public PayloadBuilder category(final String category) {
        if (category != null) {
            aps.put("category", category);
        } else {
            aps.remove("category");
        }
        return this;
    }
Sets the category of the notification for iOS8 notification
actions.  See 13 minutes into "What's new in iOS Notifications"

Passing {@code null} removes the category.

@param category the name of the category supplied to the app
             when receiving the notification
@return  this
src/main/java/com/notnoop/apns/PayloadBuilder.java