Failed to display data list in Java

Data lost in the file

The problem you're suffering is because PrintWriter , as well as PrintStream , does not automatically dump the contents of the buffer into the data stream if it is not activated in some of the constructor methods, so that data could be left unsent (in this case case, without writing to the file).

Let's take the following example:

import java.io.PrintWriter;
public class Prueba {
  public static void main(String[] args) {
    try {
      PrintWriter pw = new PrintWriter("prueba.txt");
      pw.println("hola");
    } catch (Exception e) { }
  }
}

When executed, the file would be empty.

To solve it you have three possibilities:

• Force the writing by using flush() or close() .
• Enable automatic data dump after print() or println() using a constructor that allows it.

Force explicit writing with flush()

import java.io.PrintWriter;
public class Prueba {
  public static void main(String[] args) {
    try {
      PrintWriter pw = new PrintWriter("prueba.txt");
      pw.println("hola");
      pw.flush();
    } catch (Exception e) { }
  }
}

Close the file with close()

import java.io.PrintWriter;
public class Prueba {
  public static void main(String[] args) {
    try {
      PrintWriter pw = new PrintWriter("prueba.txt");
      pw.println("hola");
      pw.close();
    } catch (Exception e) { }
  }
}

Automatic dump with the constructor OutputStream

import java.io.PrintWriter;
import java.io.FileOutputStream;
public class Prueba {
  public static void main(String[] args) {
    try {
      PrintWriter pw = new PrintWriter(new FileOutputStream("prueba.txt"), true);
      pw.println("hola");
      pw.close();
    } catch (Exception e) { }
  }
}

Prepared consultations

You should not concatenate strings to an SQL query. You could have problems if the variable contains single quotes (for example, that the parameter pais is worth Salem's Lot ).

To fix it you should use JDBC prepared queries overloading in the following way the method consultar() of your previous question :

public static ResultSet consultar(
  String sql,
  String parametro1
) throws SQLException, ClassNotFoundException {
  ConectarSingleton conexion = new ConectarSingleton();
  Connection con = ConectarSingleton.getConexion(); 
  /* Preparamos la consulta */
  PreparedStatement sentencia = con.prepareStatement(sql);
  /* Asignamos al primer ? el valor de parametro1 */
  sentencia.setString(1, parametro1);
  /* Ejecutamos la consulta como de costumbre */
  ResultSet resultados = sentencia.executeQuery();
  return resultados;
}  

public static void generarArchivo(
  String nombreArchivo,
  String pais
) throws SQLException, ClassNotFoundException, FileNotFoundException {
  ConectarSingleton conexion = new ConectarSingleton();
  Connection con = conexion.getConexion();
  PrintWriter pw = new PrintWriter(nombreArchivo);

  /* Llamamos al nuevo método indicando el primer parámetro: "pais" */
  ResultSet rs = GestionClassic.consultar(
    "SELECT * FROM oficinas WHERE pais = ?",
    pais
  );
  while(rs.next()) {
    pw.println(
      rs.getString(1) + ";"
      + rs.getString(2) + ";"
      + rs.getString(3) + ";"
      + rs.getString(4) + ";"
      + rs.getString(5) + ";"
      + rs.getString(6) + ";"
      + rs.getString(7) + ";"
      + rs.getString(8) + ";"
      + rs.getString(9)
    );
  }
}

This way, when you indicate a parameter that method will be executed that has an additional parameter of type String , assigning the first ? its value in a correct way without problems with single quotes ' and protecting your code against the Nefarious Effects of the SQL injection .