Incorrect generic types for Ordered slot

[bmorris591]

OrderedSlot should probably have the following signature:

static class OrderedSlot<T extends PresenterWidget<?> & Comparable<? extends T>> {
}

This is because it is currently impossible to have a common generic baseclass for widgets that need to be members of an OrderedSlot.

Consider the following example (sorry, it's quite horrible):

interface View {}

abstract class PresenterWidget<V extends View> {}

class OrderedSlot<T extends PresenterWidget<?> & Comparable<T>> {}

interface UiHandlers {}

interface ViewWithUiHandlers<UIH extends UiHandlers> extends View {}

abstract class BasePresenter<UIH extends UiHandlers, V extends ViewWithUiHandlers<UIH>> extends PresenterWidget<V> implements Comparable<BasePresenter<UIH, V>> {}

So the View, PresenterWidget, OrderedSlot, UIHandlers, ViewWithUiHandlers emulate existing GWTP classes.

I have added a custom base presenter class BasePresenter which is generic in the UIHandlers and the ViewWithUiHandlers. It also is Comparable to itself.

Now, in order for another Presenter to have a Slot that accepts presenters extending this base class, I would need to following OrderedSlot declaration:

final OrderedSlot<BasePresenter<?, ?>> slot = new OrderedSlot<>();

But this will result in a compiler error:

/home/boris/.../App.java
    Error:Error:line (6)java: type argument com.example.testbench.App.BasePresenter<?,?> is not within bounds of type-variable T
    Error:Error:line (6)java: incompatible types: cannot infer type arguments for com.example.testbench.App.OrderedSlot<>
    reason: inferred type does not conform to equality constraint(s)
      inferred: com.example.testbench.App.BasePresenter<capture#1 of ?,capture#2 of ?>
      equality constraints(s): com.example.testbench.App.BasePresenter<capture#1 of ?,capture#2 of ?>,com.example.testbench.App.BasePresenter<?,?>

This is because the compiler cannot find type bounds on the wildcards ?, ? such that the self type & Comparable<T> is satisfied. And this makes sense, as if I create two extension classes and add them both to the OrderedSlot<BasePresenter<?, ?>>

class PresenterOne extends BasePresenter<SomeUiHandlers, ViewWithUiHandlers<SomeUiHandlers>> {}

class PresenterTwo extends BasePresenter<OtherUiHandlers, ViewWithUiHandlers<OtherUiHandlers>> {}

Then as PresenterOne implements Comparable<BasePresenter<SomeUiHandlers, ViewWithUiHandlers<SomeUiHandlers>> and PresenterTwo implements Comparable<BasePresenter<OtherUiHandlers, ViewWithUiHandlers<OtherUiHandlers>> the type equality constraint is violated.

In order for this situation to work, the type constraint on OrderedSlot needs to be relaxed.

[olafleur]

Would you like to submit a pull request to fix that issue?

[abaranguan]

I'm facing this issue right now.

👍 to this feature

[bmorris591]

@olafleur sorry; haven't had much time recently - I'll try and push something before next week.

[olafleur]

No worries. If you need help in the process, just ping us here!