Java Generics: How Does the `super` Keyword Really Work with Bounded Wildcards?

Java Generics: Demystifying the 'Super' Keyword

Introduction:

Understanding the 'super' keyword in Java generics can be a perplexing task. This article aims to shed light on this concept by exploring its nuances through specific examples and addressing common misconceptions.

Bounded Generics with 'Super' Keyword:

The bounded wildcard List allows for capturing Number and its supertypes. However, it's crucial to note that this doesn't mean you can add any Number supertype to this list.

Unexpected Behavior of Object vs. Integer:

In the example, you mentioned, Integer(0) can be added to List, while Object() cannot. This behavior arises from the capture conversion rules for bounded wildcards. List can only capture Number, Object, and Serializable, of which Number is the lowest supertype.

Why Adding String to List Doesn't Compile:

Although String extends Object, a superclass of Number, adding String to List fails to compile. This is because bounded wildcards follow producer-extends, consumer-super (PECS) guidelines. List is a consumer of Number, so its argument type can't be a producer of String.

Why Isn't Allowed in Java Generics?

The absence of constructs in Java generics stems from the fact that they don't make sense. If a generic type can accept any supertype of its bound, there's no need for a specific type parameter.

Conclusion:

Understanding the 'super' keyword in Java generics requires careful consideration of its implications and its adherence to PECS guidelines. By grasping these concepts, you can effectively utilize bounded wildcards to enhance the flexibility of your APIs.

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