title: Java dom document.getElementsByTagName("") returns null?
body: <p>I've got a xml as below:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8" standalone="no"?&gt;
  &lt;bookshelf&gt;
      &lt;book ISBN="a01" press="AD press"/&gt;
      &lt;book ISBN="a11" press="BC press"&gt;
          &lt;book&gt;JavaSE&lt;/book&gt;
          &lt;Author&gt;John&lt;/Author&gt;
          &lt;price&gt;35.00&lt;/price&gt;
      &lt;/book&gt;
      &lt;book ISBN="b11" press="XY press"&gt;
          &lt;book&gt;Android&lt;/book&gt;
          &lt;Author&gt;Smith&lt;/Author&gt;
          &lt;price&gt;35.00&lt;/price&gt;
      &lt;/book&gt;
  &lt;/bookshelf&gt;
  </code></pre>
  
  <p>And a simple Java dom program as below:</p>
  
  <pre><code>    System.out.println(currentPath);
      DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
      DocumentBuilder builder = factory.newDocumentBuilder();
      Document document = builder.parse(currentPath + "/book.xml");
      Node node = document.getElementsByTagName("").item(1);
      System.out.println(node);
  </code></pre>
  
  <p>(1) It prints out "null", why?</p>
  
  <p>(2) I've googled and find many sample code to call document.getElementsByTagName(""), I wish to know if "" stands for root element or sth else?</p>
  
  <p>Thanks a lot.</p>