NullpointerException at startActivityForResult

Question

我目前正试图在安卓系统上实现Google API for Java并与之合作。为了测试事物，我一直在遵循两个示例(CalendarSample，TasksSample)。调用AccountManager和其他内容的代码与两个示例中的代码相同。但是，下面的代码对我不起作用，导致了一个NullPointerException

startActivityForResult(credential.newChooseAccountIntent(),2); --我是从一个普通的Activity中调用的，点击按钮后，onCreate已经被调用了。我已经尝试搜索了几个小时的谷歌和其他信息来源，但似乎没有任何效果。

清单：

<activity
android:name=".ManageAccounts"
android:label="@string/title_activity_manage_accounts"
        android:launchMode="singleTop"
        android:parentActivityName=".SettingsActivity" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.CAMERA" />
<uses-permission android:name="android.permission.GET_ACCOUNTS" />
<uses-permission android:name="android.permission.USE_CREDENTIALS" />
<uses-permission android:name="android.permission.MANAGE_ACCOUNTS" />

以下是全部活动：

public class ManageAccounts extends Activity implements GoogleLoginFragment.OnSignedIn{

DataHandler dataHandler;
//for testing of google add friend
private static final String PREF_ACCOUNT_NAME = "accountName";
Circles service;
final HttpTransport httpTransport = AndroidHttp.newCompatibleTransport();
final JsonFactory jsonFactory = GsonFactory.getDefaultInstance();
static final int REQUEST_GOOGLE_PLAY_SERVICES = 0;
static final int REQUEST_AUTHORIZATION = 1;
static final int REQUEST_ACCOUNT_PICKER = 2;
GoogleAccountCredential credential;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_manage_accounts);
    ActionBar actionBar = getActionBar();
    actionBar.setDisplayHomeAsUpEnabled(true);

    dataHandler = new DataHandler(this);

    //Add GoogleLoginFragment
    GoogleLoginFragment.attachToActivity(this);


    //testing of google add friend code
    GoogleAccountCredential credential =
            GoogleAccountCredential.usingOAuth2(this, Collections.singleton(CirclesScopes.CIRCLE_ADD +" "+ CirclesScopes.CIRCLES));
    SharedPreferences settings = this.getPreferences(Context.MODE_PRIVATE);
    credential.setSelectedAccountName(settings.getString(PREF_ACCOUNT_NAME, null));
    service =
            new Circles.Builder(httpTransport, jsonFactory, credential)
                    .setApplicationName("Social Contacts/1.0").build();
    Log.i("Activity","onCreateDone");
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.manage_accounts, 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();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    //Request Code for Twitter button: 140
    if (requestCode == 140) {
        loginButton.onActivityResult(requestCode, resultCode, data);
    }

    switch (requestCode) {
        case REQUEST_GOOGLE_PLAY_SERVICES:
            if (resultCode == Activity.RESULT_OK) {
                haveGooglePlayServices();
            } else {
                checkGooglePlayServicesAvailable();
            }
            break;
        case REQUEST_AUTHORIZATION:
            if (resultCode == Activity.RESULT_OK) {
//                    AsyncLoadTasks.run(this);

            } else {
                chooseAccount();
            }
            break;
        case REQUEST_ACCOUNT_PICKER:
            if (resultCode == Activity.RESULT_OK && data != null && data.getExtras() != null) {
                String accountName = data.getExtras().getString(AccountManager.KEY_ACCOUNT_NAME);
                if (accountName != null) {
                    credential.setSelectedAccountName(accountName);
                    SharedPreferences settings = getPreferences(Context.MODE_PRIVATE);
                    SharedPreferences.Editor editor = settings.edit();
                    editor.putString(PREF_ACCOUNT_NAME, accountName);
                    editor.commit();
//                        AsyncLoadTasks.run(this);
                }
            }
            break;
    }
}

public void onSignedIn(GoogleApiClient googleApiClient) {
    GoogleSocialActions gsa = new GoogleSocialActions(googleApiClient, dataHandler, this);
    gsa.saveAuthenticatedUser();
}

