if (!flag) {
            System.out.println("No Node with key " + intKey);
        }

        this.printNodes();
    }

    public void removeNodeAtFirst() {
        Node first = this.head;
        
        if (first != null) {
            this.head = first.next;
            System.out.println("Successful!");
        } else {
            System.out.println("No Node to remove.");
        }

        this.printNodes();
    }

    public void removeNodeAtLast() {
        Node first = this.head;
        
        if (first != null) {
            Node previous = null;
            while (first != null && first.next != null) {
                previous = first;
                first = first.next;
            }

            if (previous != null) {
                previous.next = null;
            } else {
                this.head = null;
            }

            System.out.println("Successful!");
        } else {
            System.out.println("No Node to remove.");
        }

        this.printNodes();
    }

    public void removeNodeAtKey(int intKey) {
        Node first = this.head;
        boolean flag = false;

        if (first != null) {
            if (first.intKey == intKey) {
                if (first.next != null) {
                    this.head = first.next;
                } else {
                    this.head = null;
                }

                flag = true;
            } else {
                while (first.next != null) {
                    if (first.next.intKey == intKey) {
                        if (first.next.next != null) {
                            first.next = first.next.next;
                        } else {
                            first.next = null;
                        }

                        flag = true;
                        break;
                    }

                    first = first.next;
                }
            }

            System.out.println("Successful!");
        }

        if (!flag) {
            System.out.println("No Node with key " + intKey);
        }

        this.printNodes();
    }

    public void modifyNode(Node node) {
        Node first = this.head;
        boolean flag = false;

        if (first != null) {
            if (first.intKey == node.intKey) {
                first.data = node.data;
                flag = true;
            } else {
                while (first.next != null) {
                    if (first.next.intKey == node.intKey) {
                        first.next.data = node.data;
                        flag = true;
                        break;
                    }

                    first = first.next;
                }
            }

            System.out.println("Successful!");
        }

        if (!flag) {
            System.out.println("No Node with key " + node.intKey);
        }

        this.printNodes();
    }

    public void printNodes() {
        System.out.println("List of Node(s):");
        Node node = this.head;
        if (node != null) {
            while (node.next != null) {
                System.out.println(node);
                node = node.next;
            }

            System.out.println(node);
        } else {
            System.out.println("No Node exists.");
        }
    }

    public static void main(String args[]) throws Exception {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter number of initial Node(s): ");
        int numberOfNode = input.nextInt();

        LinkedList linkedList = new LinkedList();
        linkedList.createInitialNodes(numberOfNode);

        linkedList.operation();

        System.out.println("Bye!");
    }

    private void createInitialNodes(int numberOfNode) throws IOException {
        int counter = 1;
        while (numberOfNode > 0) {
            System.out.println("Node(" + counter++ + ")");
            Node node = createNode();
            this.addNodeAtLast(node);
            numberOfNode--;
            System.out.println("-----");
        }
    }

    private void operation() {
        this.menu();
        System.out.print("\nSelect operation: ");
        int operation = input.nextInt();
        System.out.println();
        
        while (operation != 0) {
            Node node;
            int intKey;
            
            switch (operation) {
                case 1:
                    System.out.println("Add Node at the beginning");
                    node = this.createNode();
                    this.addNodeAtFirst(node);
                    break;
                case 2:
                    System.out.println("Add Node at the end/tail");
                    node = this.createNode();
                    this.addNodeAtLast(node);
                    break;
                case 3:
                    System.out.println("Add Node at key value");
                    System.out.print("Enter key value: ");
                    intKey = input.nextInt();
                    node = this.createNode();
                    this.addNodeAtKey(intKey, node);
                    break;
                case 4:
                    System.out.println("Remove Node at the beginning");
                    this.removeNodeAtFirst();
                    break;
                case 5:
                    System.out.println("Remove Node at the end/tail");
                    this.removeNodeAtLast();
                    break;
                case 6:
                    System.out.println("Remove Node by key value");
                    System.out.print("Enter key value: ");
                    intKey = input.nextInt();
                    this.removeNodeAtKey(intKey);
                    break;
                case 7:
                    System.out.println("Modify data of a Node");
                    node = createNode();
                    this.modifyNode(node);
                    break;
                case 8:
                    System.out.println("Display Nodes(s)");
                    this.printNodes();
                    break;
                default:
                    System.out.println("Please try again!");
            }

            this.menu();
            System.out.print("\nSelect operation: ");
            operation = input.nextInt();
            System.out.println();
        }
    }

    private void menu() {
        System.out.println("\nMENU");
        System.out.println("[1] Add Node at the beginning");
        System.out.println("[2] Add Node at the end/tail");
        System.out.println("[3] Add Node at key value");
        System.out.println("[4] Remove Node at the beginning");
        System.out.println("[5] Remove Node at the end/tail");
        System.out.println("[6] Remove Node by key value");
        System.out.println("[7] Modify data of a Node");
        System.out.println("[8] Display Nodes(s)");
        System.out.println("[0] Exit");
    }

    private Node createNode() {
        System.out.print("Enter data: ");
        String data = input.next();
        System.out.print("Enter key: ");
        int intKey = input.nextInt();

        return new Node(data, intKey);
    }
}