0

我要转换的 xml 文件如下：

    <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
    <question id="1">
        <answer>
            <answername>java is a programming language</answername>
            <id>101</id>
            <postedby>ravi</postedby>
        </answer>
        <answer>
            <answername>java is a platform</answername>
            <id>102</id>
            <postedby>john</postedby>
        </answer>
        <questionname>What is java?</questionname>
    </question>

我试图将文件转换为的对象如下：

import java.util.ArrayList;

import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Question {

private int id;
private ArrayList<Answer> answers;
private String questionname;

public Question() {
}

public Question(int id, ArrayList<Answer> answers, String questionname) {
    this.id = id;
    this.answers = answers;
    this.questionname = questionname;
}

@XmlAttribute
public int getId() {
    return id;
}

public void setId(int id) {
    this.id = id;
}

public ArrayList<Answer> getAnswers() {
    return answers;
}

public void setAnswers(ArrayList<Answer> answers) {
    this.answers = answers;
}

public String getQuestionname() {
    return questionname;
}

public void setQuestionname(String questionname) {
    this.questionname = questionname;
}

}

Answer对象的类如下：

public class Answer {

private int id;
private String answername;
private String postedby;

public Answer() {
}

public Answer(int id, String answername, String postedby) {
    this.id = id;
    this.answername = answername;
    this.postedby = postedby;
}

public int getId() {
    return id;
}

public void setId(int id) {
    this.id = id;
}

public String getAnswername() {
    return answername;
}

public void setAnswername(String answername) {
    this.answername = answername;
}

public String getPostedby() {
    return postedby;
}

public void setPostedby(String postedby) {
    this.postedby = postedby;
}

}

为了进行转换，我创建了以下类：

import java.io.File;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Unmarshaller;

public class App {

public static void main(String[] args) {
    File file = new File("question.xml");
    try {
        JAXBContext jaxbContext = JAXBContext.newInstance(Question.class);
        Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
        Question question = (Question) jaxbUnmarshaller.unmarshal(file);
        
        
        System.out.println(question.getAnswers().get(0).getAnswername());
        
        
    } catch (JAXBException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}
}

当我运行 main 方法时，遇到以下异常：

App.main(App.java:17) 的线程“主”java.lang.NullPointerException 中的异常

第17行指的是：System.out.println(question.getAnswers().get(0).getAnswername());

总结和咨询： 我从以下教程中获取了我的代码：[在此处输入链接描述][1]。我遵循了所有步骤，没有跳过任何内容，所以我认为代码中没有错误。我为我的项目使用的 IDE 是带有 JavaSE-1.8 的 Eclipse Photon。我没有明确导入任何库，也没有使用 Maven。Eclipse 识别注释没有问题。所以我想知道怎么了。谁能帮我弄清楚？[1]：https://www.youtube.com/watch?v=jB_3r_Cz9Ro&t=812s

4

0 回答 0