Suppose you want to include a DTD

import java.math.BigInteger;
import java.io.*;

public class FibonacciDTD {

  public static void main(String[] args) {
   
    try {
      OutputStream fout = new FileOutputStream("valid_fibonacci.xml");
      Writer out = new OutputStreamWriter(fout, "UTF-8");      
      
      BigInteger low  = BigInteger.ONE;
      BigInteger high = BigInteger.ONE;      
      
      out.write("<?xml version=\"1.0\"?>\r\n");  
      out.write("<!DOCTYPE Fibonacci_Numbers [\r\n");
      out.write("  <!ELEMENT Fibonacci_Numbers (fibonacci*)>\r\n");      
      out.write("  <!ELEMENT fibonacci (#PCDATA)>\r\n");      
      out.write("  <!ATTLIST fibonacci index CDATA #IMPLIED>\r\n");      
      out.write("]>\r\n");  
      out.write("<Fibonacci_Numbers>\r\n");  
      for (int i = 1; i <= 25; i++) {
        out.write("  <fibonacci index=\"" + i + "\">");
        out.write(low.toString());
        out.write("</fibonacci>\r\n");
        
        BigInteger temp = high;
        high = high.add(low);
        low = temp;
      }
      out.write("</Fibonacci_Numbers>");  
 
      out.close();
    }
    catch (IOException ex) {
      System.err.println(ex);
    }
  }
}