Question

Reading the Java online tutorial I haven’t understood anything about wildcard capture.
For example:

    import java.util.List;
    public class WildcardError {
     void foo(List<?> i) {
      i.set(0, i.get(0));
     }
    }

Why can’t the compiler retain the assignment safely?

It knows that, by executing for instance, the method with an Integer List, it gets from i.get an Integer value. So it tries to set an Integer value at index 0 to the same Integer list (i).

So, what’s wrong? Why write Wildcard helper?

Answer 1

why the compiler can’t retain the assignment safe?

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>