通过XML的第二个循环上的空指针？

Question

我是Java / XML的新手，正在尝试解析XML文档。 我可以浏览节点并获取我感兴趣的第一个节点的元素，但是一旦尝试遍历第二个节点，我就会得到一个空指针异常。

这是XML的示例。

<?xml version="1.0" encoding="utf-8"?>
<ICD10CM.index>
  <version>2019</version>
  <title>ICD-10-CM INDEX TO DISEASES and INJURIES</title>
  <letter>
    <title>A</title>
    <mainTerm>
      <title>Aarskog's syndrome</title>
      <code>Q87.1</code>
    </mainTerm>
    <mainTerm>
      <title>Abandonment</title>
      <see>Maltreatment</see>
    </mainTerm>
    <mainTerm>
      <title>Abasia<nemod>(-astasia) (hysterical)</nemod></title>
      <code>F44.4</code>
    </mainTerm>
    <mainTerm>
      <title>Abderhalden-Kaufmann-Lignac syndrome<nemod>(cystinosis)</nemod></title>
      <code>E72.04</code>
    </mainTerm>
    <mainTerm>
      <title>Abdomen, abdominal</title>
      <seeAlso>condition</seeAlso>
      <term level="1">
        <title>acute</title>
        <code>R10.0</code>
      </term>
      <term level="1">
        <title>angina</title>
        <code>K55.1</code>
      </term>
      <term level="1">
        <title>muscle deficiency syndrome</title>
        <code>Q79.4</code>
      </term>
    </mainTerm>
    <mainTerm>
      <title>Abdominalgia</title>
      <see>Pain, abdominal</see>
    </mainTerm>
    <mainTerm>
      <title>Abduction contracture, hip or other joint</title>
      <see>Contraction, joint</see>
    </mainTerm>
<mainTerm>
      <title>Azygos</title>
      <term level="1">
        <title>continuation inferior vena cava</title>
        <code>Q26.8</code>
      </term>
      <term level="1">
        <title>lobe<nemod>(lung)</nemod></title>
        <code>Q33.1</code>
      </term>
    </mainTerm>
  </letter>
  <letter>
    <title>B</title>
    <mainTerm>
      <title>Baastrup's disease</title>
      <see>Kissing spine</see>
    </mainTerm>
    <mainTerm>
      <title>Babesiosis</title>
      <code>B60.0</code>
    </mainTerm>
    <mainTerm>
      <title>Babington's disease<nemod>(familial hemorrhagic telangiectasia)</nemod></title>
      <code>I78.0</code>
    </mainTerm>
    <mainTerm>
      <title>Babinski's syndrome</title>
      <code>A52.79</code>
    </mainTerm>
</letter>
  <letter>
    <title>Z</title>
    <mainTerm>
      <title>Zahorsky's syndrome<nemod>(herpangina)</nemod></title>
      <code>B08.5</code>
    </mainTerm>
    <mainTerm>
      <title>Zellweger's syndrome</title>
      <code>Q87.89</code>
    </mainTerm>
    <mainTerm>
      <title>Zenker's diverticulum<nemod>(esophagus)</nemod></title>
      <code>K22.5</code>
    </mainTerm>
    <mainTerm>
      <title>Ziehen-Oppenheim disease</title>
      <code>G24.1</code>
    </mainTerm>
    <mainTerm>
      <title>Zieve's syndrome</title>
      <code>K70.0</code>
    </mainTerm>
    <mainTerm>
      <title>Zika NOS</title>
      <code>A92.5</code>
      <term level="1">
        <title>congenital</title>
        <code>P35.4</code>
      </term>
    </mainTerm>
    <mainTerm>
      <title>Zinc</title>
      <term level="1">
        <title>deficiency, dietary</title>
        <code>E60</code>
      </term>
      <term level="1">
        <title>metabolism disorder</title>
        <code>E83.2</code>
      </term>
    </mainTerm>
    <mainTerm>
      <title>Zollinger-Ellison syndrome</title>
      <code>E16.4</code>
    </mainTerm>
    <mainTerm>
      <title>Zona</title>
      <see>Herpes, zoster</see>
    </mainTerm>
    <mainTerm>
      <title>Zoophobia</title>
      <code>F40.218</code>
    </mainTerm>
    <mainTerm>
      <title>Zoster<nemod>(herpes)</nemod></title>
      <see>Herpes, zoster</see>
    </mainTerm>
    <mainTerm>
      <title>Zygomycosis</title>
      <code>B46.9</code>
      <term level="1">
        <title>specified NEC</title>
        <code>B46.8</code>
      </term>
    </mainTerm>
    <mainTerm>
      <title>Zymotic</title>
      <see>condition</see>
    </mainTerm>
  </letter>
</ICD10CM.index>

然后我的代码：

 try {
    // Create Results File
    File file = new File("ICD10-Parsed.txt");
    FileWriter fileWriter = new FileWriter(file);    

    //Build XML Document and get from url
    DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
    DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
    Document doc = dBuilder.parse("h:/Application/API/icd10trial.xml");
    doc.getDocumentElement().normalize();

