在Java中将字符串写入文件

以下两节课。我在用Java将字符串写入文件时遇到问题。为什么在我的文件xml.txt中我得到空值？为什么我不能写String a = px.readXml（url）？在xml.txt中，我只有null

package xml;

import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.*;
import java.util.StringTokenizer;
import javax.xml.parsers.*;
import org.w3c.dom.*;
import org.xml.sax.SAXException;

public class PrintXml {

    public String readXml(URL url) throws ParserConfigurationException, MalformedURLException, IOException, SAXException
    {
        //URL url = new URL("http://www.nbp.pl/kursy/xml/a093z150515.xml");
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder = factory.newDocumentBuilder();
        Document doc = builder.parse(url.openStream());
        Element root = doc.getDocumentElement();
        //System.out.println(root.getTagName());
        NodeList children = root.getChildNodes();
        int liczbaLinijek = children.getLength();
        for(int i = 0; i<children.getLength();i++)
        {
            Node child = children.item(i);
            if (child instanceof Element)
            {
                Element childElement = (Element)child;
                NodeList childrenPozycja = childElement.getChildNodes();
                for (int j = 0; j<childrenPozycja.getLength(); j++)
                {
                    Node childPozycja = childrenPozycja.item(j);
                    if (childPozycja instanceof Element)
                    {
                        String nameChf = "CHF";
                        Double kurs;
                        Element childPozycjaElement = (Element) childPozycja;
                        String listaKursow = childPozycjaElement.getTextContent();

                        //System.out.println(listaKursow);
                    }
                }
            }
        }
        return null;
    }

    public String writeXml(String toFile) throws IOException
    {   
        PrintWriter out = new PrintWriter(new FileWriter("xml.txt"));
        out.println(toFile);
        out.close();
        return null;
    }
}

这是测试类：

package xml;

import java.io.IOException;
import java.io.PrintWriter;
import java.net.*;
import java.util.StringTokenizer;
import javax.xml.parsers.*;
import org.w3c.dom.*;
import org.xml.sax.SAXException;

public class PrintXmlTester {

    public static void main(String[] args) throws MalformedURLException,
        ParserConfigurationException, SAXException, IOException {

            URL url = new URL("http://www.nbp.pl/kursy/xml/a093z150515.xml");
            PrintXml px = new PrintXml();
            String a = px.readXml(url);        
            px.writeXml(a);
        }
    }
}