The error I get is: Error: Could not find or load main class Main Caused by: java.lang.ClassNotFoundException: Main

I am new to Java and dont understand what to do in this instance or how to fix this code to run

I think this has something to do with classes due to the error description, but I am not sure

import java.util.Scanner;
import java.util.regex.*;

class Login{
    String user;
    String password;
    String firstName;
    String lastName;

    boolean checkUserName(String user){
        if(user.length()>5 || user.indexOf('_')==-1){
            return false;
        }
        else{
            return true;
        }
    }

    boolean checkPasswordComplexity(String password){
        String regex = "^(?=.*[0-9])"
                         "(?=.*[a-z])(?=.*[A-Z])"
                         "(?=.*[@#$%^& =])"
                         "(?=\\S $).{8,20}$";
        Pattern p = Pattern.compile(regex);
        Matcher m = p.matcher(password);
        return m.matches();
    }

    boolean loginUser(String user,String password){
        if(user.equals(this.user) && password.equals(this.password)){
            return true;
        }
        else{
            return false;
        }
    }

    String returnLoginStatus(String user,String password){
        if(loginUser(user, password)){
            return "Welcome " firstName lastName " it is greater to see you again.";
        }
        else{
            return "Username or passoword incorrect, please try again.";
        }
    }

    void registerUser(String firstName,String lastName,String user,String password){

        if(checkUserName(user)){
            this.user = user;
            System.out.println("Username successfully Captured");
        }
        else{
            System.out.println("Username is not correctly formatted, please ensure that your username contains an underscore and is no more than 5 characters long.");
        }
       if(checkPasswordComplexity(password)){
           this.password = password;
            System.out.println("Password successfully captured");
        }
        else{
            System.out.println("Password is not correctly formatted, please ensure that the passoword contains atleast 8 characters, a capital letter, a number and a special character.");
        }
        if(checkUserName(user) && checkPasswordComplexity(password)){
            this.firstName = firstName;
            this.lastName = lastName;
            System.out.println(returnLoginStatus(user, password));
        }
    }
}

class Registration{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        
        System.out.print("Enter the username:");
        String user = sc.nextLine();
        System.out.print("Enter the password:");
        String password = sc.nextLine();
        System.out.print("Enter your First Name:");
        String firstName = sc.nextLine();
        System.out.print("Enter Your Last Name:");
        String lastName = sc.nextLine();
        sc.close();
        Login user1 = new Login();
        user1.registerUser(firstName, lastName, user, password);

    }
   
    
}

CodePudding user response：

Whenever Java code is compiled on that website, the website's compiler looks for a class named "Main" to start execution from. Once its compiler finds that class, it calls whatever method has the signature of public static void main(String[] args) . In your case, there's no class named "Main", so the compiler doesn't know where to start running the program. An easy fix to this is changing the class name "Registration" to "Main".