Generic Methods or Wildcards in Java: When to Use Which?

Generic Methods vs. Wildcards

When encountering the concept of generic methods in Java, one may wonder about the distinction between using wildcards and generic methods themselves. Oracle's documentation provides some insights into this matter.

When to Use Wildcards

The documentation explains that wildcards should be employed when the type parameter is solely used for polymorphism, allowing various argument types at different invocations. This flexibility supports subtyping, which is the objective here.

Consideration for Generic Methods

Generic methods, conversely, are appropriate when the type parameter expresses dependencies among argument types and return values. If such dependencies do not exist, generic methods should not be utilized.

Method Signature Examples

To illustrate the distinction, in the provided example:

public static <T> void copy(List<T> dest, List<? extends T> src)

can be rewritten as:

public static <T, S extends T> void copy(List<T> dest, List<S> src)

In the second version, T and S represent type parameters. The first signature implies that types passed to dest and src do not necessarily have a relationship, while the second signature ensures their compatibility due to the relationship between T and S.

Conclusion

Ultimately, the decision of whether to use generic methods or wildcards depends on the specific requirements of the application. Generics facilitate type safety and enforce relationships between parameters, while wildcards offer flexibility for subtyping. By understanding their nuances, developers can make informed choices to optimize code design.

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