How Do Java\'s Multiple Wildcards Affect Generic Method Behavior and Type Compatibility?

Java's Multiple Wildcards: Demystifying the Compiler's Confusion

While the code snippet in the question may initially appear confusing, the underlying principles are quite simple and adhere to Java's generic type system.

The Role of Wildcards

Wildcards (*) represent unknown types. In the snippet, the use of multiple wildcards creates uncertainty for the compiler:

static void doNothing(List<?> list1, List<?> list2) { }

Since the wildcards are unrelated, you can invoke doNothing with lists of different types, e.g., List and List. However, if you attempt to modify the lists (e.g., list1.addAll(list2)), the compiler raises an error because of potential type incompatibility.

Nested Wildcards and Capture Conversion

The confusion arises primarily from misunderstanding the role of nested wildcards:

static void probablyIllegal(List<List<?>> lol, List<?> list) { }

In this case, the nested wildcard List does not capture types like List or List. Instead, it means a list whose elements can be lists of any type. To capture specific types, you need to use a bounded wildcard:

static void nowDefinitelyIllegal(List<? extends List<?>> lol, List<?> list) { }

Now, if we try to add list (which may be a List