Why adding <?> to List declaration causes error when converting from String list to Double list?

Question

I'm trying to convert a String list to Double list as follows:

List<String> myStringList = Arrays.asList("10.0", "10.5", "5.0");
List<?> myDoubleList = new ArrayList();
for(String s : myStringList){
        myDoubleList.add(Double.parseDouble(s));
}

When I remove the <?> from list description, it works. It breaks when I add it however. It gives me this error:

The method add(capture#6-of ?) in the type List is not applicable for the arguments (double)

Why is that ?

Answer 1

The idea behind this is that List<?> represents a generic List of some unknown type. This List<?> could be List<String> , List<Double> , List<File> , or anything really, all of which count as List<?> . This means if you have a variable of type List<?> , you can't just add a Double to it, because myListOfWildcard.add(someDouble) would fail if myListOfWildcard is actually a List<String> , which it could be.

The wildcards (in particular, List<?> ) aren't as useful for lists that you want to be able to contain anything. For those you generally want to use List<Object> . A wildcard would be more useful if you want to allow yourself these special cases. If you have a method and you want it to be able to accept any type of list whether it be List<String> or List<Double> , this is what List<?> is useful for, because any of these is a List<?> . However, you lose the ability to assume anything about the actual generic type parameter of the list, which means you can't just add a Double .

List<?> is a shortcut for List<? extends Object> List<? extends Object> . Wildcard lists are mostly useful as the arguments for a method; you usually don't want to create them yourself. Collection.containsAll is a good real world example of their use: In particular, Collection.containsAll accepts any Collection as its argument. It doesn't have to be a Collection<Object> , but it would if Collection<Object> where listed as its type instead of Collection<?> .

Edit: Added paragraphs from comments.