Question

我正在访问外部数据库（000webhost）以获取应用中的电子邮件地址并将其打印到ListView。我从logcat中打印的服务器得到了正确的响应，但我得到了null pointer exception。这就是我的logcat的样子：

05-11 16:00:39.891  24149-24914/info.androidhive.loginandregistration E/Entity Response:﹕ {"email":[{"email":"adeel@gmail.com"},{"email":"yamini@gmail.com"},{"email":"mona@gmail.com"}]}{"tag":"DisplayFriends","error":true,"error_msg":"Unknown 'tag' value. It should be either 'login' or 'register'"}
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<script type="text/javascript" src="http://stats.hosting24.com/count.php"></script>
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 05-11 16:00:39.895  24149-24149/info.androidhive.loginandregistration    D/AndroidRuntime﹕ Shutting down VM
 05-11 16:00:39.898  24149-24149/info.androidhive.loginandregistration  E/AndroidRuntime﹕ FATAL EXCEPTION: main
   Process: info.androidhive.loginandregistration, PID: 24149
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
            at info.androidhive.loginandregistration.FriendsList.setListAdapter(FriendsList.java:55)
        at info.androidhive.loginandregistration.FriendsList$GetAllCustomerTask.onPostExecute(FriendsList.java:74)
        at info.androidhive.loginandregistration.FriendsList$GetAllCustomerTask.onPostExecute(FriendsList.java:60)
        at android.os.AsyncTask.finish(AsyncTask.java:636)
        at android.os.AsyncTask.access$500(AsyncTask.java:177)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:653)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

以下是我的java文件： ApiConnector.java

  public class ApiConnector {

Global g=Global.getInstance();
String a=g.getLogInEmail();
String b="DisplayFriends";


public JSONArray GetAllCustomers()
{
    // URL for getting all customers


   // String url = "http://z.locationtest.comxa.com/index.php?tag=DisplayFriends&loggedInEmail="+a;
    String url = "http://z.locationtest.comxa.com/index.php";
    // Get HttpResponse Object from url.
    // Get HttpEntity from Http Response Object

    HttpEntity httpEntity = null;
    HttpClient httpClient = new DefaultHttpClient();  // Default HttpClient

    HttpPost httpPost = new HttpPost(url);
    try
    {


        List<NameValuePair> params = new ArrayList<NameValuePair>();
        params.add(new BasicNameValuePair("tag", b));
        params.add(new BasicNameValuePair("loggedInEmail", a));
        httpPost.setEntity(new UrlEncodedFormEntity(params));
       // HttpGet httpGet = new HttpGet(url);

       HttpResponse httpResponse = httpClient.execute(httpPost);

        httpEntity = httpResponse.getEntity();


    } catch (ClientProtocolException e) {

        // Signals error in http protocol
        e.printStackTrace();

        //Log Errors Here



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


    // Convert HttpEntity into JSON Array
   JSONArray jsonArray=null;
    JSONObject jsonObject =null;

    if (httpEntity != null)
    {
        try
        {
            String entityResponse = EntityUtils.toString(httpEntity);

            Log.e("Entity Response: ", entityResponse);
            jsonObject = new JSONObject(entityResponse);

            jsonArray = jsonObject.getJSONArray("email");
            // jsonArray = new JSONArray(entityResponse);
            /*for (int i = 0; i < jsonArray.length(); i++)
            {
                JSONObject c = jsonArray.getJSONObject(i);

                Log.d("TAG_LOCATIONS", jsonArray.toString(i));

            }*/
        }
        catch (JSONException e)
        {
            e.printStackTrace();
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
    }

  return jsonArray;


}


}

FriendList.java

   public class FriendsList extends Activity {

 private ListView listViewFriends;
 private JSONArray jsonArray;



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

    this.listViewFriends=(ListView)this.findViewById(R.id.listViewFriends);

    new GetAllCustomerTask().execute(new ApiConnector());




 }
void setListAdapter(JSONArray jsonArray)
 {
 //this.jsonArray=jsonArray;
  this.listViewFriends.setAdapter(new GetAllFriendsListViewAdapter(jsonArray,this));
}



private class GetAllCustomerTask extends AsyncTask<ApiConnector,Long,JSONArray>
{
    @Override
    protected JSONArray doInBackground(ApiConnector... params) {

        // it is executed on Background thread

        return params[0].GetAllCustomers();
    }

    @Override
    protected void onPostExecute(JSONArray jsonArray)
    {

        setListAdapter(jsonArray);

    }
    }
    }

GetAllFriendsListViewAdapter.java

  public class GetAllFriendsListViewAdapter extends BaseAdapter
 {
   private JSONArray dataArray;
   private Activity activity;
  private static LayoutInflater inflater= null;


  public GetAllFriendsListViewAdapter(JSONArray jsonArray, Activity a)
  {
    this.activity=a;
    this.dataArray=jsonArray;
    inflater=     (LayoutInflater)this.activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

   }
   @Override
  public int getCount()
   {
    return this.dataArray.length();

   }

   @Override
   public Object getItem(int position)
  {
    return position;

    }

    @Override
   public long getItemId(int position) {
      return position;
   }

   @Override
   public View getView(int position, View convertView, ViewGroup parent)
    {
    ListCell cell;
    if(convertView==null)
    {
        convertView=inflater.inflate(R.layout.item_layout,null);
        cell=new ListCell();
        cell.email=(TextView)convertView.findViewById(R.id.textViewFriends);

       convertView.setTag(cell);

    }
    else
    {
            cell=(ListCell)convertView.getTag();
    }
    try
    {
        JSONObject jsonObject = this.dataArray.getJSONObject(position);
        cell.email.setText(" "+jsonObject.getString("email"));
    }
    catch(JSONException e)
    {
       e.printStackTrace();
    }
    return convertView;
}


private class ListCell
{
    private TextView email;
}

}

我是Android新手。如果有人帮我在代码中找到错误，那就太好了。

Answer 1

每当您的程序运行时，都会在运行时抛出NullPointerException 尝试使用null，就好像它是一个真正的引用。

因此，错误不在于从您托管它的任何地方获取数据。这是因为你试图使用一个null的变量你已经完成了这个private JSONArray jsonArray;，但你也需要这样做variable = new variable-type();来分配内存。
请检查上面的部分，并做一些谷歌搜索。获取数据没有错误，当您尝试使用它时会发生这种情况。

您只是声明变量如下

private ListView listViewFriends;
 private JSONArray jsonArray;

但这还不够。您还需要在代码中定义它们。

在Android应用上访问外部数据库

1 个答案: