Oracle Lite & Java Database Triggers

Is there anyone who could send me an hint about how to map Oracle Lite Datatypes in Java Datatype when working with Oracle Lite 3.0 Java's Database Triggers?
Up to now I realised that:

Oracle Lite Datatype | Java Datatype

DATE                   java.sql.Date
TIMESTAMP              java.sql.Timestamp
NUMBER                 java.math.BigDecimal
VARCHAR2               String

The problem now is:
if I call the trigger with NULL String values it crashes, with the others it works fine but how do I know it's a NULL???

Thanks,

                 Giovanni Bronzini