Java Generics: What's the Difference Between `? extends T` and `? super T` Wildcards?

Java Generics: Exploring Wildcards

This article delves into the intricacies of generic wildcards in Java, addressing two fundamental questions:

1. Bounded vs. Unbounded Wildcards

Wildcards are represented with a question mark (?) followed by a bound. A bounded wildcard restricts the type of the generic, while an unbounded wildcard does not.

• Bounded Wildcards:

  • List extends T>: This indicates that the list can hold any subtype of T. It's an upper bound wildcard, meaning it restricts the type to extend T or any of its subclasses.
  • List super T>: This indicates that the list can hold any supertype of T. It's a lower bound wildcard, meaning it restricts the type to be an ancestor of T or its parent classes.

2. Difference between extends T> and super T>

• List extends T>: This type represents a list that can hold objects that are of type T or any subclass of T. For example, a list with elements of type Dog could be assigned to a list with this type parameter.
• List super T>: This type represents a list that can hold objects that are of type T or any superclass of T. For instance, a list with elements of type Animal could be assigned to a list with this type parameter.

Further Reading

For more comprehensive explanations of Java generics and wildcards, refer to the following resources:

• [Wildcards](https://docs.oracle.com/javase/tutorial/java/generics/wildcards.html)
• [More Fun with Wildcards](https://docs.oracle.com/javase/tutorial/extra/generics/morefunwithwildcards.html)

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