When Should I Use `` vs. `` in Java Generics?

Generics and the Use of vs.

In Java, generics allow for type-safe operations over a wider range of types. However, questions arise when choosing between and in certain scenarios, such as the one presented below:

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 the incorrect argument type passed to the assertThat method, which expects a Matcher instead of Matcher. To understand why this occurs, we need to delve into the concept of covariance in generic types.

Covariance states that a subtype can be safely assigned to a supertype. In other words, a Class object can be assigned to a Class object. However, the opposite is not true: a Class object cannot be assigned to a Class object.

This is because covariance only applies to the type parameter itself, not to the type arguments it contains. In our example, java.util.Date is a subtype of Serializable, but Class is not necessarily a subtype of Class.

Changing the signature of the assertThat method to Matcher addresses this issue by allowing a matcher that matches any subtype of the actual type. This provides the necessary flexibility for situations like the one in the code snippet.

However, it's important to note that there are potential downsides to using in this context. For instance, it could lead to unexpected behavior or reduced type safety in certain scenarios. As such, it should be used judiciously and with careful consideration.

Regarding the genericity of the assertThat method, it allows for type checking to ensure that the matcher used is compatible with the result type. While this does not completely eliminate the possibility of runtime errors, it adds a level of safety by preventing incompatible matches.

The above is the detailed content of When Should I Use `` vs. `` in Java Generics?. For more information, please follow other related articles on the PHP Chinese website!