在Android项目中使用Google提供的GoogleApiClient类，可以轻松获取应用程序默认凭据。以下是示例代码：

1. 在项目中添加以下依赖项：

dependencies {
  implementation 'com.google.android.gms:play-services-auth:16.0.1'
}

2. 在Application类中构建GoogleApiClient：

public class MyApp extends Application implements GoogleApiClient.ConnectionCallbacks,
    GoogleApiClient.OnConnectionFailedListener {
 
    private static final String TAG = MyApp.class.getSimpleName();
    private GoogleApiClient mGoogleApiClient;
 
    @Override
    public void onCreate() {
        super.onCreate();
        //创建GoogleApiClient
        mGoogleApiClient = new GoogleApiClient.Builder(this)
                .enableAutoManage(this /* FragmentActivity */ , this /* OnConnectionFailedListener */ )
                .addConnectionCallbacks(this)
                .addApi(Auth.GOOGLE_SIGN_IN_API, googleSignInOptions)
                .build();
    }

    @Override
    public void onConnected(@Nullable Bundle bundle) {
        Log.i(TAG, "GoogleApiClient Connected!");
 
        // 获取默认的凭据
        GoogleSignInAccount account = GoogleSignIn.getLastSignedInAccount(getApplicationContext());
        if (account != null) {
            String name = account.getDisplayName();
            String email = account.getEmail();
            String id = account.getId();
 
            Log.i(TAG, "name:" + name);
            Log.i(TAG, "email:" + email);
            Log.i(TAG, "id:" + id);
        }
    }

    @Override
    public void onConnectionSuspended(int i) {
        Log.i(TAG, "GoogleApiClient Suspended!");
    }
 
    @Override
    public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {
        Log.i(TAG, "GoogleApiClient Connection Failed!");
    }
}

3. 在需要获取凭据的Activity中获取GoogleApiClient实例，并添加相应的连接回调：

public class MainActivity extends AppCompatActivity {

    private GoogleApiClient mGoogleApiClient;
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
 
        // 获取GoogleApiClient实例
        mGoogleApiClient = new GoogleApiClient.Builder(this)
                .addApi(Auth.GOOGLE_SIGN_IN_API, googleSignInOptions)