AndroidInjection.java

/** Injects core Android types. */
@Beta
public final class AndroidInjection {
  private static final String TAG = "dagger.android";

  /**
   * Injects {@code activity} if an associated {@link AndroidInjector} implementation can be found,
   * otherwise throws an {@link IllegalArgumentException}.
   *
   * @throws RuntimeException if the {@link Application} doesn't implement {@link
   *     HasActivityInjector}.
   */
  public static void inject(Activity activity) {
    checkNotNull(activity, "activity");
    Application application = activity.getApplication();
    if (!(application instanceof HasActivityInjector)) {
      throw new RuntimeException(
          String.format(
              "%s does not implement %s",
              application.getClass().getCanonicalName(),
              HasActivityInjector.class.getCanonicalName()));
    }

    AndroidInjector<Activity> activityInjector =
        ((HasActivityInjector) application).activityInjector();
    checkNotNull(activityInjector, "%s.activityInjector() returned null", application.getClass());

    activityInjector.inject(activity);
  }

  ....
  private AndroidInjection() {}
}