? extends String

[Rodolfo Mendoza]

Hi guys,
Anyone knows why the following code don't work :

public void myMethod(List<? extends String> list){
  list.add("Foo");
}

The following work fine:

public void myMethod(List<String> list){
 list.add("Foo");
}

Thanks,

[Uttara Rishi]

 
public void myMethod(List<? extends String> list)
{
  list.add("Foo");
}
   

You cannot add anything to a List that uses <? extends> or <?>
You can add only when the method signature is <? super Dog?

In short

<? extends> - no adding.
<?> - you can pass anytype into the method but still no adding.
<? super> - adding allowed.

Hope that helps. Please correct me if I am wrong.

Uttara Rishi.

[Rodolfo Mendoza]

Uttara thanks
You are right...

[ahmed yehia]

You cannot add anything to a List that uses <? extends> or <?>

Thats correct but with little exception that you can add 'null'.

[Uttara Rishi]

Thanks Ahmed.

[Vishal Hegde]

Uttara Rishi wrote:

<? super> - adding allowed.
.

? super -adding allowed?? i didnt get it are you reffering to super class in this case

[Stephan van Hulst]

When you have a List<? extends String> you can read Strings from it, but you can't add anything to it.
When you have a List<? super String> you can add Strings to it, but you can only read Objects from it.

[Shahrukh Naqui]

Hi,

Also remember, for <? super SomeClass> means you can add only an object of SomeClass, or sub type of SomeClass.

[Vishal Hegde]

What does ? actually means is it relating to some interface or class

When i tried compiling this code with actual '?' it actually compiled... Is ? some sort of keyword in this case?

[Stephan van Hulst]

? is the generic wildcard. When you have a List<? extends String>, you could interpret it as "A list of whatever, so long as it extends String".

A Set<?> would be "A set of whatever, I don't care what it is".

http://download.oracle.com/javase/tutorial/extra/generics/wildcards.html