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我試圖將Google任務與我的應用程序同步。爲此我創建了一個名爲GTaskSyncer的類，我創建了獲取任務細節的方法。在服務對象上獲得空指針異常
我想從UI調用這些方法。現在爲了測試，我創建了一個擴展AsyncTask的類，我在這裏調用了方法doInbackground。
這是我在主要活動中調用的。
現在我在服務對象GTaskSyncer上得到空指針異常。
主要活動
public class MainActivity extends AppCompatActivity {
GoogleAccountCredential mCredential;
private TextView mOutputText;
ProgressDialog mProgress;
static final int REQUEST_ACCOUNT_PICKER = 1000;
static final int REQUEST_AUTHORIZATION = 1001;
static final int REQUEST_GOOGLE_PLAY_SERVICES = 1002;
private static final String PREF_ACCOUNT_NAME = "accountName";
private static final String[] SCOPES = { TasksScopes.TASKS };
public com.google.api.services.tasks.Tasks mService = null;
/**
* Create the main activity.
* @param savedInstanceState previously saved instance data.
*/
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
LinearLayout activityLayout = new LinearLayout(this);
LinearLayout.LayoutParams lp = new LinearLayout.LayoutParams(
LinearLayout.LayoutParams.MATCH_PARENT,
LinearLayout.LayoutParams.MATCH_PARENT);
activityLayout.setLayoutParams(lp);
activityLayout.setOrientation(LinearLayout.VERTICAL);
activityLayout.setPadding(16, 16, 16, 16);
ViewGroup.LayoutParams tlp = new ViewGroup.LayoutParams(
ViewGroup.LayoutParams.WRAP_CONTENT,
ViewGroup.LayoutParams.WRAP_CONTENT);
mOutputText = new TextView(this);
mOutputText.setLayoutParams(tlp);
mOutputText.setPadding(16, 16, 16, 16);
mOutputText.setVerticalScrollBarEnabled(true);
mOutputText.setMovementMethod(new ScrollingMovementMethod());
activityLayout.addView(mOutputText);
mProgress = new ProgressDialog(this);
mProgress.setMessage("Calling Google Tasks API ...");
Button sync = new Button(this);
sync.setText("Sync");
setContentView(activityLayout);
// Initialize credentials and service object.
SharedPreferences settings = getPreferences(Context.MODE_PRIVATE);
mCredential = GoogleAccountCredential.usingOAuth2(
getApplicationContext(), Arrays.asList(SCOPES))
.setBackOff(new ExponentialBackOff())
.setSelectedAccountName(settings.getString(PREF_ACCOUNT_NAME, null));
HttpTransport transport = AndroidHttp.newCompatibleTransport();
JsonFactory jsonFactory = JacksonFactory.getDefaultInstance();
mService = new com.google.api.services.tasks.Tasks.Builder(
transport, jsonFactory, mCredential)
.setApplicationName("Google Tasks API Android Quickstart")
.build();
}
/**
* Called whenever this activity is pushed to the foreground, such as after
* a call to onCreate().
*/
@Override
protected void onResume() {
super.onResume();
if (isGooglePlayServicesAvailable()) {
refreshResults();
} else {
mOutputText.setText("Google Play Services required: " +
"after installing, close and relaunch this app.");
}
}
/**
* Called when an activity launched here (specifically, AccountPicker
* and authorization) exits, giving you the requestCode you started it with,
* the resultCode it returned, and any additional data from it.
* @param requestCode code indicating which activity result is incoming.
* @param resultCode code indicating the result of the incoming
* activity result.
* @param data Intent (containing result data) returned by incoming
* activity result.
*/
@Override
protected void onActivityResult(
int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
switch(requestCode) {
case REQUEST_GOOGLE_PLAY_SERVICES:
if (resultCode != RESULT_OK) {
isGooglePlayServicesAvailable();
}
break;
case REQUEST_ACCOUNT_PICKER:
if (resultCode == RESULT_OK && data != null &&
data.getExtras() != null) {
String accountName =
data.getStringExtra(AccountManager.KEY_ACCOUNT_NAME);
if (accountName != null) {
mCredential.setSelectedAccountName(accountName);
SharedPreferences settings =
getPreferences(Context.MODE_PRIVATE);
SharedPreferences.Editor editor = settings.edit();
editor.putString(PREF_ACCOUNT_NAME, accountName);
editor.apply();
}
} else if (resultCode == RESULT_CANCELED) {
mOutputText.setText("Account unspecified.");
}
break;
case REQUEST_AUTHORIZATION:
if (resultCode != RESULT_OK) {
chooseAccount();
}
break;
}
super.onActivityResult(requestCode, resultCode, data);
}
/**
* Attempt to get a set of data from the Google Tasks API to display. If the
* email address isn't known yet, then call chooseAccount() method so the
* user can pick an account.