    System.out.println("Root element :" + doc.getDocumentElement().getNodeName());
    NodeList nList = doc.getElementsByTagName("letter");

    System.out.println("----------------------------");

    System.out.println("Number of nodes " + nList.getLength());
    //Testing Area

    for (int i =0; i<1 /*nList.getLength()*/;){
        Node cNode = nList.item(i);
        System.out.println("\nCurrent Element: " + cNode.getNodeName());

        if (cNode.getNodeType() == Node.ELEMENT_NODE) {

            Element eElement = (Element) cNode;
            System.out.println("Letter: " + eElement.getElementsByTagName("mainTerm").item(i).getTextContent());

            NodeList lList = eElement.getElementsByTagName("mainTerm");
             for (int x =0; x<lList.getLength();){
                 Node tNode = lList.item(x);

                if (cNode.getNodeType() == Node.ELEMENT_NODE) {
                    String condition ="";
                    Element tElement = (Element) tNode;
                    NodeList tList = tElement.getElementsByTagName("title");

                    //Get Condition
                    NodeList elements = tElement.getElementsByTagName("title");
                    Node item = elements.item(i);
                    condition = item.getTextContent();
                    //condition = tElement.getElementsByTagName("title").item(i).getTextContent();
                    System.out.println("Condition: " + condition);
                    fileWriter.write(condition +";");     
                }
               x++;
            }
        }

        System.out.println("----------------------------");
        i++;
    }


    fileWriter.flush();
    fileWriter.close();

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

如果我使用（int = 0; int <1;），则将得到以下输出，效果很好：

    Root element :ICD10CM.index
----------------------------
Number of nodes 3

Current Element: letter
Letter: 
      Aarskog's syndrome
      Q87.1

Condition: Aarskog's syndrome
Condition: Abandonment
Condition: Abasia(-astasia) (hysterical)
Condition: Abderhalden-Kaufmann-Lignac syndrome(cystinosis)
Condition: Abdomen, abdominal
Condition: Abdominalgia
Condition: Abduction contracture, hip or other joint
Condition: Azygos
----------------------------
BUILD SUCCESSFUL (total time: 0 seconds)

但是，当我更改为（int = 0; int <2;）的那一刻，我得到以下输出：

    run:
Root element :ICD10CM.index
----------------------------
Number of nodes 3

Current Element: letter
Letter: 
      Aarskog's syndrome
      Q87.1

java.lang.NullPointerException
Condition: Aarskog's syndrome
Condition: Abandonment
Condition: Abasia(-astasia) (hysterical)
Condition: Abderhalden-Kaufmann-Lignac syndrome(cystinosis)
Condition: Abdomen, abdominal
Condition: Abdominalgia
Condition: Abduction contracture, hip or other joint
Condition: Azygos
----------------------------

    at icd10parse.ICD10Parse.main(Current Element: letter
ICD10Parse.java:71)
Letter: 
      Babesiosis
      B60.0

BUILD SUCCESSFUL (total time: 0 seconds)

我想念什么？

谢谢！

Answer 1

通常，要在一行中找到一个空指针，请将该行中的所有调用更改为自己的行。

因此，此condition = tElement.getElementsByTagName("title").item(i).getTextContent(); 会变成这样：

NodeList elements = tElement.getElementsByTagName("title");
Node item = elements.item(i);
condition = item.getTextContent();

这样，您将看到导致异常的各个部分中的哪个，并且可以缩小搜索范围。 正如Colm在他的回答中提到的那样，您的错误很可能是在这种情况下在.item(i)中使用i 。 因此，此答案的目的更多是帮助您下次解决该问题。

Answer 2

可能的原因。 您正在使用索引i引用“标题”元素。 但是索引i是您用来遍历“字母”元素的内容。 如果只需要第一个标题，请在以下行中将i替换为1：

condition = tElement.getElementsByTagName("title").item(i).getTextContent();

Answer 3

弄清楚了。 我需要另一个变量用于子节点。 现在效果很好。 这是工作代码：

Node cNode = nList.item(0);
            int v = 0;

            for (int a =0; a<nList.getLength();){

                cNode = nList.item(a);
                System.out.println("\nCurrent Element: " + cNode.getNodeName());
                Element aElement = (Element) cNode;
                System.out.println("Current Letter: " + aElement.getElementsByTagName("title").item(v).getTextContent());


                //For each Node, get sub-nodes
                    NodeList lList = aElement.getElementsByTagName("mainTerm");
                     for (int x =0; x<lList.getLength();){
                         Node tNode = lList.item(x);

                        if (cNode.getNodeType() == Node.ELEMENT_NODE) {
                            String condition ="";
                            Element tElement = (Element) tNode;
                            NodeList tList = tElement.getElementsByTagName("title");

                            //Get Condition
                            NodeList elements = tElement.getElementsByTagName("title");
                            Node item = elements.item(v);
                            condition = item.getTextContent();
                            //condition = tElement.getElementsByTagName("title").item(i).getTextContent();
                            System.out.println("Condition: " + condition);
                            fileWriter.write(condition +";");     
                        }
                       x++;
                    }
                     a++; 
                }

附带说明一下，我可以不将其设置为0，而不必使用v变量。