What Does \'? extends HasWord\' Mean in Java Generics?

Understanding the Question Mark in Java Generics Type Parameters

In Java generics, you may encounter type parameters with question marks, like "? extends HasWord". This syntax indicates a bounded wildcard type. Let's delve into its meaning:

"? extends HasWord" means that the parameter represents a class or interface that extends HasWord. It encompasses HasWord itself as well as all its subclasses. In other words, it allows for any type that satisfies the condition "instanceof HasWord" or accepts null.

This concept originates from Item 31 of Effective Java 3rd Edition, "Use bounded wildcards to increase flexibility and safety". Essentially, a bounded wildcard widens the acceptable types beyond what a non-parameterized type could accept.

For example, if a method signature requires a List, passing in a List would not be allowed. However, if the signature used List, the ChildOfHasWord list would be acceptable.

The subtle difference between "? extends HasWord" and "? super HasWord" is also worth noting. The former is suitable when you need a collection from which data will be retrieved, while the latter is appropriate for collections where data will be added. This guideline is often referred to as "PEGS": Producer-extends, Consumer-super.

In summary, the question mark in Java generics type parameters, "? extends HasWord", indicates a bounded wildcard type that represents any class or interface extending HasWord, offering greater flexibility and improved type safety. Understanding this concept enhances your grasp of Java generics and enables you to optimize code design for efficiency and maintainability.

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