我希望实现一个 AsyncTask，并实现一个与 AsyncTask 的接口(interface)，以将创建的用户对象传输到 AsyncTask .execute() 之后启动的新 Activity。
该问题似乎是由接口(interface)引起的。我对数据库发送/接收数据没有任何问题。
我是否没有声明导致 NullPointerException 的结果？有人看到任何实现不正确的地方吗？
创建用户
public static class CreateUser extends AsyncTask<User, Void, User> {
Context mContext;
public interface CreateUserInterface {
public void getCreatedUser(User user);
public void getUserCreateErrorMessage(String error);
}
CreateUserInterface callback;
public CreateUser(Context context) {
// TODO Auto-generated constructor stub
this.mContext = context;
// callback = (CreateUserInterface) context;
}
@Override
protected User doInBackground(User... params) {
ObjectMapper mapper = new ObjectMapper(); // create once, reuse
User user = params[0];
String url = ROUTE_USER_CREATE;
HttpPost httppost = new HttpPost(url);
HttpClient httpclient = new DefaultHttpClient();
String UserJSONResponse = null;
try {
String jsonString = mapper.writeValueAsString(user);
StringEntity m_stringEntity = new StringEntity(jsonString);
httppost.setEntity(m_stringEntity);
httppost.addHeader("Content-type", "application/json");
HttpResponse postResponse = httpclient.execute(httppost);
UserJSONResponse = EntityUtils.toString(postResponse
.getEntity());
user = mapper.readValue(UserJSONResponse, User.class);
} catch (JsonProcessingException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (UnsupportedEncodingException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ClientProtocolException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return user;
}
@Override
protected void onPostExecute(User result) {
// TODO Auto-generated method stub
super.onPostExecute(result);
//User user = new User();
Log.e("USERNAME", result.getUser_name());
callback.getCreatedUser(result);
}
}
这是我尝试从中检索用户并实现接口(interface)的 Activity
public class UserFragment extends Fragment implements OnClickListener, UserREST.CreateUser.CreateUserInterface {
UserREST.CreateUser createUserAsyncTask;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup content_frame,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_user, content_frame, false);
createUserAsyncTask = new UserREST.CreateUser(getActivity());
Spinner spinner = (Spinner) view.findViewById(R.id.spinnerUserChoice);
// Create an ArrayAdapter using the string array and a default spinner
// layout
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(
getActivity(), R.array.user_choice_array,
android.R.layout.simple_spinner_item);
// Specify the layout to use when the list of choices appears
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
// Apply the adapter to the spinner
spinner.setAdapter(adapter);
setHasOptionsMenu(true);
return view;
}
@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
// TODO Add your menu entries here
// inflater = getActivity().getMenuInflater();
inflater.inflate(R.menu.user_navigation, menu);
super.onCreateOptionsMenu(menu, inflater);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle presses on the action bar items
switch (item.getItemId()) {
case R.id.action_createUser:
// update the main content by replacing fragments
FragmentManager fragmentManager = getFragmentManager();
Fragment mCreateUserFragment = new CreateUserFragment();
fragmentManager.beginTransaction()
.replace(R.id.content_frame, mCreateUserFragment).commit();
}
return true;
}
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
switch (v.getId()) {
default:
break;
}
}
@Override
public void getCreatedUser(User user) {
// TODO Auto-generated method stub
Log.e("USERNAME", user.getUser_name());
}
@Override
public void getUserCreateErrorMessage(String error) {
// TODO Auto-generated method stub
}
堆栈跟踪:
10-23 14:16:08.265: E/AndroidRuntime(463): FATAL EXCEPTION: main
10-23 14:16:08.265: E/AndroidRuntime(463): Process: com.e.main, PID: 463
10-23 14:16:08.265: E/AndroidRuntime(463): java.lang.NullPointerException
10-23 14:16:08.265: E/AndroidRuntime(463): at com.e.rest.UserREST$CreateUser.onPostExecute(UserREST.java:166)
10-23 14:16:08.265: E/AndroidRuntime(463): at com.e.rest.UserREST$CreateUser.onPostExecute(UserREST.java:1)
10-23 14:16:08.265: E/AndroidRuntime(463): at android.os.AsyncTask.finish(AsyncTask.java:632)
10-23 14:16:08.265: E/AndroidRuntime(463): at android.os.AsyncTask.access$600(AsyncTask.java:177)
10-23 14:16:08.265: E/AndroidRuntime(463): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
10-23 14:16:08.265: E/AndroidRuntime(463): at android.os.Handler.dispatchMessage(Handler.java:102)
10-23 14:16:08.265: E/AndroidRuntime(463): at android.os.Looper.loop(Looper.java:136)
10-23 14:16:08.265: E/AndroidRuntime(463): at android.app.ActivityThread.main(ActivityThread.java:5105)
10-23 14:16:08.265: E/AndroidRuntime(463): at java.lang.reflect.Method.invokeNative(Native Method)
10-23 14:16:08.265: E/AndroidRuntime(463): at java.lang.reflect.Method.invoke(Method.java:515)
10-23 14:16:08.265: E/AndroidRuntime(463): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:792)
最佳答案
这是我为解决此问题而进行的更改。
CreateUser 内部
public CreateUser(Context context) {
// TODO Auto-generated constructor stub
this.mContext = context;
callback = (CreateUserInterface) context;
}
以及我正在使用的 NavigationActivity
public class NavigationActivity extends ActionBarActivity implements UserREST.CreateUser.CreateUserInterface
及其相关方法
@Override
public void getCreatedUser(User user) {
// TODO Auto-generated method stub
Log.e("USERNAME", user.getUser_name());
}
@Override
public void getUserCreateErrorMessage(String error) {
// TODO Auto-generated method stub
}
关于java - AsyncTask 接口(interface)中的 NullPointerException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/26534717/