im new in mobile app development ...

Can you please assist, im trying to get wordpress post on my android app but it happens only to post categories and failing to load the actual posts

Following the source code given on the link below:

public static final String url = "/?filter[category_name]=4&per_page=50&fields=id,date,link,title,better_featured_image";

public static final String get_categories_url = ",description,name";

public static final String get_all_posts_of_category_url = ";fields=id,date,link,title,better_featured_image";

public static final String get_content_by_id = "";

public static final String pages = "";

Please see the below error im getting:

{"code":"rest_invalid_param","message":"Invalid parameter(s): categories","data":{"status":400,"params":{"categories":"categories[0] is not of type integer."}}}