使用Java从XML文件删除节点时出现问题

Question

currently I'm required to remove a specific node and its child in an XML file , however I always encountered null pointer exception whenever I'm trying to remove the nodes. The "position" parameter would be the # of node to remove. eg position 3 should remove reservation id(04113049)and everything under it.

public void removeReservation(int position){

    try{

        File file = new File("reservations.xml");
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder = factory.newDocumentBuilder();
        Document doc = builder.parse(file);
        Element element = (Element)doc.getElementsByTagName("reservation").item(position);
        element.getParentNode().removeChild(element);

        TransformerFactory transformerFactory = TransformerFactory.newInstance();
        Transformer transformer = transformerFactory.newTransformer();
        DOMSource source = new DOMSource(doc);

        StreamResult result = new StreamResult(new File("reservations.xml"));
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        transformer.transform(source, result);


    }
    catch(Exception e){
        e.printStackTrace();
    }
}

Here are the contents of the xml file:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<data>
<reservation_list>

<reservation>
<resID>01014664</resID>
<roomNo>0101</roomNo>
<roomType>VIPSuite</roomType>
<noOfGuest>3</noOfGuest>
<bedType>Master</bedType>
<smoking>Y</smoking> 
<startDate>121313</startDate>
<endDate>121316</endDate>
<wifi>Y</wifi>
<roomView>Y</roomView>
<availability>Reserved</availability>
<name>Johnny depp</name>
<address>NTU Hall 17 #01-111</address>
<country>Singapore</country>
<gender>Male</gender>
<nationality>Singaporean</nationality>
<contact>92003239</contact>
<creditCardNo>1234567812345678</creditCardNo>
<creditCardCSV>432</creditCardCSV>
<creditCardExpDate>11/16</creditCardExpDate>
<identity>U0000000I</identity>
</reservation>

<reservation>
<resID>11025652</resID>
<roomNo>1102</roomNo>
<roomType>Double</roomType>
<noOfGuest>3</noOfGuest>
<bedType>Master</bedType>
<smoking>Y</smoking>
<startDate>1212</startDate>
<endDate>1213</endDate>
<wifi>Y</wifi>
<roomView>Y</roomView>
<availability>Reserved</availability>
<name>Thomas</name>
<address>Mountbatten #2-12 Garden ave</address>
<country>Singapore</country>
<gender>Male</gender>
<nationality>Singaporean</nationality>
<contact>93482032</contact>
<creditCardNo>1234567812345678</creditCardNo>
<creditCardCSV>588</creditCardCSV>
<creditCardExpDate>3/16</creditCardExpDate>
<identity>U1234567I</identity>
</reservation>

<reservation>
<resID>04113049</resID>
<roomNo>0411</roomNo>
<roomType>VIPSuite</roomType>
<noOfGuest>7</noOfGuest>
<bedType>Master</bedType>
<smoking>Y</smoking>
<startDate>121112</startDate>
<endDate>232333</endDate>
<wifi>Y</wifi>
<roomView>Y</roomView>
<availability>Reserved</availability>
<name>elaine</name>
<address>punggol</address>
<country>Singapore</country>
<gender>Female</gender>
<nationality>Singaporean</nationality>
<contact>12345672</contact>
<creditCardNo>1234123412341234</creditCardNo>
<creditCardCSV>123</creditCardCSV>
<creditCardExpDate>1212</creditCardExpDate>
<identity>S96777777777F</identity>
</reservation>
</reservation_list>
</data>

Answer 1

First, filtering using item() is zero-based ( starts from index 0 ), there is no item(3) in your file.

Second, you should always check that you are able to find a reservation for that position before you are trying to remove it. In your case, I think you're trying to do .getParentNode() on a null element which is why you're seeing the NullPointer.

Element element = (Element)doc.getElementsByTagName("reservation").item(position);
if ( null != element) {
   element.getParentNode().removeChild(element);
  //etc 
    }