Can use bounded wildcard

Reports generic method parameters that can make use of bounded wildcards.

Example:

void process(Consumer<Number> consumer);

should be replaced with:

void process(Consumer<? super Number> consumer);

This method signature is more flexible because it accepts more types: not only Consumer<Number>, but also Consumer<Object>.

Likewise, type parameters in covariant position:

T produce(Producer<T> p);

should be replaced with:

T produce(Producer<? extends T> p);

To quote Joshua Bloch in Effective Java third Edition:

tip

Using wildcard types in your APIs, while tricky, makes the APIs far more flexible. If you write a library that will be widely used, the proper use of wildcard types should be considered mandatory. Remember the basic rule: producer-extends, consumer-super (PECS). Also remember that all Comparables and Comparators are consumers.

Use the inspection options to toggle the reporting for:

• invariant classes. An example of an invariant class is java.util.List<T> because it both accepts values (via the List.add(T) method) and produces values (via the T List.get() method).

  On the other hand, contravariant classes only receive values, for example, java.util.function.Consumer<T> with the only method accept(T). Similarly, covariant classes only produce values, for example, java.util.function.Supplier<T> with the only method T get().

  People often use bounded wildcards in covariant/contravariant classes but avoid wildcards in invariant classes, for example, void process(List<? extends T> l). Disable this option to ignore such invariant classes and leave them rigidly typed, for example, .

  void
        process(List<T> l)

• private methods, which can be considered as not a part of the public API

• instance methods