Convert read data to String or JSON format

Hello,

I need to read specific Obis codes and convert their values to an JSON Object.
Server Sends Obis code and Object type like Data, Register or Profile generic.

I am reading like this:
GXDLMSRegister d = new GXDLMSRegister(obStruct.obis_code);
reader.read(d,3);
reader.read(d,2);

GXDLMSProfileGeneric d = new GXDLMSProfileGeneric(obStruct.obis_code);
reader.read(d,3);
reader.read(d,4);
reader.read(d,5);
reader.read(d,6);
reader.read(d,7);
reader.read(d,8);

Then Now Convert Obtain value to String or JSON.

Please suggest best way to convert those values to JSON or String.

Thank You