Java仪表板登录错误

Question

我一直在做java项目。我在仪表板登录代码中遇到了麻烦。我的登录代码如下。

String fieldtxtname=txtusername.getText();
String fieldtxtpw=txtpassword.getText();
String fieldtxtutype=txttypeof.getText();
try{

    Connection cnn=DriverManager.getConnection("jdbc:mysql://localhost/student","root","");
    Statement stm=cnn.createStatement();

    String sqlQuery="select username,password,typeof from tbl_user";

    ResultSet rs=stm.executeQuery(sqlQuery);

      while(rs.next()){
        String unamedb=rs.getString("username");
        String pworddb=rs.getString("password");
        String tofdb=rs.getString("typeof");
         if(fieldtxtname.equals ("")||fieldtxtpw.equals ("")||fieldtxtutype.equals ("")){
            System.out.println("The fields arenot filled properly");
            }
          else if(fieldtxtname.equals(unamedb)&&fieldtxtpw.equals(pworddb)&&fieldtxtutype.equals(tofdb)){
            System.out.println("Access granted");
            }
         else
            {
            System.out.println("Unknown username"); 
            }


      } 

}
catch(SQLException ex){
    System.out.println(ex);
} 

代码工作正常，除了当授予访问权限时，它打印为

 Access granted
    Unknown username
    Unknown username
    Unknown username
    Unknown username

字段为空时的情况类似，即它们打印五次，如下所示。

The fields arenot filled properly
The fields arenot filled properly
The fields arenot filled properly
The fields arenot filled properly
The fields arenot filled properly

.我不知道问题的原因。请帮忙，我提前非常感谢。

Answer 1

向您发布的当前代码实现添加注释，以及如何克服在输出上多次打印的问题。

ResultSet rs=stm.executeQuery(sqlQuery); // this is returning 5 elements

while(rs.next()) {
    String unamedb=rs.getString("username");
    String pworddb=rs.getString("password");
    String tofdb=rs.getString("typeof");
    if(fieldtxtname.equals ("")||fieldtxtpw.equals ("")||fieldtxtutype.equals ("")){
        System.out.println("The fields are not filled properly.");
        break;
// you might want to break the loop if the fields are empty in db
    } else if(fieldtxtname.equals(unamedb)&&fieldtxtpw.equals(pworddb)&&fieldtxtutype.equals(tofdb)) {
        System.out.println("Access granted");
        break;
// you might want to break the loop once the username, passwrod is found for access to be granted
    } else {
        System.out.println("Unknown username"); // just to display information
    }
} 

PS - 理想情况下，对于用户名和密码的唯一组合，我希望它仅返回 1 个元素。

编辑 - 要使用 PreparedStatement 来解决上述问题而不使用循环，请查看 answer from @Arvind