/** Check that Google Play services APK is installed and up to date. */
private boolean checkGooglePlayServicesAvailable() {
    final int connectionStatusCode = GooglePlayServicesUtil.isGooglePlayServicesAvailable(this);
    if (GooglePlayServicesUtil.isUserRecoverableError(connectionStatusCode)) {
        showGooglePlayServicesAvailabilityErrorDialog(connectionStatusCode);
        return false;
    }
    return true;
}

void showGooglePlayServicesAvailabilityErrorDialog(final int connectionStatusCode) {
    runOnUiThread(new Runnable() {
        public void run() {
            Dialog dialog =
                    GooglePlayServicesUtil.getErrorDialog(connectionStatusCode, ManageAccounts.this,
                            REQUEST_GOOGLE_PLAY_SERVICES);
            dialog.show();
        }
    });
}

private void haveGooglePlayServices() {
// check if there is already an account selected
    if (credential.getSelectedAccountName() == null) {
// ask user to choose account
        chooseAccount();
    } else {
// load calendars
//            AsyncLoadTasks.run(this);
    }
}
private void chooseAccount() {
    startActivityForResult(credential.newChooseAccountIntent(), 2);
}

public void chooser(View view) {
    if (view.getId() == R.id.chooser) {
        startActivityForResult(credential.newChooseAccountIntent(), REQUEST_ACCOUNT_PICKER);
    }
}

}

Stacktrace如下所示。在第204行的NullPointer是上面所示的一行。

java.lang.IllegalStateException: Could not execute method of the activity at android.view.View$1.onClick(View.java:3602) at android.view.View.performClick(View.java:4095) at android.view.View$PerformClick.run(View.java:17078) at android.os.Handler.handleCallback(Handler.java:615) at android.os.Handler.dispatchMessage(Handler.java:92) at android.os.Looper.loop(Looper.java:137) at android.app.ActivityThread.main(ActivityThread.java:4872) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557) at dalvik.system.NativeStart.main(Native Method) Caused by: java.lang.reflect.InvocationTargetException at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at android.view.View$1.onClick(View.java:3597) at android.view.View.performClick(View.java:4095) at android.view.View$PerformClick.run(View.java:17078) at android.os.Handler.handleCallback(Handler.java:615) at android.os.Handler.dispatchMessage(Handler.java:92) at android.os.Looper.loop(Looper.java:137) at android.app.ActivityThread.main(ActivityThread.java:4872) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557) at dalvik.system.NativeStart.main(Native Method) Caused by: java.lang.NullPointerException at com.example.martin.contacts.ManageAccounts.chooser(ManageAccounts.java:204) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at android.view.View$1.onClick(View.java:3597) at android.view.View.performClick(View.java:4095) at android.view.View$PerformClick.run(View.java:17078) at android.os.Handler.handleCallback(Handler.java:615) at android.os.Handler.dispatchMessage(Handler.java:92) at android.os.Looper.loop(Looper.java:137) at android.app.ActivityThread.main(ActivityThread.java:4872) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557) at dalvik.system.NativeStart.main(Native Method)

Answer 1

你从credential.newChooseAccountIntent()那里得到什么意图了吗？？堆栈跟踪表明您没有得到任何意图，因此无法调用startActivityForResult()。

更新答复

您将GoogleAccountCredential credential;作为实例成员。但是您还没有初始化这个成员。在您的onCreate中，您已经声明了另一个本地credential，但是您的成员credential从未正确初始化，下面是您的有问题的行-

//testing of google add friend code
    GoogleAccountCredential credential =
            GoogleAccountCredential.usingOAuth2(this, Collections.singleton(CirclesScopes.CIRCLE_ADD +" "+ CirclesScopes.CIRCLES));

相反，你应该只写

    //testing of google add friend code
 credential =GoogleAccountCredential.usingOAuth2(this, Collections.singleton(CirclesScopes.CIRCLE_ADD +" "+ CirclesScopes.CIRCLES));