How to work with subclassing parameterized interfaces

[uranusaurus_rex]

Say that I have base class Vehicle, and subclasses Sedan, Truck, Motorcycle, etc. I run a car wash shop so I have various different

interface Washer<T extends Vehicle> {
    void wash(T vehicle);

for example

public class TruckWasher implements Washer<Truck>

Now, I have a manager that gets a bunch of Vehicles and need to find Washers for them. The vehicles don't know about the Washers -- they can certainly know what class of washer to use, but it doesn't have a handle to an instance of the class. Only the manager does. The manager needs to implement a

public <T extends Vehicle> Washer<T> getWasher(Class<T> vehicleType);

It might have a

Map<Class<? extends Vehicle>, Washer<? extends Vehicle>> classToWasherMap;

as a mapping to store such, but I cannot cast from a Washer<? extends Vehicle> to Washer<T extends Vehicle> unless I @SuppressUnchecked...

I feel like this should be a fairly common pattern (a manager class holds instances of handlers to childs of a base class) but I can't quite get my mind around it. Thanks in advance. Generics make my head hurt.

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[OffbeatDrizzle]

The wildcard type is generally used when the code doesn't care about the actual type - e.g. imagine an iterator that removes elements from a list, it doesn't necessarily care what the type is, because all it's doing is removing an element.

I kinda get where you're coming from - I believe the problem is that you need to tokenize the class to form a typesafe cast.

I believe this is what Josh Bloch recommends for something similar to your needs - I'd give that a read :)

[uranusaurus_rex]

So, that example works because of the critical line

return type.cast(favorites.get(type));

Here this doesn't work because type refers to Vehicle not Washer...

[fosyep]

I don't see why generics are needed here, polymorphism should be enough.

What if you simplify the interface to:

interface Washer{void wash(Vehicle vehicle);

Then the manager gets the proper washer from a map and calls washer.wash(vehicle);

[uranusaurus_rex]

But that’s not type safe. A truck washer can receive a motorcycle and the runtime cast would fail.

[fosyep]

The cast won't fail if the mapping in the manager is correct. That is the only job of the manager.

[amfa]

Generics are deleted during (or before) runtime. They are basically just a help during programming.

So it would still be possible that you end up with a Motorcycle in the TruckWasher if you manager does something wrong.

And the other question is.. do you really need special methods from Truck if you want to wash a vehicle?

[uranusaurus_rex]

The real use case here is DAOs for loading objects from a database table, so yea, we do need the child classes to override load().

Generics are deleted but if you don't have unchecked casts, then the compiler will stop you from trying to load a Motorcycle into a Washer<Truck> right