Why Can't I Add Elements to a Java `List

Using Wildcards in Java List Structures: Understanding the Implications of List extends Number>

The scenario described involves an attempt to add data to a List declared as List extends Number> named foo3. However, this operation triggers a compile-time error. This article aims to clarify why such operations are not allowed and provide a comprehensive explanation of the implications of using wildcards, specifically List extends Number>`.

In Java, wildcards are generic types that represent unknown types. In the context of collections, wildcards can be used to specify either upper bounds (? extends T) or lower bounds (? super T) for the type of elements that can be contained in the collection.

In the case of List extends Number>, the wildcard declaration indicates that the List can hold any type that is a subclass of Number. This means that the list could potentially contain Integers, Doubles, Floats, or any other subclasses of Number. However, the exact type of the list is unknown.

Given this uncertainty, it becomes impossible to determine which specific type of object can be safely added to the list without violating the constraints of the wildcard declaration. For example, if the list is actually a List, adding a Double would violate the constraints of the list, as it is not an Integer.

Conversely, if the list is a List, adding an Integer would also violate the list's constraints. The problem lies in the fact that the exact type of the list is unknown at compile time.

Therefore, the only safe operation that can be performed on a List extends Number> is to read from it, as any attempt to add or modify its elements could potentially violate its constraints. This restriction ensures the integrity of the list and prevents runtime errors.

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