Possible OCPJP8 correction, chapter 3 page 120?

[Peter Deak]

In the explanation below the middle code block on page 120, the book states:

"Line 7 doesn't compile because we can't add a Sparrow to List<Bird>"

I'm not sure if that's correct or effectively explaining that case, since based on the code above, a Sparrow could technically be added to a List<Bird> like:

List<Bird> birds = new ArrayList<>();
birds.add(new Sparrow());

[Guillaume Bailly]

Hi Peter,

Technically, what you say is true. Since the List is a list of Bird, we could add instances of Sparrow to it (because of inheritance). So, while the surrounding explanation is correct, the sentence you mention has to be understood this way:

Line 7 doesn't compile because at compile time Java doesn't know what the real type of the list is (it could be Bird, Sparrow, or even another subclass of Bird or Sparrow)

And you agree that if Parrot is another subclass of Bird (next to Sparrow), then I can't possibly add an instance of Sparrow to it.

But you're right, while Java doesn't allow that, it should be clearer as to why it doesn't.

Guillaume

[Jeanne Boyarsky]

Peter,
You are correct that is an errata. It should say line 7 is incorrect because you can't add a Sparrow to a a List<? extends Bird>. Good catch!

[Juerg Bauman]

I don't really understand the explanation of the first compiler error.

import java.util.*;
class Birdies  {
    static class Sparrow extends Bird { }
    static class Bird { }   
    
    public static void main(String[] args) {
        List<? extends Bird> birds = new ArrayList<Bird>();
        birds.add(new Sparrow());             // DOES NOT COMPILE
        birds.add(new Bird());                // DOES NOT COMPILE
    }
}

birds can be: (1)List<Bird> or (2)List<Sparrow> or (3)List<"another possible subclass of Bird">
birds.add(new Sparrow()):  List<Bird> and List<Sparrow> are ok, so the line doesn't compile because birds could be of List<"another possible subclass of Bird">
birds.add(new Bird()): does not compile because birds could be of List<Sparrow> or List<"another possible subclass of Bird">

Line 7 doesn’t compile because we can’t add a Sparrow to List<? extends Bird>...

Is this explanation correct?

[Jeanne Boyarsky]

Yes, your explanation is correct

[Juerg Bauman]

Thank you. For me then this explanation isn't helpful at all. It describes the result - a compiler error, but doesn't explain anything.

Line 7 doesn’t compile because we can’t add a Sparrow to List<? extends Bird>...

I suggest something like:
Line 7 doesn't compile because we can't add a Sparrow to a List of some other subclass of Bird, like f.e. List<Parrot>.