Generics Problem

[Rajul Konkar]

Can anyone explain me why does 1st scriptlet fails and 2nd works,

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


public void takeList(List<String> list ) {
	list.add("rajul");
}

Thanks in advance

[Mohamed Sanaulla]

Did you try executing?

[Matthew Brown]

Ignore for the moment the fact that String is a final class, and imagine we have a subclass MyString. And we have something like this:

List<MyString> list = new ArrayList<MyString>();
takeList(list);

Now, that would be allowed according to the first version of your method. But then you're adding a String to a list that can only take MyStrings. You've broken type safety. Which is why, whenever you have a List<? extends Something> as a method argument, you can't add anything to it within the method. The compiler can't guarantee that it's safe.

In the second case, the compiler knows inside the method that adding a String is completely safe.

[Rob Spoor]

Matthew Brown wrote:Which is why, whenever you have a List<? extends Something> as a method argument, you can't add anything to it within the method. The compiler can't guarantee that it's safe.

Anything except null that is.