Why Can't I Add Values to a Java Collection with a Wildcard Generic Type (List

Java Collection with Wildcard Generic Type: Adding Value

When attempting to add a value to a Java collection declared with a wildcard generic type, such as List extends Parent>, the compiler may raise an error. This occurs when the type of the value being added (e.g., Parent p) does not match the expected type of the collection.

This restriction exists for safety reasons. Consider the following example:

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); // Unexpected type!</child></child>

Here, we define a list of Child objects and then create a wildcard generic list parentList from it. We can safely retrieve an item from parentList and convert it to Parent. However, attempting to assign a Parent object to the list is not allowed, as it may not be compatible with the actual type of the list's elements.

This restriction prevents potential runtime type errors and ensures that the integrity of the collection is maintained. It allows the use of wildcard types for reading from unknown collections while preventing modifications that could compromise the collection's intended behavior.

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