The Unclear Impact

programming, java

is there a name for the #ExistentialType pattern in #java to avoid wildcards (especially to reduce confusion about where the quantifiers are when returning a #HeterogenousCollection like Set<Foo<?>>)? I’m thinking along the lines of

public sealed interface AnyFoo permits Foo {
    // Methods of Foo<T> that do not involve any T type variable
}

public non-sealed class Foo<T> implements AnyFoo {
    // Methods of Foo<T> that involve some T
}

I had students get very confused when the T type variable degrades to Object / Void based on the #covariant / #contravariant position, so enforcing the presence of an explicit cast might be beneficial. What is extra nice is that

AnyFoo anyFoo;
Foo<?> foo = (Foo<?>) anyFoo;

is permitted without an unsafe cast warning due to the sealed keyword (although, to be honest, something like

<T> void processFoo(Foo<T> foo);
processFoo((Foo<?>) anyFoo);

is much preferable – if you have a Skolem type variables lying around, the least you can do is naming them! blobfoxscience)

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