NullPointerException 并且不确定如何修复

Posted 2023-04-19

【中文标题】NullPointerException 并且不确定如何修复

【英文标题】：NullPointerException and not sure how to fix

【发布时间】：2017-12-15 17:00:13

【问题描述】：

我收到java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.mycompany.alawamhm.hellobutton.Comic.toString()' on a null object reference，但我不确定为什么漫画在这里是空的。我假设comic = om.readValue(new URL(urlStrings[0]),Comic.class); 没有做我希望它做的事情。

import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.BufferedOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.URL;
import java.util.Locale;

public class MainActivity extends AppCompatActivity 

private int mIssueNumber = 1;
private TextView mMessageTextView;

@Override
protected void onCreate(Bundle savedInstanceState) 
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mMessageTextView = (TextView)findViewById(R.id.message_text);
    Button incrementButton = (Button)findViewById(R.id.button);

    incrementButton.setOnClickListener(new View.OnClickListener() 

        @Override
        public void onClick(View v) 
            mIssueNumber++;
            String urlString = String.format(Locale.US, "http://xkcd.com/%d/info.0.json", mIssueNumber);
            new GetComicTask().execute((urlString));
        
    );


public class GetComicTask extends AsyncTask<String, Void, Comic> 

    @Override
    protected Comic doInBackground(String... urlStrings) 
        Comic comic = null;
        ObjectMapper om = new ObjectMapper();
        try 
            comic = om.readValue(new URL(urlStrings[0]),Comic.class);
         catch (IOException e) 
            e.getMessage();
        
        return comic;
    

    @Override
    protected void onPostExecute(Comic comic) 
        super.onPostExecute(comic);                
        mMessageTextView.setText(comic.toString());


    
  

更新问题 漫画课：

public class Comic 
private int num;
private int month;
private int day;
private int year;
private String link;
private String news;
private String transcript;
private String safe_title;
private String alt;
private String img;
private String title;


public Comic() 


public int getNum() 
    return num;


public void setNum(int num) 
    this.num = num;


public int getMonth() 
    return month;


public void setMonth(int month) 
    this.month = month;


public int getDay() 
    return day;


public void setDay(int day) 
    this.day = day;


public int getYear() 
    return year;


public void setYear(int year) 
    this.year = year;


public String getLink() 
    return link;


public void setLink(String link) 
    this.link = link;


public String getNews() 
    return news;


public void setNews(String news) 
    this.news = news;


public String getTranscript() 
    return transcript;


public void setTranscript(String transcript) 
    this.transcript = transcript;


public String getSafe_title() 
    return safe_title;


public void setSafe_title(String safe_title) 
    this.safe_title = safe_title;


public String getAlt() 
    return alt;


public void setAlt(String alt) 
    this.alt = alt;


public String getImg() 
    return img;


public void setImg(String img) 
    this.img = img;


public String getTitle() 
    return title;


public void setTitle(String title) 
    this.title = title;


@Override
public String toString() 
    return "Comic" +
            "num=" + num +
            ", month=" + month +
            ", day=" + day +
            ", year=" + year +
            ", link='" + link + '\'' +
            ", news='" + news + '\'' +
            ", transcript='" + transcript + '\'' +
            ", safe_title='" + safe_title + '\'' +
            ", alt='" + alt + '\'' +
            ", img='" + img + '\'' +
            ", title='" + title + '\'' +
            '';

【问题讨论】：

debug GetComicTask doInBackground 你会得到答案

使用e.printStackTrace() 而不是e.getMessage();。它会显示错误

urlStrings[0] 很可能不是有效的 URL。因此new URL(urlStrings[0]) 失败。你也确定URL.class可以映射到Comic.class

当我打印 ulString[0] 时，我得到 link 这是一个有效的 URL，正是我想要的。我按照分步教程进行操作，但我不知道为什么会出现此问题。我不知道它是否可能，但我添加的依赖项或打包选项可能不是最新的。 compile( [group: 'com.fasterxml.jackson.core', name: 'jackson-core', version: '2.4.1'], [group: 'com.fasterxml.jackson.core', name: 'jackson-annotations', version: '2.4.1'], [group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: '2.4.1'] )

你能粘贴你的Comic.class文件吗？

【参考方案1】：

问题是您在 try-catch 块之外返回 comic。您正在进行网络调用，因此它肯定会在 try 或 catch 块中消耗一些时间，但您正在该块之外返回，因此该方法将立即返回 null comic 实例，而无需等待 try-catch 块的完成。您可以通过以下方式修改代码。

public class GetComicTask extends AsyncTask<String, Void, Comic> 

    @Override
    protected Comic doInBackground(String... urlStrings) 
        Comic comic = null;
        ObjectMapper om = new ObjectMapper();
        try 
            comic = om.readValue(new URL(urlStrings[0]),Comic.class);
            return comic;
         catch (IOException e) 
            e.getMessage();
            return null;
        

     

    @Override
    protected void onPostExecute(Comic comic) 
        super.onPostExecute(comic);                
        mMessageTextView.setText(comic.toString());
    
  

这样，只有在发生异常时才会得到 null。 我希望这能解决您的问题。