How to programmatically create xml with VTD-XML?

I want to create an xml with this shape where I insert the inner elements /a/b in a loop and also set the attributes on element b.

<ROOT>
  <a>
    <b attr1="1" attr2="a"/>
  </a>
  <a>
    <b attr1="1" attr2="b"/>
  </a>
  <a>
    <b attr1="2" attr2="a"/>
  </a>
  <a>
    <b attr1="2" attr2="b"/>
  </a>
</ROOT>

This is the code I have so far:

  public static String createXML(Collection<Integer> numbers, Collection<String> words) {
    String charsetName = "UTF-16";
    byte[] root = "<ROOT></ROOT>".getBytes(charsetName);
    VTDGen vg = new VTDGen();
    AutoPilot ap = new AutoPilot();
    ap.selectXPath("/ROOT");
    XMLModifier xm = new XMLModifier();
    vg.setDoc(root);
    vg.parse(false);
    VTDNav vn = vg.getNav();
    ap.bind(vn);
    xm.bind(vn);

    byte[] aTag = "<a></a>".getBytes(charsetName);
    byte[] bTag = "<b />".getBytes(charsetName);

    int i;

    String collect = numbers.stream().flatMap(number -> words.stream().map(word -> {
      try {
        xm.insertAfterHead(aTag);
        ap.selectXPath("a");
        xm.insertAfterHead(bTag);
        ap.selectXPath("b");
        xm.insertAttribute(String
          .format(" attr1=\"%d\" attr2=\"%s\"",
            number,
            word));
        return xm.outputAndReparse().toNormalizedString(0);
      } catch (ModifyException | NavException | ParseException | IOException | TranscodeException | XPathParseException e) {
        throw new RuntimeException(e);
      }
    }))
      .collect(Collectors.joining(""));

    return collect;
  }

I get a ModifyExcpetion because I call insertAfterHead twice. How can I generate the desired xml shape? I do not fully understand how to put the offset to the right place yet.

Solution

I think I may know what you are trying to accomplish. There are a few sugggestions

selectXPath(a) merely compiles an xpath into internal format... It does not evaluate into a node set for you. To evaluate it, you need to call evalXPath().
You want to insert as many into under root node as possible as a single string concatenation. The actually string concatenation operation should occur as an independent part of your app logic. In VTD-XML, you think in terms of bits bytes, byte arrays and int/long arrays.

Below is my mod of your code.

public static void main(String[] args) throws VTDException,IOException,
    UnsupportedEncodingException{
        String charsetName = "UTF-16";
        byte[] root = "<ROOT><a><b/></a><a><b/></a><a><b/></a><a><b/></a></ROOT>"
    .getBytes(charsetName); // that is template you want to start with
        VTDGen vg = new VTDGen();
        AutoPilot ap = new AutoPilot();
        ap.selectXPath("/ROOT/a/b");
        XMLModifier xm = new XMLModifier();
        vg.setDoc(root);
        vg.parse(false);
        VTDNav vn = vg.getNav();
        ap.bind(vn);
        xm.bind(vn);
        int i=0;
        int[] ia = new int[4];
        ia[0]=1;ia[1]=1;ia[2]=2;ia[3]=2;
        String[] sa = new String[4];
        sa[0]="a";sa[1]="b";sa[2]="a";sa[3]="b";
        int k=0;
        while((i=ap.evalXPath())!=-1){
            xm.insertAttribute( String.format(" attr1=\"%d\" attr2=\"%s\"",
                    ia[k],
                    sa[k]));
            k++;
        }
        XMLByteOutputStream xbos = new XMLByteOutputStream(xm.getUpdatedDocumentSize());
        xm.output(xbos);
        System.out.println(new String(xbos.getXML(),"UTF-16"));
    }