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.