The documentation says

It is perfectly acceptable for the named persistent class to be an interface. You would then declare implementing classes of that interface using the <subclass> element.
com.microsoft.northwind.Product is an interface
com.microsoft.northwind.impl.ProductImpl is the only implementing class

So I try
<class name="com.microsoft.northwind.Product" table="Products" polymorphism="explicit" >
<id name="ProductID" type="int" unsaved-value="null" >
<column name="ProductID" sql-type="int identity" not-null="true" />
<generator class="hilo" />
</id>
<property name="productName" column="ProductName" type="string" not-null="true" />
<property name="quantityPerUnit" column="QuantityPerUnit" type="string" not-null="true" />
<subclass name="com.microsoft.northwind.impl.ProductImpl" />
</class>
NO Luck a discriminator is required
Using productName - a required column as value="non null"
means productName is not read
I have not figured out how to use ProductID value="non null"
as a discriminator
If I declare the class name as ProductImpl hibernate will not handle my properties which are declared as the interface Product
How do I do this???

No! In this case there is no reason to declare the interface at all.

OK - so When I have only one implementation I can map the implementation not the interface.
Ther problem comes with associations. I want the Procuct interface to
declare that the return from the one-to-many association to Subblier is the
supplier interface not the implementation. This is reasonable - the interfaces should not need to reference implementations.
The problem is that if I do not map the interface Supplier I get the error listed below - Supplier is unmapped. If I map the interface (as I think is proper) then I need a way to tell the persistor to ALWAYS instantiate the implementation. I suppose a custom persistor could do this but do not believe this is needed - again one good sample would make the issue very clear

<hibernate-mapping>
<class name="com.microsoft.northwind.impl.ProductImpl" table="Products" >
<id name="ProductID" type="int" unsaved-value="null" >
<column name="ProductID" sql-type="int identity" not-null="true" />
<generator class="hilo" />
</id>
<property name="productName" column="ProductName" type="string" not-null="true" />
<many-to-one name="Supplier" column="SupplierID" not-null="false" />
...
</class>
<class name="com.microsoft.northwind.impl.SupplierImpl" table="Suppliers" >
<id name="SupplierID" type="int" unsaved-value="null" >
<column name="SupplierID" sql-type="int identity" not-null="true" />
<generator class="hilo" />
</id>
...
<set name="Products" table="Products" inverse="true" cascade="all" >
<key column="SupplierID" />
<one-to-many class="com.microsoft.northwind.impl.SupplierImpl" />
</set>
</class>
</hibernate-mapping>

public interface Product ...
{
public long getProductID();
...
public com.microsoft.northwind.Supplier getSupplier();
public void setSupplier(com.microsoft.northwind.Supplier in);
}


public interface Supplier ...
{
public long getSupplierID();

public Set getProducts();
}


net.sf.hibernate.MappingException: An association from the table Products refers to an unmapped class: com.microsoft.northwind.Supplier
at net.sf.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:643)
at net.sf.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:743)
at com.babel17.northwind.HibernateTest.createSessionFactory(HibernateTest.java:53)
at com.babel17.northwind.HibernateTest.testProduct(HibernateTest.java:59)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

This is why we have metadata. Obviously if the Java property had to be exactly the same as the type declared in the metadata, we could determine everything using reflection!

Just tell Hibernate what the concrete type is, and declare whatever interface type you like in your JavaBean. Easy.

"Just tell Hibernate what the concrete type is, and declare whatever interface type you like in your JavaBean. Easy."

That is EXACTLY what I am doing in the sample above -
I get an error because the interface is not mapped - the interface is the property (as a foreign key an association) but the mapping declares the impl file and I get the error

net.sf.hibernate.MappingException: An association from the table Products refers to an unmapped class: com.microsoft.northwind.Supplier

Supplier is the interface SupplierImpl is declared in the map and it does not work

One good piece of sample code would help alot and I promise to pose a sample if and when I get this working