Question

我创建了一个应用程序，显示从json对象从web到我的电影列表视图的数据。错误是它说有

 java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference.

这是我用来从web加载电影参数并将其显示为列表的完整代码。

MainActivity.java

package com.example.shailendra.jsonmovies;

import android.os.AsyncTask;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.widget.ListView;
import android.widget.TextView;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

import model.MovieModel;

public class MainActivity extends AppCompatActivity {
   ListView listView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        listView=(ListView)findViewById(R.id.listView);

     //   new JsonTask().execute("http://jsonparsing.parseapp.com/jsonData/moviesDemoList.txt");



        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }


    public  class JsonTask extends AsyncTask<String,String,List<MovieModel>>
    { MovieAdapter adapter=null;
        @Override
        protected void onPostExecute(List<MovieModel> s) {
           try{
            super.onPostExecute(s);
            adapter= new MovieAdapter(getApplicationContext(),R.layout.row,s);
            listView.setAdapter(adapter);}
                    catch (Exception e)
            {
                e.printStackTrace();
            }




        }

        @Override
        protected List<MovieModel> doInBackground(String... params) {
            HttpURLConnection connection = null;
            BufferedReader reader = null;
            try {
                URL url= new URL(params[0]);
                connection=(HttpURLConnection)url.openConnection();
                connection.connect();
                InputStream is= connection.getInputStream();
                InputStreamReader isr= new InputStreamReader(is);
                reader= new BufferedReader(isr);
                StringBuffer buffer= new StringBuffer();

                String line=null;

                while ((line=reader.readLine())!=null)
                {
                    buffer.append(line);
                }

                String finalJson=  buffer.toString();

                JSONObject parentObject= new JSONObject(finalJson);
                JSONArray parentArray= parentObject.getJSONArray("movies");

                 List<MovieModel> movieModelList = new ArrayList<>();
                MovieModel movieModel= new MovieModel();
                  for(int i=0;i<parentArray.length();i++)
                {

                JSONObject finalObject= parentArray.getJSONObject(i);
                String movieName= finalObject.getString("movie");
                    movieModel.setMovie(finalObject.getString("movie"));
                     int year= finalObject.getInt("year");
                    movieModel.setYear(finalObject.getInt("year"));
                    movieModel.setRating((float)finalObject.getDouble("year"));
                    movieModel.setDirector(finalObject.getString("director"));
                    movieModel.setDuration(finalObject.getString("duration"));
                    movieModel.setTaglline(finalObject.getString("tagline"));
                    movieModel.setStory(finalObject.getString("story"));
                    movieModel.setImage(finalObject.getString("image"));

                    List<MovieModel.Cast> castList = new ArrayList<>();
                    for(int j=0;j<finalObject.getJSONArray("cast").length();j++)
                    {
                        JSONObject castObject= finalObject.getJSONArray("cast").getJSONObject(j);

                         MovieModel.Cast cast= new MovieModel.Cast();
                        cast.setName(castObject.getString("name"));
                        castList.add(cast);
                    }   movieModel.setCastList(castList);

                    movieModelList.add(movieModel);
                }


                     return   movieModelList;

            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } catch (JSONException e) {
                e.printStackTrace();
            } finally {
                if(connection!=null)
                {
                    connection.disconnect();}
                try {
                    if(reader!=null)
                    {
                        reader.close();}
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return null;
        }
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            new JsonTask().execute("http://jsonparsing.parseapp.com/jsonData/moviesDemoList.txt");
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

movieAdapter.java

package com.example.shailendra.jsonmovies;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.RatingBar;
import android.widget.TextView;

import java.util.List;

import model.MovieModel;

/**
 * Created by Shailendra on 10/21/2016.
 */
public class MovieAdapter extends ArrayAdapter{
   public List<MovieModel> movieModelList;
    int resource;
    LayoutInflater inflater;
    public MovieAdapter(Context context, int resource, List<MovieModel> objects) {
        super(context, resource, objects);
        movieModelList=objects;
        this.resource=resource;


