Why Can't I Add Values to a Java Collection with a Wildcard Generic Type like `? extends Parent`?

Issues with Adding Values to Java Collections with Wildcard Generic Types

Consider the following code, where Parent is an interface:

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

Why does this code fail?

The issue lies in the use of the wildcard generic type ? extends Parent. This syntax indicates that the list can hold objects of any type that extends Parent. While this provides flexibility, it also adds a level of uncertainty regarding the actual type of objects in the list.

To understand why the code fails, consider the following scenario:

List<Child> childList = new ArrayList<Child>();
childList.add(new Child());

List<? extends Parent> parentList = childList;
parentList.set(0, new Parent());

Child child = childList.get(0); // No! It's not a child! Type safety is broken...

The wildcard type ? extends Parent allows us to assign the childList to the parentList. However, when we try to set an object of type Parent in the parentList, the compiler cannot guarantee that the corresponding object in the childList is also of type Child. This could lead to a runtime error or unexpected behavior.

To maintain type safety, the compiler restricts the operations that can be performed on a collection with a wildcard generic type. In this case, assigning an object to the list is forbidden because the compiler cannot guarantee the compatibility of the object with the unknown type of the list.

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