Why is this such use of generic and wildcard not allowed?

Question

This code

static void writeTo(List<? super Apple> apples) {
        apples.add(new Apple());
        apples.add(new Jonathan());
    }

The author of this code stated that

The argument apples is a List of some type that is the base type of Apple; thus you know that it is safe to add an Apple or a subtype of Apple. Since the lower bound is Apple,

Jonathan is a subclass of Apple.

But when I tried this

    List<Jonathan> loj = new ArrayList<Jonathan>();
    listSuper(loj);

It gave me this error

The method listSuper(List<? super Apple>) in the type Basket<T> is not applicable for the arguments (List<Jonathan>)

Where listSuper looks like this

static void listSuper (List<? super Apple> cont) {}

How does the two differ?

Also what confuses me on the first code that I posted is that I thought ? super T means that any base type of T. but from the looks of it he added a subtype of T. I am confused.

Could't understand the question. Both functions seem to have the same signature. What is the problem? — vainolo, Sep 06 '12 at 13:56
@vainolo I added a little information about the error. please do check it out — KyelJmD, Sep 06 '12 at 13:59
[Producer Extends, Consumer Super](http://stackoverflow.com/questions/2723397/java-generics-what-is-pecs/2723538#2723538) — Michael Myers, Sep 06 '12 at 14:06

axtavt · Accepted Answer · 2012-09-06T14:11:51.017

6

List<? super Apple> means a List you can add an Apple to (and since Jonathan is an Apple, you can put Jonathans into a List of that type as well).

It can be List<Apple>, List<Fruit> or List<Object>, but not List<Jonathan>, since you cannot put arbitrary Apples into List<Jonathan>. As you can see, in this case ? can be an Apple or any of its superclasses.

List<? extends Apple> means a List you can get an Apple from. It can be List<Apple> or List<Jonathan>, but not List<Fruit>, since List<Fruit> is not guaranteed to contain only Apples.

This explanation is known as "producer - extends, consumer - super" rule: if parameter acts as a consumer of elements, it should be declared with super, and vice versa.

edited Sep 06 '12 at 14:11

answered Sep 06 '12 at 14:05

axtavt

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but how come he added a Jonathan object inside a List ? super Apple – KyelJmD Sep 06 '12 at 14:11
@KyelJmD: Updated. `Jonathan` is an `Apple`, therefore you can put it into a `List` that can contain `Apple`s. – axtavt Sep 06 '12 at 14:12

score 4 · Answer 2 · answered Sep 06 '12 at 14:04

4

Jonathan is a subtype of Apple, not a supertype. It would match <? extends Apple> but does not match <? super Apple>

answered Sep 06 '12 at 14:04

verdesmarald

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But how come he added a Jonathan inside the a list of apples? in the writeTo method? – KyelJmD Sep 06 '12 at 14:08
There is no problem adding an instance of `Jonathan` to a `List`, why would there be? That is totally different to what you are trying to do... – verdesmarald Sep 06 '12 at 14:13

score 0 · Answer 3 · answered Sep 06 '12 at 14:04

0

The author of the code was wrong. You can't pass a subclass of Apple to a method that takes ? super Apple, only Apple itself and superclasses of Apple. If you want to be able to add subclasses of Apple, you need to use ? extends Apple.

answered Sep 06 '12 at 14:04

Tobb

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You are wrong, read the top answers. A list of type ? super Apple can take Apple and all its subtypes. A list of type ? extends Apple cannot have anything added to it at all, but you are always getting apples from it. – Stefan Sep 06 '12 at 15:30

score 0 · Answer 4 · answered Sep 06 '12 at 14:05

0

The type parameter must be a supertype of Apple, not a subclass, which is what Jonathan is. So, for instance, this would be valid:

List<Fruit> loj = new ArrayList<Fruit>();     
listSuper(loj);

answered Sep 06 '12 at 14:05

GriffeyDog

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blackcompe · Answer 5 · 2012-09-06T15:04:24.547

How does the two differ?

Also what confuses me on the first code that I posted is that I thought ? super T means that any base type of T. but from the looks of it he added a subtype of T. I am confused.

You must distinguish between (1) what you can insert into a generic list and (2) what can be sent as an argument to method with a generic list parameter.

You can insert subtypes of Apple into apples because the bound type parameter is a base class which has the ability to reference all its subtypes.
If Jonathan isn't a super type of Apple, then it's generically incorrect to try and send a list of Jonathan to that method, since I'd be allowed to insert Apples into a list of Jonathans. Then you'd have references of type Jonathan accessing the properties and methods of objects it knows nothing about, which isn't type-safe.

Why is this such use of generic and wildcard not allowed?

5 Answers5