Can Multiple Wildcards in Generic Methods Refer to Different Types?

Multiple Wildcards on Generic Methods

Question: Can different wildcards in generic methods refer to completely different types, and how does nested wildcard behave?

Answer:

Nested wildcards are the source of confusion, not multiple wildcards.

• Nested Wildcards: A List> is NOT a List> because Java generics is invariant, meaning a List is not a List. Similarly, a List> is not a List>.

• Wildcards in Generic Methods:

  • Multiple unrelated wildcards in generic methods are allowed, e.g., List> list1, List> list2.
  • However, the method addAll(list2) on list1 is permitted because the generic type captured by ? in both lists can be different.
  • If the wildcards have explicit upper bounds (e.g., ? extends List>>), then additional restrictions apply.

Snippet Explanations:

• LOLUnknowns1: ProbablyIllegal adds a List> list to a List> lol. This compiles because ? in List> can refer to any type, while List> can contain lists of any type.
• LOLUnknowns1a: When the same ? is used for both List> types, compilation fails because the capture conversion rules for nested wildcards prohibit the use of a single ? for both lists.
• LOLUnknowns1b: When one argument is explicitly null, compilation succeeds because null is applicable to any generic type and does not participate in capture conversion.
• DoubleLOL: Adding two List> instances is permitted because the capture conversion does not apply recursively, meaning the ? can refer to different types in each list.
• IntoTheWild: Creating a List> for a List> is not permitted because the capture conversion is not applied recursively.
• IntoTheWild2: The capture conversion cannot be applied to the inner list type, so List> is not a List>.

In summary, multiple wildcards in generic methods allow for flexibility in handling unknown types, while nested wildcards have specific restrictions related to capture conversion.

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