JDBC & custom type of variable types (!)

Hello,

I'd like to retrieve a custom type from a result set,
but problem: I don't know exactly the structure.
It may be float/text or int/text, so I'd need to map it to one of those java custom types:

deciType <Double,String>;
intType <Long, String>;

Can I rely on a ClassCastException as below to find the correct matching?
even when the custom type has NULL(s) in it ?

Or is there a better approach ?

case Types.STRUCT:

try
{
foo = (intType)resultset1.getObject(1);
}
catch (ClassCastException e)
{
blah = (deciType)resultset1.getObject(1);
}

(Postgres 9.3 , JDBC4 Postgresql Driver, Version 9.3-1100)

many thanks,

Marc Mamin