在Android中调用notifyDataSetChanged（）时出现空指针异常

Question

我有一个默认的ListView项目，我希望通过点击浮动操作按钮让用户能够添加自己的项目。一旦他们在对话框上点击OK，我就会在itemAdapter.notifyDataSetChanged（）行上获得一个空指针异常。我现在花了几个小时研究这个，并且没有理由为什么这不起作用。如果我删除此行，则不会出现错误，但是当项目添加到ArrayList时列表不会更新。

这似乎已被标记为重复，但Android的原因与链接问题等标准Java应用程序完全不同。我特意得到这个错误，因为Android活动和片段的交互方式。

MainActivity.java

package com.example.josh.project3;

import android.content.DialogInterface;
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.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.text.InputType;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.EditText;

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

import java.io.IOException;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

public class MainActivity extends AppCompatActivity {
    private MainActivityFragment mainActivityFragment = new MainActivityFragment();
    private String m_Text = "";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        if (fab != null) {
            fab.setOnClickListener(new View.OnClickListener(){

                @Override
                public void onClick(View v) {
                    AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
                    builder.setTitle("Add Company");

                    final EditText input = new EditText(MainActivity.this);
                    input.setInputType(InputType.TYPE_CLASS_TEXT);
                    builder.setView(input);

                    builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                            m_Text = input.getText().toString();
                            String name = mainActivityFragment.getTitle(m_Text);
                            String desc = mainActivityFragment.getDescription(m_Text);
                            System.out.println(name);
                            System.out.println(desc);
                            mainActivityFragment.updateList(name, desc);
                        }
                    });

                    builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                            dialog.cancel();
                        }
                    });

                    builder.show();
                }
            });
        }
    }


    @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) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    private class CallWebTask extends AsyncTask<String, Void, String>
    {
        String change, companySymbol, biddingPrice;


        protected String doInBackground(String... urls)
        {

            try {
                String jsonData = mainActivityFragment.run(urls[0]);
                JSONObject jsonObject = new JSONObject(jsonData);
                JSONObject query = jsonObject.getJSONObject("query");
                JSONObject results = query.getJSONObject("results");
                JSONObject quote = results.getJSONObject("quote");
                change = quote.getString("Change");
                companySymbol = quote.getString("symbol");
                biddingPrice = quote.getString("Bid");


            }
            catch (IOException e) {
                e.printStackTrace();
            }
            catch(JSONException e)
            {
                e.printStackTrace();
            }
            return companySymbol + "    " + change + "    (" + biddingPrice + ")";
        }

    }

}

MainActivityFragment.java

package com.example.josh.project3;
import android.annotation.SuppressLint;
import android.app.AlertDialog;
import android.app.ListFragment;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.StrictMode;
import android.provider.Settings;
import android.support.design.widget.Snackbar;
import android.support.v4.app.Fragment;
import android.view.View;
import android.widget.ListView;
import android.widget.Toast;

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

import java.io.Console;
import java.io.IOException;
import java.util.ArrayList;
import java.util.concurrent.ExecutionException;

import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.Response;

/**
 * A simple {@link Fragment} subclass.
 */
public class MainActivityFragment extends ListFragment {

    OkHttpClient client = new OkHttpClient();
    String desc;
    public ArrayList<ListItems> items = new ArrayList<ListItems>();
    public ListItemsAdapter itemAdapter;
    public final static String EXTRA_TITLE = "com.example.josh.project3.title";
    public final static String EXTRA_MESSAGE =
            "com.example.josh.project3.message";
    @Override
    public void onActivityCreated(Bundle savedInstanceState){
        super.onActivityCreated(savedInstanceState);
 /*
 //You can use a simple array like this to populate the list:
 String [] values = new String [] {"Tennis", "Football", "Basketball", "Baseball",
"Vollyball", "Swimming"};
 ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity(),
android.R.layout., values);
 setListAdapter(adapter);
 */
           //Here I am creating a list of objects from a class called ListItems using ArrayList
        items.add(new ListItems("Yahoo", getDescription("YHOO")));
        items.add(new ListItems("Google", getDescription("GOOG")));
        items.add(new ListItems("Apple", getDescription("AAPL")));
        items.add(new ListItems("Microsoft", getDescription("MSFT")));
        items.add(new ListItems("Netflix", getDescription("NFLX")));
        itemAdapter = new ListItemsAdapter(getActivity(), items);
        setListAdapter(itemAdapter);

    }
    //this is to create a simple response when a list item is clicked.

    @Override
    public void onListItemClick(ListView l, View v, int position, long id){
        super.onListItemClick(l, v, position, id);
        Intent intent = new Intent(getActivity(), SelectedItemActivity.class);
        ListItems item = (ListItems) getListAdapter().getItem(position);
        intent.putExtra(EXTRA_TITLE, item.getTitle());
        intent.putExtra(EXTRA_MESSAGE, item.getMessage());
        startActivity(intent);
    }

    public String run(String url) throws IOException {
        Request request = new Request.Builder()
                .url(url)
                .build();

        Response response = client.newCall(request).execute();
        return response.body().string();
    }

    public String getDescription(String companySymbol)
    {
        try {
            desc = new CallWebTask().execute("http://query.yahooapis.com/v1/public/yql?q=select%20*%20from%20yahoo.finance.quotes%20where%20symbol%20in%20%28%22" +
                    companySymbol + "%22%29&format=json&env=store://datatables.org/alltableswithkeys").get();
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        catch (ExecutionException e)
        {
            e.printStackTrace();
        }

        return desc;

    }

    public String getTitle(String companySymbol)
    {
        try {
            desc = new CallWebTask().execute("http://query.yahooapis.com/v1/public/yql?q=select%20*%20from%20yahoo.finance.quotes%20where%20symbol%20in%20%28%22" +
                    companySymbol + "%22%29&format=json&env=store://datatables.org/alltableswithkeys1").get();
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        catch (ExecutionException e)
        {
            e.printStackTrace();
        }

        return desc;
    }

    public void updateList(String title, String desc)
    {
        ListItems listItem = new ListItems(title, desc);
        items.add(listItem);
        //itemAdapter.notifyDataSetChanged();
    }

private class CallWebTask extends AsyncTask<String, Void, String>
{
    String change, companySymbol, biddingPrice, name;


    protected String doInBackground(String... urls) {

        if (urls[0].endsWith("1")) {
            urls[0] = urls[0].substring(0, urls[0].length() - 1);
            try {
                String jsonData = run(urls[0]);
                JSONObject jsonObject = new JSONObject(jsonData);
                JSONObject query = jsonObject.getJSONObject("query");
                JSONObject results = query.getJSONObject("results");
                JSONObject quote = results.getJSONObject("quote");
                name = quote.getString("Name");


            } catch (IOException e) {
                e.printStackTrace();
            } catch (JSONException e) {
                e.printStackTrace();
            }
            return name;
        } else {
            try {
                String jsonData = run(urls[0]);
                JSONObject jsonObject = new JSONObject(jsonData);
                JSONObject query = jsonObject.getJSONObject("query");
                JSONObject results = query.getJSONObject("results");
                JSONObject quote = results.getJSONObject("quote");
                change = quote.getString("Change");
                companySymbol = quote.getString("symbol");
                biddingPrice = quote.getString("Bid");


            } catch (IOException e) {
                e.printStackTrace();
            } catch (JSONException e) {
                e.printStackTrace();
            }
            return companySymbol + "    " + change + "    (" + biddingPrice + ")";
        }
    }
}
}

ListItems.java

package com.example.josh.project3;

public class ListItems {
    private String title, message;

    public ListItems(String title, String message){
        this.title = title;
        this.message = message;
    }
    public String getTitle(){
        return title;
    }
    public String getMessage(){
        return message;
    }


}

ListItemsAdapter.java

package com.example.josh.project3;
import android.content.Context;
import android.provider.ContactsContract;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;

import java.util.ArrayList;


public class ListItemsAdapter extends ArrayAdapter<ListItems> {
    public ListItemsAdapter(Context context, ArrayList<ListItems> items){
        super(context, 0, items);
    }
    @Override
    public View getView(int position, View convertView, ViewGroup parent){
        // Get the data item for this position
        ListItems note = getItem(position);
        // Check if an existing view is being reused, otherwise inflate a new view from// custom row layout
        if (convertView == null) {
            convertView =
                    LayoutInflater.from(getContext()).inflate(R.layout.row_item_layout, parent, false);
        }
        // Grab references of views so we can populate them with specific note row data
        TextView itemTitle = (TextView) convertView.findViewById(R.id.listItemTitle);
        TextView itemText = (TextView) convertView.findViewById(R.id.listItemBody);
        // Fill each new referenced view with data associated with note it's referencing
        itemTitle.setText(note.getTitle());
        itemText.setText(note.getMessage());
        // now that we modified the view to display appropriate data,
        //return it so it will be displayed.
        return convertView;
    }
}

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context="com.example.josh.project3.MainActivity">

    <fragment

        android:name="com.example.josh.project3.MainActivityFragment"
        android:id="@+id/MainActivityFrag"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:layout_marginTop="45dp"
        tools:layout="@layout/row_item_layout" />

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>



    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        android:src="@android:drawable/ic_input_add" />

</android.support.design.widget.CoordinatorLayout>