Substraction of two database-fields

Question

Hello everybody,
I'm busy with a java-assignment for school. We have to build an application that functions like a ATM. We have to get and save information in a database.
We're working with several files, and the following is one of them.
import java.awt.*;
import java.awt.event.*;
import java.sql.*;
import javax.swing.*;
public class UpdateRecord implements ActionListener {
private ScrollingPanel fields;
private JTextArea output;
private Connection connection;
public UpdateRecord( Connection c, ScrollingPanel f,
JTextArea o )
connection = c;
fields = f;
output = o;
public void actionPerformed( ActionEvent e )
try {
Statement statement = connection.createStatement();
if ( ! fields.id.getText().equals( "" ) ) {
          String query = "UPDATE pintabel SET " +
          "saldo='" + fields.saldo.getText() +     "', pin='" + fields.pin.getText() +
          "' WHERE id=" + fields.id.getText();
          output.append( "\nAktie verzonden: " +
          connection.nativeSQL( query ) + "\n" );
          int result = statement.executeUpdate( query );
          if ( result == 1 )
          output.append( "\nGeld is opgenomen\n" );
          else {
          output.append( "\nGeld opnemen mislukt\n" );
          fields.saldo.setText( "" );
          fields.pin.setText( "" );
statement.close();
else
output.append( "\nYou may only update an " +
"existing record. Use Find to " +
"locate the record, then " +
"modify the information and " +
"press Update.\n" );
catch ( SQLException sqlex ) {
sqlex.printStackTrace();
output.append( sqlex.toString() );
The bold line is the problem ("saldo='" + fields.saldo.getText() +).
There's a value in the field saldo, but we also have a certain value in a field called opnemen. What we want to do is substract the value in the opnemen-field from the value in the saldo-field. We've tried several things, but we can't work it out. Can anybody help me?
Many thanks,
Michael Roos

Answer 1

First, you should know that there is a set of updateXXX() methods in Java 2.0. So, when you have the correct row of the resultSet in hand you can use:
updateString(3,"Hello world!")method to update the field in the column 3 to a String with value "Hello world!". This is just an example of course.
To make this example complete see this:
// assume you have the right row from the resultSet in hand.
// assume rs is your resultSet
int foo = rs.getInt(1);
foo += foo*0.05;
// and other calculations here
rs.updateInt(1,foo);
rs.updateRow(); // This line is here so as to make the changes
                // permanent.
                // No need to mention that you cannot change primary
                // keys...Second, I think I've given you a very clear example of what you can do. And since this is a Java assignment you don't want to be spoon-feeded, do you? :)
Hope this helped.
afotoglidis