Postgre SQL / Java比较数据库中的用户名和散列密码

Question

I am trying to implement a function which asks the user to enter their username. Then checks if the user name entered is in the database. The below code is a test code before I start coding more functions. My problem is that the code doesn't print out when the username entered doesn't exist in the database. However, it does print out when the username does exist. For example, ohm34 exists in the database, so when the program is ran it prints: User Name Does Exist . brian23 doesn't exist in the database so it should print User Name Does Not Exist but it doesn't. The program is ran simply by using java ProgramName

 public void letLogin() throws SQLException
 {
    Scanner stdin;
    String sql;
    Statement select = connection.createStatement();
    String userNames;

    stdin = new Scanner(System.in);

    System.out.print("Enter your user name: ");
    user_name = stdin.next();

  sql = "SELECT " + "user_name " + "FROM" + " users8" + " where 
        user_name = "
        + "'" + user_name + "'";

  r = select.executeQuery(sql);
  if(r.next())
  {
     userNameCounter = r.getString("user_name");
     if(userNameCounter.equals(user_name))
     {
        System.out.println("User Name Does Exist");

     }

     else
     {
        System.out.println("User Does Not Exist");
     }
  }

  } // method letLogin

Answer 1

When you create the query, if the user exists, it will enter on the if(r.next()). But, if the user does not exists, it will not enter. You'll have to put the else with the if(r.next()).

Example:

if(r.next())
{
   System.out.println("User Name Does Exist");
}
else
{
   System.out.println("User Does Not Exist");
}

You don't need to get the user with getString(), because you are testing the user on the query, so if the query returns something (r.next() == true), the user exists.