public static void main(String args[]) {
        Scanner in = new Scanner(System.in);
        String[] userinfo = new String[]{"a","b"};
        while (true) {
            boolean rename = false; // 重命名回复默认值
            System.out.print("请输入用户名：");
            String user = in.next();

            int c = 0;//是这个c的问题吧？你每次都置c=0，c++就相当于没用了。放外面。
            for (int i = 0; i<9; i++) {
                if (user.equals(userinfo[c])) {
                    rename = true;
                    break;
                }
                c++;
            }
            if (rename){
                System.out.println("您的用户名已经被人注册，请选择其他用户名注册。");
                continue;
            }

            System.out.print("请输入密码：");
            String password = in.next();
            System.out.print("请再次输入密码：");
            String passwordagain = in.next();
            if (!passwordagain.equals(password)) {
                System.out.println("密码不一致，请重新输入：");
                continue;
            }
            
            System.out.println("退出请输入0");
            int tuichu = in.nextInt();
            if (tuichu == 0) {
                break;
            }
        }
    }