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.

Why not to use ArrayList<Validation> without any extends? — Mikita Belahlazau
– Mikita Belahlazau, Commented Jan 4, 2013 at 20:35
? extends Validation means some specific but unknown subclass of Validation. For all the compiler knows, it could be some other subclass completely unrelated to Email or Address. — Louis Wasserman
– Louis Wasserman, Commented Jan 4, 2013 at 20:37

Community · Accepted Answer · 2020-06-20 09:12:55Z

7

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>.

edited Jun 20, 2020 at 9:12

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answered Jan 4, 2013 at 20:38

Marko Topolnik

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Comments

jlordo · Accepted Answer · 2013-01-04 20:35:16Z

5

You can use:

List<Validation> myValidationObjects = new ArrayList<>();           // Java 7
List<Validation> myValidationObjects = new ArrayList<Validation>(); // pre Java 7

Now you can add any instance of a class that implements Validation to that list.

answered Jan 4, 2013 at 20:35

jlordo

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Comments

Henry · Accepted Answer · 2013-01-04 20:37:12Z

2

The declaration ArrayList<? extends Validation> means a list of an unknown class that extends Validation. Email is not compatible with this unknown class.

You can use ArrayList<Validation> for your list.

answered Jan 4, 2013 at 20:37

Henry

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Comments

yshavit · Accepted Answer · 2013-01-04 20:37:57Z

If a generic class's T is <? extends Foo>, then the only thing you can pass to a method that takes T is null -- not any subclass that extends Foo.

The reason is that List<? extends Validation> doesn't mean "a list of things that extend Validation". You can get that with just List<Validation>. Instead, it means "a list of some type, such that that type extends Validation."

It's a subtle distinction, but basically the idea is that List<? extends T> is a subtype of List<T>, and you therefore don't want to be able to insert anything into it. Think of this case:

List<FooValidation> foos = new ArrayList<>();
List<? extends Validation> validations = foos; // this is allowed
validations.add(new BarValidation()); // not allowed! this is your question
FooValidation foo = foos.get(0);

If the third line were allowed, then the last line would throw a ClassCastException.

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