So implementieren Sie eine einfache Anmeldung und Registrierung in Java

import java.util.Random;
import java.util.Scanner;

public class Login {
    public static void main(String[] args) {
        //定义内置账号、密码
        String account = "xiaojian";
        String passwd = "123456";
        String code = "";
        //新建键盘输入对象
        Scanner scan = new Scanner(System.in);
        Login login = new Login();
        //调用输入账号
        String accountinput = login.accountinput(scan);
        //调用输入密码
        String passwdinput = login.passwdinput(scan);
        //验证码生成
        code = login.code(4);
        //输出验证码
        System.out.println("验证码为(请注意大小写):"+code);
        //调用输入验证码
        String codeinput = login.codeinput(scan);
        //调用验证方法
        login.Contrast(account,passwd,code,accountinput,passwdinput,codeinput);
    }

    //账号输入
    public String accountinput(Scanner scan) {
        System.out.print("请输入账号:");
        String accountinput = scan.next();
        return accountinput;
    }

    //密码输入
    public String passwdinput(Scanner scan) {
        System.out.print("请输入密码:");
        String passwdinput = scan.next();
        return passwdinput;
    }

    //随机验证码生成
    public String code(int length) {
        //定义一个空字符串
        String code = "";
        //定义一个scanner
        //for循环
        for (; length > 0; length--) {
            //随即判断生成数字还是字母
            // 字母怎么生成？65~90大写字母 97~122小写字母
            Random rand = new Random();
            int flag = rand.nextInt(3);
            //判断字母大写还是小写
            switch (flag) {
                case 0:
                    Random random = new Random();
                    code += rand.nextInt(10);
                    break;
                case 1:
                    Random random1 = new Random();
                    code += (char) (rand.nextInt(26) + 65);
                    break;
                case 2:
                    Random random2 = new Random();
                    code += (char) (rand.nextInt(26) + 97);
                    break;
            }
        }
        return code;
    }
    //验证码输入
    public String codeinput(Scanner scan) {
        System.out.print("请输入验证码:");
        String codeinput = scan.next();
        return codeinput;
    }

    //账号密码验证码对比
    public void Contrast(String account, String passwd, String code,
                         String accountinput, String passwdinput, String codeinput){
        if (!code.equals(codeinput)){
            System.out.println("验证码错误");
            return;
        }
        if (!account.equals(accountinput)){
            System.out.println("账号错误");
            return;
        }
        if (!passwd.equals(passwdinput)){
            System.out.println("密码错误");
            return;
        }
        System.out.println("你好，"+"【"+account+"】"+"欢迎回来!");
    }
}