*/
private void refreshResults() {
if (mCredential.getSelectedAccountName() == null) {
chooseAccount();
} else {
if (isDeviceOnline()) {
new TestAsyncTask(mCredential).execute();
} else {
mOutputText.setText("No network connection available.");
}
}
}
/**
* Starts an activity in Google Play Services so the user can pick an
* account.
*/
private void chooseAccount() {
startActivityForResult(
mCredential.newChooseAccountIntent(), REQUEST_ACCOUNT_PICKER);
}
/**
* Checks whether the device currently has a network connection.
* @return true if the device has a network connection, false otherwise.
*/
private boolean isDeviceOnline() {
ConnectivityManager connMgr =
(ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();
return (networkInfo != null && networkInfo.isConnected());
}
/**
* Check that Google Play services APK is installed and up to date. Will
* launch an error dialog for the user to update Google Play Services if
* possible.
* @return true if Google Play Services is available and up to
* date on this device; false otherwise.
*/
private boolean isGooglePlayServicesAvailable() {
final int connectionStatusCode =
GooglePlayServicesUtil.isGooglePlayServicesAvailable(this);
if (GooglePlayServicesUtil.isUserRecoverableError(connectionStatusCode)) {
showGooglePlayServicesAvailabilityErrorDialog(connectionStatusCode);
return false;
} else if (connectionStatusCode != ConnectionResult.SUCCESS) {
return false;
}
return true;
}
/**
* Display an error dialog showing that Google Play Services is missing
* or out of date.
* @param connectionStatusCode code describing the presence (or lack of)
* Google Play Services on this device.
*/
void showGooglePlayServicesAvailabilityErrorDialog(
final int connectionStatusCode) {
Dialog dialog = GooglePlayServicesUtil.getErrorDialog(
connectionStatusCode,
MainActivity.this,
REQUEST_GOOGLE_PLAY_SERVICES);
dialog.show();
}
}
TestAsyncTask
public class TestAsyncTask extends AsyncTask<Void, Void, List<TaskList>>{
private com.google.api.services.tasks.Tasks mService = null;
private Exception mLastError = null;
private MainActivity activity;
private com.google.api.services.tasks.Tasks client = null;
public TestAsyncTask(GoogleAccountCredential credential) {
HttpTransport transport = AndroidHttp.newCompatibleTransport();
JsonFactory jsonFactory = JacksonFactory.getDefaultInstance();
mService = new com.google.api.services.tasks.Tasks.Builder(
transport, jsonFactory, credential)
.setApplicationName("Google Tasks API Android Quickstart")
.build();
}
protected List<TaskList> doInBackground(Void... params) {
GTaskSyncer gTaskSyncer = new GTaskSyncer(activity);
List<TaskList> result = new ArrayList<>();
try {
result = gTaskSyncer.getAllTaskList();
} catch (IOException e) {
Toast.makeText(activity,"IO Exception",Toast.LENGTH_SHORT).show();
}
return result;
}
}
GTaskSyncer
public class GTaskSyncer
{
final MainActivity activity;
final com.google.api.services.tasks.Tasks mService;
GTaskSyncer(MainActivity activity) {
this.activity = activity;
mService = activity.mService;
}
public List<TaskList> getAllTaskList() throws IOException
{
List<TaskList> result = new ArrayList<TaskList>();
TaskLists taskLists = mService.tasklists().list().execute();
for (TaskList taskList : taskLists.getItems()) {
result.add(taskList);
}
return result;
}
public TaskList createList() throws IOException
{
TaskList taskList = new TaskList();
taskList = activity.mService.tasklists().insert(taskList).execute();
return taskList;
}
public Task createTask(String listId) throws IOException
{
Task task = new Task();
task = activity.mService.tasks().insert(listId, task).execute();
return task;
}
public Task getTask(String listId,String taskId) throws IOException
{
Task task = activity.mService.tasks().get(listId, taskId).execute();
return task;
}
public List<Task> getTaskList(String listId) throws IOException
{
List<Task> result = new ArrayList<Task>();
List<Task> tasks = mService.tasks().list(listId).execute().getItems();
if (tasks != null) {
for (Task task : tasks) {
result.add(task);
}
} else {
Toast.makeText(activity,"No tasks.",Toast.LENGTH_SHORT).show();
}
return result;
}
}
請幫助..
請出示您的日誌輸出。跟蹤這個問題可能會有用。 –
可能重複[什麼是空指針異常，以及如何解決它？]（http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-how-do -i-fix-it） – beresfordt