public class EmailException extends Exception {

  EmailException(String msg) {
    super(msg);
  }


}

import java.util.LinkedList;
import java.util.List;

public class UserRegiste {


  public List usr;

  UserRegiste() {
    usr = new LinkedList<>();
  }

  public void regist(String email) throws EmailException {
    if (email.equals("123@qq.com")) {//抛出自定义的异常
      throw new EmailException("该用户已经存在了");
    } else {//如果不存在错误，则将信息添加进集合中
      usr.add(email);
    }
  }


}

import java.io.Console;
import java.util.Scanner;
 
 
public class one {
  public static void main(String amg[]) {
    UserRegiste userRegiste = new UserRegiste();
 
    Scanner scanner = new Scanner(System.in);
    System.out.println("邮箱:");
    String get = scanner.next();
 
 
    try {
      userRegiste.regist(get);
    } catch (EmailException e) {
      e.printStackTrace();
    }
 
    System.out.println("得到信息:");
    System.out.println(userRegiste.usr.get(0));
 
  }
}