Difference Java Generics between T and? extends T

Question

it works.

Method Declaration

<T> Session<T> start(Criteria<T> criteria, List<Property<T>> orders);

Using

Criteria<? extends Product> criteria = Utils.getCriteria();
Session<? extends Product> session = null;
session = Service.start(criteria, null);

he does not work.

Method Declaration

<T> List<Session<T>> start(Criteria<T> criteria, List<Property<T>> orders)

Using

Criteria<? extends Product> criteria = Utils.getCriteria();
List<Session<? extends Product>> sessions = null;
sessions = Service.start(criteria, null);

error message

Type mismatch: cannot convert from List<Session<capture#2-of ? extends Product>> to List<Session<? extends Product>>

If I change the method declaration to this, it works.

<T> List<Session<? extends T>> start(Criteria<? extends T> criteria, List<Property<? extends T>> orders)

Why is this method declaration not working?

<T> List<Session<T>> start(Criteria<T> criteria, List<Property<T>> orders)

Answer 1

Here's what happens: you call start()with actual parameters that have a wildcard type ? extends Product, so the compiler infers that type by replacing a capture variable, such as CAP#2for ?, for Tin the call to run. So the type goes out of type List< Session< CAP#2 > >, where all we know is that CAP#2 is-a Product.

List< Session< ? extends Product > >, . , List< ? extends Session< ? extends Product > >, .

:

( ∀S : S is-a T ) G< S > is-a G< ? extends T >                       [1]

:

( ∀S : S is-a T ) Session< S > is-a Session< ? extends T >           [2]

, CAP#2 is-a Product, [2] :

Session< CAP#2 > is-a Session< ? extends Product >                   [3]

List< Session< CAP#2 > > is-NOT-a List< Session< ? extends Product > >

[1] [3] :

List< Session< CAP#2 > > is-a List< ? extends Session< ? extends Product > >

:

< T > List< Session< T > > start(
    Criteria< ? extends T > criteria,
    List< ? extends Property< ? extends T > > orders
)