XML Datei einlesen und anschließen formatiert ausgeben in Datei

[AssELAss]

Hallo würde gerne diese zwei Methoden verbinden. Aber irgendwie bekomm ich es nicht hin.
Es handelt sich ums einlesen einer XML Datei, die anschließend formatiert ausgegeben wird.

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class HostXMLStatementConv
{
	private BufferedReader fi;
	private BufferedWriter fo;
	private String line;

	public HostXMLStatementConv(String datei_ein, String datei_aus)
	{
		try
		{
			StringBuilder all_content = new StringBuilder();
			this.fi = new BufferedReader(new FileReader(datei_ein));
			this.fo = new BufferedWriter(new FileWriter(datei_aus));

			while ((this.line = this.fi.readLine()) != null)
			{
				all_content.append(line);

			}

			
			

			this.fi.close();
			this.fo.close();
		}
		catch (IOException e)
		{
			System.out.println("Fehler beim Lesen/Schreiben einer Datei " + e);
		}
	}
}

      import org.w3c.dom.Node;
      import org.w3c.dom.bootstrap.DOMImplementationRegistry;
      import org.w3c.dom.ls.DOMImplementationLS;
      import org.w3c.dom.ls.LSSerializer;
      import org.xml.sax.InputSource;

      import javax.xml.parsers.DocumentBuilderFactory;
      import java.io.StringReader;

      /**
      * Pretty-prints xml, supplied as a string.
      * <p/>
      * eg.
      * <code>
      * String formattedXml = new XmlFormatter().format("<tag><nested>hello</nested></tag>");
      * </code>
      */
      public class Main {

          public String format(String xml) {

              try {
                  final InputSource src = new InputSource(new StringReader(xml));
                  final Node document = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(src).getDocumentElement();
                  final Boolean keepDeclaration = Boolean.valueOf(xml.startsWith("<?xml"));

              //May need this: System.setProperty(DOMImplementationRegistry.PROPERTY,"com.sun.org.apache.xerces.internal.dom.DOMImplementationSourceImpl");


                  final DOMImplementationRegistry registry = DOMImplementationRegistry.newInstance();
                  final DOMImplementationLS impl = (DOMImplementationLS) registry.getDOMImplementation("LS");
                  final LSSerializer writer = impl.createLSSerializer();

                  writer.getDomConfig().setParameter("format-pretty-print", Boolean.TRUE); // Set this to true if the output needs to be beautified.
                  writer.getDomConfig().setParameter("xml-declaration", keepDeclaration); // Set this to true if the declaration is needed to be outputted.

                  return writer.writeToString(document);
              } catch (Exception e) {
                  throw new RuntimeException(e);
              }
          }

          public static void main(String[] args) {
              String unformattedXml =""; 
                      

              System.out.println(new Main().format(unformattedXml));
          }
      }