Generic question with ?

[L Yan]

Code Fragment: List <?> l = new ArrayList <Integer>();

Why nothing can be added to this list? For example it's a compile error for l.add(new Object());

if List <? extends Object> l = new ArrayList <Integer>(); Why it's still a compile error for l.add(new Integer());

Is it a rule that if generic type definition has a ?, then no object can be added to the collection, but other operations still work?

Thanks.

[Naseem Khan]

Originally posted by L Yan
Is it a rule that if generic type definition has a ?, then no object can be added to the collection, but other operations still work?

Collection<?> c = new ArrayList<String>();
c.add("hello"); // compile time error

Since we don�t know what the element type of c stands for, we cannot add objects to it. The add() method takes arguments of type E, the element type of the collection.

When the actual type parameter is ?, it stands for some unknown type. Any parameter we pass to add would have to be a subtype of this unknown type. Since we don�t know what type that is, we cannot pass anything in. The sole exception is null, which is a member of every type.

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List <? extends Object> is an example of bounded wildcard. The ? stands
for an unknown type. However, in this case, we know that this unknown type is in fact a subtype of Object. We say that Object is the upper bound of the wildcard.
It seems that compiler will allow adding any object in this case as ny object is subclass of Object.

But its wrong. Still u can't add nything here except null.

Take this case...

    
public void addRectangle(List<? extends Shape> shapes){ 
   shapes.add(0, new Rectangle()); // compile-time error!
}

The type of the second parameter to shapes.add() is ? extends Shape - an unknown subtype of Shape. Since we don�t know what type it is, we don�t know if it is a supertype of Rectangle; it might or might not be such a supertype, so it isn�t safe to pass a Rectangle there.

regards

[Naseem Khan]

If u find problem understanding my explanation then u need to understand bout wildcard first.


One such link i m giving below...


wildcard in generics


regards

[Firas Zuriekat]

Code Fragment: List <?> l = new ArrayList <Integer>();

Why nothing can be added to this list? For example it's a compile error for l.add(new Object());

if List <? extends Object> l = new ArrayList <Integer>(); Why it's still a compile error for l.add(new Integer());

List <?> is EXACTLY the same as List <? extends Object>

Also, when you say "<? super AddMe>", this typically means adding AddMe obj is possible into the list that's passed in the method like this:

void PassAnyList(ArrayList <? super AddMe> a){
AddMe b =new AddMe();
a.add(b);
}

And when you see extends instead of super, then you could say it's a rule that you can't add anything to the list passed in. So if you see List <?>, which actually means <? extends Object>, then because of this rule, you can't add anything to the list passed in.

See my response here for more.
[ June 10, 2006: Message edited by: Firas Zureikat ]

[L Yan]

Thank you for your reply.

Is it also a rule that List<? super String> can only take a String?

List<? super String> list = new ArrayList <String> ();
list.add(new Object());

Why this code cannot compile? Is it always or sometimes that List <? super MyClass> can only add MyClass but not MySuperClass (MyClass extends MySuperClass)?

[Firas Zuriekat]

When using <? super rightHandOperantType>, means you could only add obj instances of the rightHandOperantType (and it's subtypes).

Since you are asking about <? super String>, this means you could add only String objects to the list.

So ofcourse list.add(new Object()) results in compiler error would come up because String is in your right hand side and not object.
[ June 10, 2006: Message edited by: Firas Zureikat ]