abstract class BaseEventConsumer<EV : TBase<...

# announcements

[Sandy]

abstract class BaseEventConsumer<EV : TBase<EV, out TFieldIdEnum>, ID>(val topic: String) we are using kafka, and I don't understand the meaning of EV, EV: TBase<EV, out TFieldEnum> .

[Shawn]

so to start, EV is declared as a generic parameter that is required to be a type that is or inherits from TBase and is parameterized with EV and a covariantly-specified (i.e. “this type or a subtype”) TFieldEnum

[Shawn]

if this is confusing, you’re not alone lmao

[Shawn]

it both involves variance and is a recursively-defined generic

[streetsofboston]

I don’t know about kafka programming per se, but this typedef requires that you can create your own implementation of a BaseEventConsumer and your own event-type that implements TBase and the first type-parameter of BaseEventConsumer needs to be such a event-type:

class MyFieldIdEnum : TFieldIdEnum { ...}

class MyEvent : TBase<MyEvent, MyFieldIdEnum> { ...  }

class MyConsumer: BaseEventConsumer<MyEvent, String>("topic") { .... }

[streetsofboston]

The recursive type-def of EV: TBase<EV, ...> is similar to that of C: Comparable<C>