What's the difference between `` and `

Contrasting and in Java Generics

In object-oriented programming, generics play a crucial role in enhancing type safety and code reusability. In Java generics, the use of wildcards such as and is essential for handling covariance and contravariance scenarios.

Example Scenario

Consider the following example using JUnit with Hamcrest matchers:

Map<String, Class<? extends Serializable>> expected = null;
Map<String, Class<java.util.Date>> result = null;
assertThat(result, is(expected));  

This code fails to compile due to a mismatch between the generic type parameters in the assertThat method signature and the actual types involved.

Why Resolves the Issue

To understand why using resolves the issue, let's delve into the concept of covariance. In Java, arrays are covariant, meaning that if a type T is a subtype of S, then the corresponding arrays T[] and S[] are also subtypes.

In the given example, the expected map can hold instances of Class objects representing subclasses of Serializable. On the other hand, the result map can only hold Class objects representing Date class. By using , the assertThat method can accept matchers that check if a value is an instance of any class that extends Serializable, including Date class.

Underscoring the Role of Generics in assertThat

The assertThat method in JUnit is generic to ensure that an appropriate matcher is passed in for the result type. By generically typing it as , the method enforces that the matcher provided conforms to the type T, preventing potential type mismatch errors.

Drawbacks of Widening the Generic Type

While switching assertThat's parameter list to Matcher addresses the compilation issue, it does introduce a potential drawback. This broadened parameter list permits any matcher that operates on a type extending T, which may lead to incorrect matching logic in certain scenarios.

Conclusion

Understanding the distinction between and in Java generics is crucial for handling covariance correctly. In the example provided, allows for flexibility in accepting matchers for supertypes of Serializable, resolving the compilation error. However, it's essential to consider the potential implications of widening the generic type to prevent unintended mismatches in matcher logic.

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