I used this small servlet to convert RGB values to Hue's specific XY encoding.

HueRGBConverter.java

package com.hypernephelist;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.philips.lighting.hue.sdk.utilities.PHUtilities;

/**
 * Servlet implementation class HueRGBConverter
 */
@WebServlet("/convert")
public class HueRGBConverter extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public HueRGBConverter() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// Get Parameters
		int r = Integer.parseInt(request.getParameter("r"));
		int g = Integer.parseInt(request.getParameter("g"));
		int b = Integer.parseInt(request.getParameter("b"));
		String modelId = request.getParameter("modelid");
		
		// Calculate XY
		// Color is "complicated", see http://www.developers.meethue.com/documentation/core-concepts#color_gets_more_complicated
		// For Model ID, see http://www.developers.meethue.com/documentation/supported-lights
		float xy[] = PHUtilities.calculateXYFromRGB(r, g, b, modelId);
		
		// Return JSON
		// It can be sent directly to the Hue API, see http://www.developers.meethue.com/documentation/lights-api#16_set_light_state
		String x = Float.toString(xy[0]);
		String y = Float.toString(xy[1]);
		response.getWriter().append("{\"xy\":[").append(x).append(",").append(y).append("]}");
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}