Hi Stefan,
yes, an XSTRING is a raw byte array and you can use a Java byte[] type to fill in the values.
Please see the JCo API JavaDoc at class com.sap.conn.jco.JCoRecord for further info.
The first row in the shown table [method "Object getValue()"] tells which is the default Java type representation for the various ABAP RFC types.
So simply use JCoRecord.getByteArray([index|name]) and JCoRecord.setValue([index|name], byte[] value) for the appropriate XSTRING field.
Best regards,
Stefan