Why Can't I Add Elements to a `List

Wildcards and Mutability in Java Collections

When working with Java collections, it's important to understand how wildcards affect the behavior of the collection. Consider the following code:

List extends Parent> list = ...;
Parent p = factory.get();   // returns concrete implementation
list.set(0, p);   // fails: set(int, ? extends Parent) cannot be applied to (int, Parent)

This code fails to compile due to a restriction related to wildcards and mutability.

Why the Restriction?

The wildcard syntax, extends Parent>, indicates that the collection contains some subtype of Parent. However, this restriction prevents adding a Parent object to the collection because the specific subtype might not allow it.

For example, if list were actually a List, adding a Parent object would violate the type safety of the collection. This is because Child is a subtype of Parent, and a Parent object might not be compatible with the operations allowed on Child.

To maintain type safety, Java restricts adding elements to wildcard collections. This restriction ensures that the collection remains consistent and prevents unexpected behavior.

Conclusion

When using wildcards, it's crucial to consider the potential implications for mutability. While wildcards provide flexibility when working with collections, they also introduce certain constraints to ensure the integrity of the collection and its contents.

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