Base Callback to centralize the management of errors if you use Appsly Android REST.

BaseApiCallback.java

public abstract class BaseApiCallback<Result> extends Callback<Result> {
  private final static String CLASS_NAME = "BaseApiCallback";

  @Override
  public final void onResponse(Response<Result> response) {
    if (response.getStatusCode() == 0) {
      LogManager.error(CLASS_NAME, "onResponse", response.getError().getMessage());
      notifyError(ErrorType.ERROR_CONECTION);
      return;
    }

    if (isChildOfBaseResponse(response.getResult())) {
      manangerResponse((Response<BaseApiResponse>) response);
    } else {
      throw new IllegalArgumentException("response must be extends BaseApiResponse");
    }
  }

  private void manangerResponse(Response<BaseApiResponse> response) {
    BaseApiResponse result = response.getResult();
    if (ErrorManager.isValidResponse(response.getStatusCode(), result)) {
      LogManager.verbose(CLASS_NAME, "managerResponse", "Response success");
      notifyDataLoaded(result);
    } else {
      ErrorType error = ErrorManager.managerError(response);
      notifyError(error);
    }
  }

  private boolean isChildOfBaseResponse(Result instance) {
    return BaseApiResponse.class.isAssignableFrom(instance.getClass());
  }

  public abstract void notifyDataLoaded(BaseApiResponse response);

  public abstract void notifyError(ErrorType error);
}

BaseApiResponse.java

public class BaseApiResponse {
  private String message;
  private String code;

  public String getMessage() {
    return message;
  }

  public void setMessage(String message) {
    this.message = message;
  }

  public String getCode() {
    return code;
  }

  public void setCode(String code) {
    this.code = code;
  }
}

ErrorManager.java

public class ErrorManager {
  private final static String CLASS_NAME = "ErrorManager";

  public static boolean isValidResponse(int statusCode, BaseApiResponse response) {
    boolean res = statusCode == HttpStatus.SC_OK
        && response != null
        && response.getCode() == null;
    return res;
  }

  public static ErrorType managerError(Response<BaseApiResponse> response) {
    if (response.getStatusCode() != HttpStatus.SC_OK) {
      return managerHttpError(response.getStatusCode(), response.getError().getMessage());
    } else {
      return managerApiError(response.getResult());
    }
  }

  public static ErrorType managerApiError(BaseApiResponse response) {
    LogManager.error(CLASS_NAME, "managerHttpError", response.getCode() + " " + response.getMessage());
    return ErrorType.ERROR_SERVER;
  }

  public static ErrorType managerHttpError(int statusCode, String message) {
    LogManager.error(CLASS_NAME, "managerHttpError", statusCode + " " + message);
    ErrorType error;
    switch (statusCode) {
      case HttpStatus.SC_FORBIDDEN:
        error = ErrorType.ERROR_CONECTION;
        break;
      case HttpStatus.SC_GONE:
        error = ErrorType.ERROR_CONECTION;
        break;
      case HttpStatus.SC_NOT_FOUND:
        error = ErrorType.ERROR_CONECTION;
        break;
      case HttpStatus.SC_INTERNAL_SERVER_ERROR:
        error = ErrorType.ERROR_SERVER;
        break;
      default:
        error = ErrorType.ERROR_CONECTION;
        break;
    }
    return error;
  }
}

ErrorType.java

public enum ErrorType {
  ERROR_UNDEFINED,
  ERROR_CONECTION,
  ERROR_SERVER,
}