generics - Java ArrayList of ? extends Interface

Question

I have a group of classes that all implement a validation interface which has the method isValid(). I want to put a group of objects--all of different classes--into an ArrayList, loop through them and call isValid() on each.

Here's my code

Email email = new email();
Address address = new Address();

ArrayList<? extends Validation> myValidationObjects = new ArrayList();

But when I try to do:

myValidationObjects.add(email);

I get:

The method add(capture#2-of ? extends Validation) in the type ArrayList is not applicable for the arguments (Email)

Both Email and Address implement Validation.

According to this document, I should be able to use extends for both interfaces and subclasses.

Answer 1

List<? extends Validation> myValidationObjects

Incorrect reading

"myValidationObjects is list of objects that extend Validation."

Correct reading

"myValidationObjects can be a list of any type that extends Validation. For example, it could be a List<RangeValidation> or a List<RegexValidation>."

Since there is no object you can legitimately add to both a List<RangeValidation> and a List<RegexValidation>, Java prevents you to call add on a variable of such type.

Your case is in fact the simpler one: you need the definite type List<Validation>.