Question

I’m not sure why the last statement in the following code is illegal. Integer should be a subtype of ?, so why can’t I assign it to b?

List<String> a = new ArrayList<String>();
a.add("foo");

// b is a List of anything
List<?> b = a;

// retrieve the first element
Object c = b.get(0);
// This is legal, because we can guarantee
// that the return type "?" is a subtype of Object

// Add an Integer to b.
b.add(new Integer (1)); 

Answer 1

The point is that b refers to a list of some type, but the compiler doesn’t know what the type is, so it doesn’t know whether or not it’s valid to add an Integer to it. And a good thing too, given your example – you’d be adding an Integer to an object initially created to hold a list of strings. Sure, that information is lost at execution time in Java – but the compiler tries to keep you as safe as it can.

See the Java generics FAQ for a lot more information.