CODEWITHSUNDEEP
javagenericstypes
Mario Ishac
Mario Ishac

Reputation: 5877

Wildcard Bounds if Class Generic Type is Already Bounded?

Given:

class Foo {
    ...
}

class Bar<T extends Foo> {
    ...
}

Assuming I want to make a list of instances of Bar, with any generic type for Bar allowed, the type of that list could be one of two options:

List<Bar<?>>
List<Bar<? extends Foo>>

1) I'd assume that these lists can contain the exact same objects, namely instances of Bar of any generic type. Is this true?

2) Which type is preferred? Should I be more explicit and restate the bound on the generic type of Bar (second option), or just opt for a wildcard that is unbounded in the list, with the guarantee that instances of Bar can only be constructed with a certain bound on the generic type before being put into the list (first option)?

Upvotes: 2

Views: 54

Answers (1)

Jacob G.
Jacob G.

Reputation: 29680

1) I'd assume that these lists can contain the exact same objects, namely instances of Bar of any generic type. Is this true?

Both lists can only contain Bar<T> where T is either Foo or any of its children.

2) Which type is preferred?

Both are equivalent in this case. However, reading code is more important than writing it; users might think "Oh, it's an unbounded wildcard, so I can use any type.", but they'd be wrong due to the bounded wildcard of Bar. For that reason, the latter may be preferred in some cases, but at the end of the day, it's your call.

Upvotes: 1

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