Generic extend can't add the same type

Question

I\'m new to these generic types. In the below code, I created a method that accepts a List of items that extends \"String\".

My Question? - When the list can be assigned

Answer 1

There's a subtle difference. It takes a list that contains one type of thing (a thing that extends string). This list may be a subclass of String and therefore not be a String iyswim. See http://docs.oracle.com/javase/tutorial/java/generics/upperBounded.html Upper bounded wildcards.

If it was

     public void takeList(List<? *super* String> list){

Then you could add strings to it, because the list is guaranteed to be able to accept Strings.

Answer 2

Because it's not the runtime type that's relevant here. list is still of type List<? extends String>, you've just happened to assign it to a new ArrayList<String>(). Consider this:

list = rand() ? new ArrayList<String>() : new ArrayList<NotString>();

The compiler could not possibly tell if list.add("test") will be valid -- it only makes decisions based on the compile-time type of list.

Note that in reality nothing extends String, it's a final class.

Answer 3

When you have a variable with a wildcard, and a method that takes the generic type parameter, Java cannot ensure type safety. It must disallow this call.

Consider a List<? extends Animal> for example. You may have assigned it List<Dog>, but the variable could be assigned a List<Squid>. You shouldn't be allowed to add a Dog to such a list.

To allow the add method to be called, remove the wildcard.

public void takeList(List<String> list){

Besides, String is final, so there really is no point to saying ? extends String.