how to change new value from PropertyChangeEvent

I have a preferences page in eclipse (IDE) and every time user change value from a property and click ok, those values are saved in a preferences files. I'm using a listener for getting new values. But what I need to do is get this new value -> process -> and save this new value processed and store in preference file. The implementation that I have is this:

public class PreferenceUtil extends FieldEditorPreferencePage implements IWorkbenchPreferencePage {

private StringFieldEditor stringUser;

@Override
public void init(IWorkbench arg0) {
    setPreferenceStore(Activator.getDefault().getPreferenceStore());
}

@Override
protected void createFieldEditors() {
stringUser = new StringFieldEditor("user", "&User:\t\t", getFieldEditorParent());

addField(stringUser);

Activator.getDefault().getPreferenceStore().addPropertyChangeListener(new IPropertyChangeListener() {


@Override
public void propertyChange(PropertyChangeEvent event) {
            if (event.getProperty() == "user") {
                user = event.getNewValue().toString();                  
                //ConnectionValues.convert(user) is an algorithm that process a string
                stringUser.setStringValue(ConnectionValues.convert(user));                                      
            }
}
}
}

I found the solution, what I had to do is, create a StringFieldEditor and then use it in a listener for get the new value, process this value using my own algorithm for encrypt, then set this value encrypted to StringFieldEditor.

public class PreferenceUtil extends FieldEditorPreferencePage implements IWorkbenchPreferencePage {

StringFieldEditor secretSFE;

@Override
public void init(IWorkbench arg0) {
    setPreferenceStore(Activator.getDefault().getPreferenceStore());
}


@Override
protected void createFieldEditors() {

        secretSFE = new StringFieldEditor("secret", "&Secret:\t\t",getFieldEditorParent()){

        @Override
        protected void doFillIntoGrid(Composite parent, int numColumns) {
            super.doFillIntoGrid(parent, numColumns);

            getTextControl().setEchoChar('*');
        }

    };
    addField(secretSFE);

    Activator.getDefault().getPreferenceStore().addPropertyChangeListener(new IPropertyChangeListener() {

        @Override
        public void propertyChange(PropertyChangeEvent event) {
            if("secret".equalsIgnoreCase(event.getProperty())){
                String valueToEncrypt = (String)event.getNewValue();
                valueToEncrypt = ConnectionValues.convert(valueToEncrypt);
                secretFFE.setStringValue(valueToEncrypt);
            }
        }
    });      
  }
}