Hi Scott:
Thank you very much for the reply. I have got it.
2012/5/8 Scott Carey <[email protected]>
> Avro schemas can represent Union types, but not abstract types. It does
> not make sense to serialize an abstract class, since its data members are
> not known.
> By definition, an abstract type does not define all of the possible sub
> types in advance, which presents another problem -- in order to make sense
> of serialized data, the universe of types serialized need to be known.
>
> You can model an abstract type with union types with a little bit of work.
> For example, if you have type AbstractThing, with children Concrete1 and
> Concrete2, you can serialize these as a union of Concrete1 and Concrete2.
> When reading the element with this union, you will need to check the
> instance type at runtime and cast or if you know the super type is
> AbstractThing, you can blindly cast to AbstractThing. As new types are
> added, your schema will change to include more branches in the union. If
> you remove a type, you will need to provide a default in case the removed
> type is encountered while reading data.
>
> If you are using the Java Specific API the above will not work without
> wrapper classes that contain the hierarchy, and the ability to create these
> from the serialized types.
>
> Serialization deals only with data stored in member variables, and
> interfaces have no data. An Avro Protocol maps to a Java Interface, but it
> is never serialized, it represents a contract for exchanging serialized
> data.
>
> -Scott
>
> On 5/6/12 9:55 PM, "Gavin YAO" <[email protected]> wrote:
>
> Hello:
> I am very new to the Apache Avro community so I hope I am doing
> right in just sending a mail to this address.
> Is it possible to represent abstract as in Java language we can do
> it by abstract class or interface?
>
> Thanks a lot!
>
>
>
