Why Can't We Assign ArrayList<Child> to List<Parent> in Java?

Why can't we assign ArrayList<Child> to List<Parent> in Java?

List<Parent> myList = new ArrayList<Child>();

The inability to assign an `ArrayList<Child>` to a `List<Parent>` in Java is a consequence of the way Java implements generics and type safety. Java generics are invariant, meaning that even if `Child` is a subclass of `Parent`, `ArrayList<Child>` is not a subclass of `ArrayList<Parent>`.

List<? extends Parent> myList = new ArrayList<Child>();
// This works because it allows myList to be a list of any type that extends Parent.

Causes

• Java generics are invariant, meaning `List<Child>` and `List<Parent>` are treated as completely separate types.
• Allowing `List<Child>` to be assigned to `List<Parent>` would violate type safety, potentially leading to runtime errors.
• In a type-safe environment, invoking methods expecting `Parent` objects on `Child` objects could lead to unexpected behavior.

Solutions

• Use wildcard types to allow flexibility: `List<? extends Parent>` could be used in certain contexts, although it has its own limitations.
• Consider using raw types (not recommended due to type safety concerns) or redesigning your class structures to better reflect type hierarchies.
• Utilize generics in a way that ensures type compatibility, such as creating a common interface or abstract class.

Mistake: Assuming that List<Child> can substitute for List<Parent> due to inheritance.

Solution: Remember that Java generics are invariant. Always specify the correct types.

Mistake: Using raw types to work around the type compatibility issue.

Solution: Avoid using raw types; they bypass the safety features provided by Java generics.