错误-对空结果集的非法操作

Question

我正在尝试运行此代码块，但是当我运行代码时，它显示错误-对空结果集的非法操作。

Connection conn =null;
Statement st =null;
ResultSet rs =null;
String url ="jdbc:mysql://localhost:3306/smartmall?autoReconnect=true&useSSL=false";
String unam="root";
String pass="MARtena123@";

try{
    conn =DriverManager.getConnection(url, unam, pass);
    st =conn.createStatement();
    rs = st.executeQuery("select ssn from customer where ssn='"+ssntex.getText()+"'");
    rs.next();
    String strng =rs.getString("ssn");

    JOptionPane.showMessageDialog(null,"query executed");
}catch(Exception ex)
{
    JOptionPane.showMessageDialog(null,ex.toString());
}