My config class. Uses Java properties instead of XML. Works fine when bundled in a JAR and run in the OS (as opposed to being run in the IDE). I made it a while ago so I don't really remember how everything works, or how efficient it is...

gistfile1.java

package com.moomoohk.ThreeD;

import java.util.prefs.Preferences;

public class Config
{
  public static Preferences prefs = Preferences.userNodeForPackage(Config.class);

	public void save(String key, int val)
	{
		System.out.println("Saving " + val + " to " + key);
		int def = 0;
		if (key.equalsIgnoreCase("width"))
			def = 800;
		if (key.equalsIgnoreCase("height"))
			def = 600;
		prefs.putInt(key, val);
		System.out.println("Saved " + prefs.getInt(key, def) + " to " + key);
	}

	public void loadDimensions()
	{
		Preferences prefs = Preferences.userNodeForPackage(getClass());
		int w = prefs.getInt("Width", 800);
		int h = prefs.getInt("Height", 600);
		setRes(w, h);
	}

	public int getSel()
	{
		Preferences prefs = Preferences.userNodeForPackage(getClass());
		int w = prefs.getInt("Width", 800);
		if (w == 640)
			return 1;
		if (w == 800)
			return 2;
		return 3;
	}

	public void setRes(int width, int height)
	{
		Display.dim.height = height;
		Display.dim.width = width;
	}
}