java - 使用数组列表提取数据时尝试在空对象引用上调用虚拟方法 'java.lang.String'

Question

我是这个平台的新手，我已经被这个错误困扰了一段时间，我在网上搜索了这个错误，但似乎没有一个答案能满足我的情况。我正在尝试在 Android 上使用 XmlPullparser 提取数据，并将提取的数据添加到 model 类型的数组列表(我定义的“model”对象)。当我尝试将数据存储在 String 类型的数组列表中并且它起作用时，XmlPullparser 正在正确解析文档。当我存储到前者并出现以下错误时出现问题

  java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.newsmodel.model.getTitles()' on a null object reference
        at com.example.newsmodel.MainActivity$GetStoriesInBackground.onPostExecute(MainActivity.java:121)
        at com.example.newsmodel.MainActivity$GetStoriesInBackground.onPostExecute(MainActivity.java:49)
        at android.os.AsyncTask.finish(AsyncTask.java:755)
        at android.os.AsyncTask.access$900(AsyncTask.java:192)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:772)
        at android.os.Handler.dispatchMessage(Handler.java:107)

下面是解析Xml文档的代码:

import androidx.appcompat.app.AppCompatActivity;

import android.annotation.SuppressLint;
import android.content.Context;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.AsyncTask;
import android.os.Bundle;
import android.widget.TextView;

import org.xmlpull.v1.XmlPullParser;
import org.xmlpull.v1.XmlPullParserException;
import org.xmlpull.v1.XmlPullParserFactory;

import java.io.IOException;
import java.io.InputStream;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {
    TextView tvtitle;
    TextView tvDescription;
    ArrayList<String> titles;
    ArrayList<String> descriptions;
    ArrayList<model> items;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tvtitle = (TextView) findViewById(R.id.tvTitle);
        tvDescription = (TextView) findViewById(R.id.tvDescription);
        titles = new ArrayList<>();
        descriptions = new ArrayList<>();
        items = new ArrayList<model>();
        new GetStoriesInBackground().execute();
    }
    private InputStream getInputStream(URL url) {
        try {
            return url.openConnection().getInputStream();
        } catch (IOException e) {
            return null;
        }
    }


    @SuppressLint("StaticFieldLeak")
    public class GetStoriesInBackground extends AsyncTask<Void, Void, String>
    {

        @Override
        protected String doInBackground(Void... params) {

            try {

                URL url = new URL("https://www.businessdailyafrica.com/539444-539444-view-asFeed-bfdflfz/index.xml");

                XmlPullParserFactory factory = XmlPullParserFactory.newInstance();
                factory.setNamespaceAware(false);
                XmlPullParser xpp = factory.newPullParser();

                // get the XML from an input stream
                xpp.setInput(getInputStream(url), "UTF_8");

                //variable to check if parser is inside <item> tag
                boolean insideItem = false;

                // Returns the type of current event: START_TAG, END_TAG, etc..
                int eventType = xpp.getEventType();
                model item = null;
                while (eventType != XmlPullParser.END_DOCUMENT) {
                    if (eventType == XmlPullParser.START_TAG) {

                        if (xpp.getName().equalsIgnoreCase("item")) {
                            insideItem = true;
                            item = new model();
                        } else if (xpp.getName().equalsIgnoreCase("title")) {
                            if (insideItem)
                                //titles.add(xpp.nextText()); //extract the headline
                                item.setTitles(xpp.nextText());
                        }
                        else if (xpp.getName().equalsIgnoreCase("description")) {
                            if (insideItem)
                                //descriptions.add(xpp.nextText());//extract the link of article
                               item.setDescriptions(xpp.nextText());
                          //add item to the arraylist after the last required tag has been extracted
                                items.add(item);
                        }
                    }else if(eventType==XmlPullParser.END_TAG && xpp.getName().equalsIgnoreCase("item")){
                        insideItem=false;
                    }

                    eventType = xpp.next(); //move to next element
                }

            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (XmlPullParserException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }

            return null;

        }


        @Override
        protected void onPostExecute(String result) {
          //  tvtitle.setText(titles.get(1));
          //  tvDescription.setText(descriptions.get(1));
            tvtitle.setText(items.get(0).getTitles());
            tvDescription.setText(items.get(0).getDescriptions());

        }

    }
}

对象类:

public class model {
    public String titles;
    public String descriptions;

    public String getTitles() {
        return titles;
    }

    public void setTitles(String titles) {
        this.titles = titles;
    }

    public String getDescriptions() {
        return descriptions;
    }

    public void setDescriptions(String descriptions) {
        this.descriptions = descriptions;
    }
}

证明解析器在使用描述/标题Arraylist时提取数据: emulator screenshot with text extracted from xml

Answer 1

找到了我的问题的解决方案...我刚刚将 'item=new model()' 从最初的 if 语句( 'if (xpp.getName().equalsIgnoreCase("item"))') 移至顶部但在 while 循环内部。新的代码块现在如下所示:

URL url = new URL("https://www.businessdailyafrica.com/539444-539444-view-asFeed-bfdflfz/index.xml");

XmlPullParserFactory factory = XmlPullParserFactory.newInstance();
factory.setNamespaceAware(false);
XmlPullParser xpp = factory.newPullParser();

// We will get the XML from an input stream
xpp.setInput(getInputStream(url), "UTF_8");

boolean insideItem = false;

// Returns the type of current event: START_TAG, END_TAG, etc..
int eventType = xpp.getEventType();
String Title="";
String Description = "";

while (eventType != XmlPullParser.END_DOCUMENT) {
    model item = new model();

    if (eventType == XmlPullParser.START_TAG) {

        if (xpp.getName().equalsIgnoreCase("item")) {
            insideItem = true;

      }

        else if (xpp.getName().equalsIgnoreCase("title")) {
            if (insideItem)
                Title= xpp.nextText();
        }
        else if (xpp.getName().equalsIgnoreCase("description")) {
            if (insideItem)
                Description = xpp.nextText();

               item.setDescriptions(Description);
                item.setTitles(Title);
                //add item to the arraylist after the last required tag has been extracted
                items.add(item);

        }
    }else if(eventType==XmlPullParser.END_TAG && xpp.getName().equalsIgnoreCase("item")){
        insideItem=false;
    }

    eventType = xpp.next(); //move to next element
}