Right now the only way to use RQL queries is to use the GravitonApi rest methods that take a Url as argument, for example

GravitonApi gravitonApi = new GravitonApi();
gravitonApi.get("https://graviton-base-url/person/customer/?eq(name,string:Herbert)&select(zip, city)").execute();

RQL support should be better integrated into the GDK that allows the user to rely on method instead of plain RQL syntax. Probably the easiest way to integrate the base RQL support, is to consider all set attributes in the POJO while linking them with the AND modifier. This might look like

GravitonApi gravitonApi = new GravitonApi();
Customer customer = new Customer();
customer.setName("Herbert");
customer.setAge(45);
gravitonApi.query(customer).execute();

Which would give us all customers with the name Herbert that have the age 45.

The next step would be to support selectors like limit and select. As a new part (maybe similar to the HeaderBag) of the Request.Builder this could look like

GravitonApi gravitonApi = new GravitonApi();
Customer customer = new Customer();
customer.setName("Herbert");
gravitonApi
    .query(customer)
    .setQuery(new Rql.Builder()
        .setLimit(1)
        .addSelect("zip")
        .addSelect("city")
        .build())
    .execute();

With #12 RQL support was introduced for string and date fields.
Since the RQL syntax varies depending on the field type, integer and float and boolean are currently not supported, since they get treated as regular string fields.

Lets have a look at a typical query against a string field fieldname with the value value
?eq(fieldname,string:value)
In this case the string: prefix is not required. It has the same result as
?eq(fieldname,value)

But lets look at another example (again a string field)
?eq(fieldname,string:20)
At this point the string: prefix is required, since the Graviton RQL parser needs to know it's dealing with a string. Omitting string: would lead to an empty result

On the other hand, if we look at an integer field
?eq(integerFieldName,string:20)
would lead to an empty result. In this case the query needs to look like
?eq(integerFieldName,20)

The part that needs changing is https://github.com/libgraviton/gdk-java/blob/develop/gdk-core/src/main/java/com/github/libgraviton/gdk/api/query/rql/Rql.java#L141 where currently every field is always treated as string.