在 Java 中导致编译错误的上限通配符

Question

I cannot understand why I am getting these compilation errors:

1:

The method add(capture#1-of ? extends Exec.Bird) in the type List is not applicable for the arguments (Exec.Sparrow)

2:

The method add(capture#2-of ? extends Exec.Bird) in the type List is not applicable for the arguments (Exec.Bird)

static class Bird{}
static class Sparrow extends Bird{}

public static void main(String[] args){
    List<? extends Bird> birds = new ArrayList<Bird>();
    birds.add(new Sparrow()); //#1 DOES NOT COMPILE
    birds.add(new Bird());// //#2 DOES NOT COMPILE
}

Answer 1

With List<? extends Bird> List<? extends Bird> you actually say any type that is a subtype of Bird , or, more accurately, an unknown, but specific type that is a subtype of Bird . It's not the same as saying every type that extends Bird .

That means that ? can be a Sparrow , but it can also be a Blackbird . If you try to add a Sparrow to a list that could contain only Blackbird s, it won't work. For that same reason you cannot add a Bird to a list that could be a List of Sparrow s.

In order to make things work, you just change the declaration of the list to this:

List<Bird> birds = new ArrayList<>();

or use lower bound:

List<? super Bird> birds = new ArrayList<>();

About this lower bound example: the declaration actually says any type that is a Bird or one of its superclasses . That means that you can safely add a Sparrow or a Bird , because both meet those criteria.

Generally speaking, you should use ? super ... ? super ... when you are writing to the list, and ? extends ... ? extends ... when you are reading from the list. If you are both reading and writing, you shouldn't use bounds.

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This answer provides very useful information about generics. You definitely should read it.

Answer 2

You can instantiate the birds list like this:

List<Bird> birds = new ArrayList<>();

Full code:

import java.util.ArrayList;
import java.util.List;

public class Main {
    static class Bird{}
    static class Sparrow extends Bird{}

    public static void main(String[] args) {
        List<Bird> birds = new ArrayList<>();
        birds.add(new Sparrow());
        birds.add(new Bird());
    }
}