MainActivity.java

public class MainActivity extends ActionBarActivity {

    public Tasks service;
    private static final int REQUEST_ACCOUNT_PICKER = 2;
    GoogleAccountCredential mCredential;

    private static String PREF_ACCOUNT_NAME = "accountName";
    static final String TAG = "TasksSample";
    final HttpTransport mHttpTransport = AndroidHttp.newCompatibleTransport();
    final JsonFactory mJsonFactory = GsonFactory.getDefaultInstance();

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

        mCredential = GoogleAccountCredential.usingOAuth2(this, Collections.singleton(TasksScopes.TASKS));
        SharedPreferences settings = getPreferences(Context.MODE_PRIVATE);
        mCredential.setSelectedAccountName(settings.getString(PREF_ACCOUNT_NAME, null));

        service = new Tasks.Builder(mHttpTransport, mJsonFactory, mCredential)
                .setApplicationName("Google-TasksAndroidSample/1.0").build();

        Button b = (Button)findViewById(R.id.button_user_info);
        b.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                chooseAccount();
            }
        });
    }

    private void chooseAccount(){
        startActivityForResult(mCredential.newChooseAccountIntent(), REQUEST_ACCOUNT_PICKER);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data){
        super.onActivityResult(requestCode, resultCode, data);

        if(requestCode == REQUEST_ACCOUNT_PICKER && resultCode == RESULT_OK &&
                data != null && data.getExtras() != null ){
            String accountName = data.getExtras().getString(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);
                Log.d(TAG, accountName);
            }
        }
    }
}