求教数据流哪里写错了

package 注册登录;import java.io.*;
import java.util.*;import static java.lang.System.in;
/**
 * Created by Administrator on 2018/3/25.
 */
public class test {
    public static Scanner sc;
    public static List<user> c;
    //File f = new File("G:/java.txt");
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        c = new ArrayList();
        while (true) {            Scanner sc = new Scanner(System.in);
            System.out.println("1.注册账户  2.登录账户");
            int input = sc.nextInt();            switch (input){
                case 1 :
                    register();
                    break;
                case 2 :
                    login();
                    break;
            }        }
    }
    public static void register() throws IOException {
        File f = new File("G:/java.txt");
        FileOutputStream fo = new FileOutputStream(f,true);
        ObjectOutputStream oo = new ObjectOutputStream(fo);
        Scanner sc = new Scanner(System.in);
        System.out.println("1.请输入用户名和密码，用逗号隔开.");
        String s = sc.nextLine();
        String[] arr = s.split(",");//        c.add(new user(arr[0],arr[1]));
        System.out.println("注册成功");
        oo.writeObject(s);
        oo.close();    }
    public static void login() throws IOException, ClassNotFoundException {
        File f = new File("G:/java.txt");
        FileInputStream fi = new FileInputStream(f);
        ObjectInputStream oi = new ObjectInputStream(fi);
        Scanner sc = new Scanner(System.in);
        System.out.println("1.请输入用户名和密码");
        String s = sc.nextLine();//        user u = (user)oi.readObject();
//        while (u !=null){
//            c.add(u);
//        }
        c = (List<user>)oi.readObject();
        String[] arr = s.split(",");        if(c.contains(new user(arr[0],arr[1])))
            System.out.println("welcome"+arr[0]);
        else
            System.out.println("wrong");
        oi.close();    }
}
1.注册账户  2.登录账户
2
1.请输入用户名和密码
1,
Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.util.List
at 注册登录.test.login(test.java:64)
at 注册登录.test.main(test.java:31)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)Process finished with exit code 1