求大神解答，用最基础的java代码，，，java小白一个。。。

http://download.csdn.net/detail/caoyang1405/3270044

http://www.pudn.com/downloads162/sourcecode/java/detail737260.html

http://zhidao.baidu.com/link?url=pmVCcr5PE9MuVNFAQkL7K0G74YqH2b6g2PDjM_iFbiB1J92MSxg0tinNcesNfvLzTi32HrtJK_2GaZqgtA09Ua

参考下

 public static final String info1 = "登录成功！";
    public static final String info2 = "请输入用户名密码！";
    public static final String info3 = "用户名不能为空！";
    public static final String info4 = "密码不能为空！";
    public static final String info5 = "密码错误";
    public static final String info6 = "用户不存在";



    /**
     * 获取已经存在的用户信息
     * @return
     */
    public static List<Map<String,String>> getExistsUser(){
        List<Map<String,String>> userList = new ArrayList<>();
        Map<String,String> user1 = new HashMap<String,String>();
        user1.put("username", "张三");
        user1.put("password", "123");
        userList.add(user1);
        Map<String,String> user2 = new HashMap<String,String>();
        user2.put("username", "李四");
        user2.put("password", "456");
        userList.add(user2);
        Map<String,String> user3 = new HashMap<String,String>();
        user3.put("username", "王五");
        user3.put("password", "789");
        userList.add(user3);

        return userList;
    }

    public Map<String,String> userLogin(String loginInfo,int logintime){


        Map<String,String> resultMap = new HashMap<>();
        if(null == loginInfo || loginInfo.indexOf(",") < 0){
            resultMap.put("success", "false");
            resultMap.put("describe", info2);
        }else{
            if(null == loginInfo.split(",")[0] || "" == loginInfo.split(",")[0]){
                resultMap.put("success", "false");
                resultMap.put("describe", info3);
            }else if(null == loginInfo.split(",")[1] || "" == loginInfo.split(",")[1]){
                resultMap.put("success", "false");
                resultMap.put("describe", info4);
            }else{
                List<Map<String,String>> userList = getExistsUser();
                String[] userInfo = loginInfo.split(",");
                String username = userInfo[0];
                String password = userInfo[1];

                Map<String,String> loginUser = new HashMap<>();
                loginUser.put("username", username);
                loginUser.put("password", password);

                if(userList.contains(loginUser)){
                    resultMap.put("success", "true");
                    resultMap.put("describe", info1);
                    return resultMap;
                }

                Set<String> userSet = new HashSet<>();
                Set<String> nameSet = new HashSet<>();
                for(Map<String,String> user : userList){
                    userSet.add(user.get("username")+","+user.get("password"));
                    nameSet.add(user.get("username"));
                }
                if(!nameSet.contains(username)){
                    resultMap.put("success", "true");
                    resultMap.put("describe", info6);
                }else if(!userSet.contains(loginInfo)){
                    resultMap.put("success", "true");
                    resultMap.put("describe", info5);
                }

            }


        }


        return resultMap;
    }

自己再优化一下吧

import java.util.Scanner;
public class Login {
public static void main(String[] args) {
Users a = new Users("张三","123");
Users b = new Users("李四","456");
Users c = new Users("王五","789");
int temp=0;
Scanner input = new Scanner(System.in);
while(temp<3){
System.out.print("请输入用户名：");
String username = input.next();
if(username.equals("")||username==null){
System.out.println("用户名不能为空！");
temp++;
continue;
}
System.out.print("请输入密码:");
String password = input.next();
if(password.equals("")){
System.out.println("密码不能为空");
temp++;
continue;
}
if((!username.equals(a.getUsername()))&&(!username.equals(b.getUsername())
)&&(!username.equals(c.getUsername()))){
System.out.println("不存在名为"+username+"的用户！");
temp++;
continue;
}else if((!password.equals(a.getPassword()))&&(!password.equals(b.getPassword())
)&&(!password.equals(c.getPassword()))){
System.out.println("密码错误！");
temp++;
continue;
}

        if((username.equals(a.getUsername())&&password.equals(a.getPassword()))
                ||(username.equals(b.getUsername())&&password.equals(b.getPassword()))
                        ||(username.equals(c.getUsername())&&password.equals(c.getPassword()))){
            System.out.println("登录成功！");
            break;
        }
    }

    if(temp==3){

        System.out.println("退出程序！");
        System.exit(0);
    }
}

}
class Users{

private String username;
private String password;

public Users(String username,String password){
    this.username = username;
    this.password = password;
}

public String getUsername() {
    return username;
}

public void setUsername(String username) {
    this.username = username;
}

public String getPassword() {
    return password;
}

public void setPassword(String password) {
    this.password = password;
}

}

这该不会楼主的大学计算机编程作业吧