JDBC: why is PGobject class instead of interface?

In the JDBC binding, why is PGobject a class instead of an interface?

This is a moot issue when creating a type from scratch, but Java
contains a large number of standard classes (not interfaces) for
PKIX objects so I'm in a bind when trying to create my own JDBC
extensions.

Specifically, some of the key mappings (no pun intended) are:

hugeint <-> java.language.BigInteger
principal <-> java.security.Principal
x509 <-> java.security.cert.X509Certificate
x509_crl <-> java.security.cert.X509CRL

and some additional metamappings between pkcs8 and java.security.KeyStore.

I can implement the mapping by casting between the objects and text,
but if a type extension mechanism is available it would be nice to be
able to hide those details from the user.