Why cannot add String to type to List<?>?

Question

The error:

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

Code:

List<?> to = new ArrayList<Object>();
to.add(new String("here"));

Since List<?> is a generic type List and therefore can be of any type, then why it is not accepting String in add method?

Answer 1

A List<?> is a list of some type, which is unknown. So you can't add anything to it except null without breaking the type-safety of the list:

List<Integer> intList = new ArrayList<>();
List<?> unknownTypeList = intList;
unknownTypeList.add("hello"); // doesn't compile, now you should see why

Answer 2

should a String not be acceptable ?

No. <?> means the type is unknown and the compiler cannot be sure that any type is acceptable to add (this include String)

Answer 3

You can specify a lower bound:

List<? super Object> to = new ArrayList<Object>();
to.add(new String("here")); // This compiles

Now the compiler is sure that the list can contain any Object

Answer 4

According to wildcards, the question mark (?), called the wildcard, represents an unknown type and not a generic type, since its an unknown type the compiler cannot accept String in your case.