    }


    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        inflater=LayoutInflater.from(getContext());
        View customView=inflater.inflate(R.layout.row,parent,false);
        ImageView ivMovieIcon=(ImageView)customView.findViewById(R.id.ivIcon);
        TextView tvMovie=(TextView)customView.findViewById(R.id.tvMovie);
        TextView tvTagLine=(TextView)customView.findViewById(R.id.tvTagline);
        TextView tvYear=(TextView)customView.findViewById(R.id.tvYear);
        TextView tvDuration=(TextView)customView.findViewById(R.id.tvDuration);
        TextView tvDirector=(TextView)customView.findViewById(R.id.tvDirector);
        TextView tvCast=(TextView)customView.findViewById(R.id.tvCast);
        TextView tvStory=(TextView)customView.findViewById(R.id.tvStory);
        RatingBar ratingBare=(RatingBar) customView.findViewById(R.id.ratingBar);

        tvMovie.setText(movieModelList.get(position).getMovie());
        tvTagLine.setText(movieModelList.get(position).getTaglline());
        tvYear.setText("Year: "+movieModelList.get(position).getYear());
        tvDuration.setText(movieModelList.get(position).getDuration());
        tvDirector.setText(movieModelList.get(position).getDirector());
       StringBuffer stringBuffer= new StringBuffer();
       for(MovieModel.Cast cast:movieModelList.get(position).getCastList())
       {
           stringBuffer.append(cast.getName() +", ");}
        tvCast.setText(stringBuffer);
        tvStory.setText(movieModelList.get(position).getStory());
        ratingBare.setRating(movieModelList.get(position).getRating()/2);

        return customView;
    }
}

MovieModel.java

package model;

import java.util.List;

/**
 * Created by Shailendra on 10/21/2016.
 */
public class MovieModel {

    private String movie;
    private int year;
    private float rating;
    private String duration;
    private String director;
    private String taglline;
    private String image;
    private String story;
    private List<Cast> castList;

   public static class Cast {

     private String name;

       public String getName() {
           return name;
       }

       public void setName(String name) {
           this.name = name;
       }
   }


    public String getMovie() {
        return movie;
    }

    public void setMovie(String movie) {
        this.movie = movie;
    }

    public int getYear() {
        return year;
    }

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

    public float getRating() {
        return rating;
    }

    public void setRating(float rating) {
        this.rating = rating;
    }

    public String getDuration() {
        return duration;
    }

    public void setDuration(String duration) {
        this.duration = duration;
    }

    public String getDirector() {
        return director;
    }

    public void setDirector(String director) {
        this.director = director;
    }

    public String getTaglline() {
        return taglline;
    }

    public void setTaglline(String taglline) {
        this.taglline = taglline;
    }

    public String getImage() {
        return image;
    }

    public void setImage(String image) {
        this.image = image;
    }

    public String getStory() {
        return story;
    }

    public void setStory(String story) {
        this.story = story;
    }

    public List<Cast> getCastList() {
        return castList;
    }

    public void setCastList(List<Cast> castList) {
        this.castList = castList;
    }
}

Answer 1

首先将您的MovieModel定义为全局。然后在将适配器设置为ListView之前将其初始化。在您的情况下，您不会初始化ArrayList。

这样做。

全球宣言：

private ArrayList<MovieModel> movieModel;

然后在OnCreate方法内部初始化它。

movie = new ArrayList<MovieModel>();

示例：

见这个例子：

http://androidexample.com/How_To_Create_A_Custom_Listview_-_Android_Example/index.php?view=article_discription&aid=67

Answer 2

在适配器类中重写getCount（）方法

@Override
public int getCount() {
    if (movieModelList == null)
        return 0;
   movieModelList.size()
}

还要检查从AsyncTask的doInBackground（）返回的值。

Answer 3

如果json链接正确，请查看json链接，如果json是否正确，您可以在www.jsonlint.com查看格式。 ---＆GT; 1）必须保留方法getCount？ put get Count并查看它返回的内容

@Override
public int getCount() {
    if (movieModelList == null)
        return (0);
   movieModelList.size();
}

对于数组，只需全局定义它 ---＆gt;第2点应用程序开始工作后，您可能会获得白色文本，就好像文本位于当前布局之后。为此使用getBaseContext（）;而不是getApplicationContext（）;

列表视图大小错误，虚拟方法接口说

3 个答案: