Listview 无法使用 volley

Question

我正在尝试使用 Volley 使用数据库中的数据填充列表视图（不太熟悉）。

这是java代码

package com.example.wackyroad.internannouncement;

import android.app.ListActivity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;

import com.android.volley.Request;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;

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

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;

public class MainActivity extends ListActivity {

private static final String GET_URL = "http://intern.alcfoundation.ph/mobile/get_announcement";
private JSONArray listArray = null;
private static final String TAG_ID = "id";
private static final String TAG_TITLE = "title";
private static final String TAG_CONTENT = "content";
private static final String TAG_DATE = "date";
private ArrayList<HashMap<String, String>> listArrayList;
private ProgressDialog pDialog;

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

    pDialog.dismiss();
}

@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.refresh) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}


private void getAnnouncement() {
    StringRequest postRequest = new StringRequest(Request.Method.POST, GET_URL,
            new Response.Listener<String>() {
                @Override
                public void onResponse(String response) {
                    try {
                        JSONObject jsonResponse = new JSONObject(response);
                        Log.d("JSONOBject", ""+jsonResponse);
                        try {
                            JSONArray data = jsonResponse.getJSONArray("announcements");

                            for (int i = 0; i < data.length(); i++) {
                                JSONObject c = data.getJSONObject(i);

                                HashMap<String, String> map = new HashMap<String, String>();
                                map.put(TAG_ID, c.getString("announcement_id"));
                                map.put(TAG_TITLE,  c.getString("announcement_title"));
                                map.put(TAG_CONTENT, c.getString("announcement_content"));
                                map.put(TAG_DATE, c.getString("announcement_date"));
                                listArrayList.add(map);

                                Log.d("JSONArray", "" + map);
                            }

                        } catch (Exception e) {
                            e.printStackTrace();

                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            },
            new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    error.printStackTrace();
                }
            }

    ) {
    };

    pDialog = new ProgressDialog(MainActivity.this);
    pDialog.setMessage("Getting Announcements..");
    pDialog.show();

    Volley.newRequestQueue(getApplication()).add(postRequest);



}

private void updateList() {
    ListAdapter adapter = new SimpleAdapter(this, listArrayList,
            R.layout.activity_lv_list, new String[]{TAG_ID, TAG_TITLE, TAG_CONTENT,
            TAG_DATE}, new int[]{R.id.tv_id, R.id.tv_title, R.id.tv_content, R.id.tv_date});


    setListAdapter(adapter);

    ListView lv = getListView();
    lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> parent, View view,
                                int position, long id) {

            Intent i = new Intent(getApplicationContext(), Announcement_Details.class);
            String tv_id = ((TextView) view.findViewById(R.id.tv_id)).getText().toString();
            i.putExtra("tv_id", tv_id);
            startActivity(i);

        }
    });
}


}

这是日志

02-04 08:36:51.975    1380-1380/com.example.wackyroad.internannouncement E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.wackyroad.internannouncement, PID: 1380
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.wackyroad.internannouncement/com.example.wackyroad.internannouncement.MainActivity}: java.lang.NullPointerException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
        at android.app.ActivityThread.access$800(ActivityThread.java:135)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5001)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
        at android.widget.SimpleAdapter.getCount(SimpleAdapter.java:93)
        at android.widget.ListView.setAdapter(ListView.java:480)
        at android.app.ListActivity.setListAdapter(ListActivity.java:265)
        at com.example.wackyroad.internannouncement.MainActivity.updateList(MainActivity.java:134)

这是来自数据库的 json 数据

{"success":1,"message":"Post Available!","announcements":[{"announcement_id":"1","announcement_title":"Sample Title Here","announcement_content":"Sample Content","announcement_date":"2016-02-04"},{"announcement_id":"2","announcement_title":"Sample Title","announcement_content":"Sample Content Again","announcement_date":"2016-02-04"}]}

我尝试在 logcat 中显示 jsonobject 和 jsonarray 值，它就在那里。所以我认为问题在于列表视图中的数据数量。 我希望有人可以帮助我

Answer 1

定义

ListAdapter adapter;

在班级层面，然后改变

private ArrayList<HashMap<String, String>> listArrayList;

到

private ArrayList<HashMap<String, String>> listArrayList = new ArrayList<HashMap<String, String>>();

一旦数据可从onResponse() 中的响应调用adapter.notifyDataSetChanged() 获得。