? extends Number - doesn't allow Integer

[Tim Bant]

Morning,

why doesn't the add line here work?

public static void add (List <? extends Number> list) {
		list.add(1234);
	}

I would have thought since Integer extends Number it should allow it?

Thanks as always

[Stephan van Hulst]

It would work if list was a List<Number>. Because Integer extends Number, you can add an Integer to a List<Number>.

But list isn't a List<Number>. It is a List<? extends Number>, meaning a list of some unknown type. At runtime, it could refer to a List<Byte>. I hope you agree that adding an Integer to a List<Byte> is a bad idea.