Does anyone know why the following code does not compile? Neither add() nor addAll() works as expected. Removing the "? extends" part makes everything work, but then I would not be able to add subclasses of Foo.
List<? extends Foo> list1 = new ArrayList<Foo>();
List<? extends Foo> list2 = new ArrayList<Foo>();
/* Won't compile */
list2.add( new Foo() ); //error 1
list1.addAll(list2); //error 2
error 1:
IntelliJ says:
add(capture<? extends Foo>) in List cannot be applied to add(Foo)
The compiler says:
cannot find symbol
symbol : method addAll(java.util.List<capture#692 of ? extends Foo>)
location: interface java.util.List<capture#128 of ? extends Foo>
error 2:
IntelliJ gives me
addAll(java.util.Collection<? extends capture<? extends Foo>>) in List cannot be applied to addAll(java.util.List<capture<? extends Foo>>)
Whereas the compiler just says
cannot find symbol
symbol : method addAll(java.util.List<capture#692 of ? extends Foo>)
location: interface java.util.List<capture#128 of ? extends Foo>
list1.addAll(list2);
(I assume here that Bar and Baz are both subtypes of Foo.)
List<? extends Foo> means a list of elements of some type, which is a subtype of Foo, but we don't know which type. Examples of such lists would be a ArrayList<Foo>, a LinkedList<Bar> and a ArrayList<Baz>.
As we don't know which subtype is the type parameter, we can't put Foo objects into it, neither Bar or Baz objects. But we still know that the type parameter is a subtype of Foo, so every element already in the list (and which we can get from the list) must be a Foo object, so we can use Foo f = list.get(0); and similar things.
Such a list can only be used for taking elements out of the list, not to adding elements at all (apart from null, but I don't know if the compiler actually allows this).
A List<Foo> on the other hand allows adding any object which is a Foo object - and as Bar and Baz are subtypes of Foo, all Bar and Baz objects are Foo objects, so they can be added, too.