Java 8 generics compatibility

Question

I have the following method declaration:

public <T extends Serializable,
        Method extends BotApiMethod<T>,
        Callback extends SentCallback<T>>
    void executeAsync(Method method, Callback callback)

When I use it in the following way:

executeAsync(editMessage, new SentCallback() {

it works fine but shows the type safety warnings.

But when I provide the generic type to the SentCallback, like:

executeAsync(editMessage, new SentCallback<Message>()

it show the following compilation error:

The method executeAsync(Method, Callback) in the type AbsSender is not applicable for the arguments (EditMessageReplyMarkup, new SentCallback(){})

Why I can't use it with new SentCallback<Message> ?

Answer 1

First, let us take a look at the inferred type of T. Given the types of your parameters (BotApiMethod<Serializable> and SentCallback<Message>), T should be either inferred to Serializable or to Message. But at the same time the current implementation forces that both parameters have the exact same T. This is the reason the presented attempt does not work. Keep in mind that in Java, generics are invariant.

If one cannot change the given definition of executeAsync(...), the problem is not solvable with the parameters provided.

If parameter T is essential and the design allows for different generic parameters of Method and Callback, which only need to have a common supertype T, the definition of executeAsync(...) could be changed to:

public <T extends Serializable,
        Method extends BotApiMethod<? extends T>,
        Callback extends SentCallback<? extends T>>
    void executeAsync(Method method, Callback callback)

If parameter T is not essential it could be dropped completely, leading to the following definition of executeAsync(...):

public <Method extends BotApiMethod<? extends Serializable>,
        Callback extends SentCallback<? extends Serializable>>
    void executeAsync(Method method, Callback callback)