What Does \'?\' Mean in Java Generics\' Wildcard Type Parameters?

Comprehending Wildcard Parameter Type in Java Generics

Java provides generics to enhance type safety and flexibility in code. Within these generics, wildcards offer a versatile way to represent unknown or unbounded types. This article delves into the meaning of the question mark (?) within generic type parameters.

In the example provided, the question mark accompanied by "extends" indicates a bounded wildcard. This means the generic type represents a class or interface that extends a specified supertype, in this case, "HasWord." Hence, the expression "? extends HasWord" signifies a type that is either "HasWord" itself or any of its subclasses, including null.

Technically, this bounded wildcard ensures that type parameters are compatible with the expected type. It allows you to pass in objects of subclasses of the specified type. For instance, in the provided code, you could initialize the "wordList" variable with a "List" instance because it still satisfies the required type.

It's crucial to note the difference between "? extends HasWord" and "? super HasWord." The former is suitable when the method produces elements from the collection, while the latter is appropriate for methods that add elements to the collection. This principle, known as PECS (producer-extends, consumer-super), helps maintain type safety and prevent unintended assignments.

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