generic type

wildcard capture
The compiler doesn't know anything about the type of elements in List<?> i, by definition of ?. Wildcard does not mean "any type;" it means "some unknown type."

It knows that,by executing for instance, the method with an Integer List, it gets from i.get an Integer value.
That's true, but as I said above: the compiler can only know – at compile time, remember – that i.get(0) returns an Object, which is the upper bound of ?. But there's no guarantee that ? is at runtime Object, so there is no way for the compiler to know that i.set(0, i.get(0)) is a safe call. It's like writing this:

List<Foo> fooz = / init /;
Object foo = fooz.get(0);
fooz.set(0, foo); // won't compile because foo is an object, not a Foo
More reading:

Can't add value to the Java collection with wildcard generic type
Java Collections using wildcard
Generic collection & wildcard in java
Generics - Cannot add to a List with unbounded wildcard
What is the difference betwen Collection<?> and Collection<T>

from
https://stackoverflow.com/que...