java.sql.SQLException : Illegal operation on empty result set. Else 语句不起作用

Question

try {
                    //Class.forName("org.h2.Driver");
                    //Connection conn = DriverManager.getConnection("jdbc:h2:~/test","admin","admin");
                    String m=textField_1.getText();
                    Connection conn = DriverManager.getConnection("jdbc:mysql://localhost/logindetails","root","root");
                    //Connection conn1 = DriverManager.getConnection("jdbc:mysql://localhost/logindetails","root","root");


                    String query1="Select * from Details where Username='"+m+"'";
                    PreparedStatement statement1 = conn.prepareStatement(query1);
                    ResultSet res=statement1.executeQuery(query1);
                    res.next();

                    String user;
                    user=res.getString("Username");

                    {

                        if(m.compareTo(user)==0)
                        {

                            JOptionPane.showMessageDialog(null, "CONNECTION SUCCESSFULL & DATA SAVED");

                            DataEntry.this.dispose();
                            login ing= new login();
                            ing.setVisible(true);



                        }

                        else
                        {   

                            String query = "insert into Details values(?,?,?,?,?)";
                            PreparedStatement statement=conn.prepareStatement(query);
                            statement.setString(1, textField.getText());


                            statement.setString(2, textField_2.getText());
                            statement.setString(3, textField_3.getText());
                            statement.setString(4, textField_1.getText());
                            statement.setString(5,String.valueOf(passwordField.getPassword()));
                            statement.execute();

                            }


                    }



                }   

在这个 else 部分不起作用。它抛出错误“对空结果集进行非法操作”。我还交换了两个 if-else 语句的主体，但这对我没有帮助。If 语句主体工作正常。我认为错误是 ResultSet 语句。提前致谢。

Answer 1

您没有检查 ResultSet 是否包含结果。因此，在空 ResultSet 上调用 res.getString("Username") 时会出现异常。

更改:

res.next();

String user;
user=res.getString("Username");

至:

String user = null;
if (res.next())
    user=res.getString("Username");

然后，代替:

if(m.compareTo(user)==0) {
    ...
}

写:

if (user != null) {
    ...
}