Because of inheritance, a leaf-class has to have a Function<T, R> which it uses as a Supplier<R>. (Ignore the passed parameter)

What is the best way to convey that the generic parameter type is not used and that null should get passed?

CodePudding user response：

To convey the notion that some variable, return type, parameter etc. can only be null the type Void can be used - there's only one valid value for this: null.

So in your case you could use Function<Void, R> to indicate apply(null) will always be called, so the function needs to be mapped to a supplier of sorts. In general you'd want to use Supplier<R> instead but in cases where Function is required this could be an option.

The same would be true for consumers that would need to be represented as functions: use Function<T, Void> in those cases and return null.

CodePudding user response：

Use java.lang.Void

The Void class is an uninstantiable placeholder class to hold a reference to the Class object representing the Java keyword void.

It can't be instantiated, so only null can be passed as parameter.

class SomeClass implement Function<Void, YourResultClass> {
  //implement apply
}