名单 <? extends Object> #add（new MyClass（））无法编译

Question

MyClass e = new MyClass();

List<Object> ok = new ArrayList<Object>();
List<? extends Object> ko = new ArrayList<Object>();

ok.add(e);
ko.add(e); // doesn't compile

Why does it doesn't compile? MyClass is whatever a subclass of Object ...

For information, I get the following message:

The method add(capture#1-of ? extends Object) in the type List<capture#1-of ? extends Object> is not applicable for the arguments (MyClass)

Answer 1

This is your problem:

List<? extends Object>

That means "it's a list of some type T which extends Object, but I don't care what T is".

So this would be valid:

List<? extends Object> ko = new ArrayList<Banana>();

... but you wouldn't want:

ko.add(e);

to compile at that point, would you? Because a MyClass isn't a Banana .

See the Java generics FAQ for much more information.

Answer 2

Josh Block, in Effective Java , teaches us PECS : Producer = extends , Consumer = super .

Since you are using ko as a consumer (you add an object to it) you should declare is as:

List<? super Object> ko = new ArrayList<Object>();

Perhaps a better illustration of the concept would be:

List<? super MyClass> ko = new ArrayList<Object>();

You may think of it in the same way that you can only assign a MyClass object to a variable whose type is MyClass, or any super type -- you can only add a MyClass object to a List whose type is MyClass, or any super type.

Going back to your example, List<? extends Object> ko List<? extends Object> ko can't be the right type declaration because it would also accept this:

List<? extends Object> ko = new ArrayList<String>();

And you see here that, based on the generic type of ko , the compiler cannot be sure that the actual List accepts instances of MyClass.