Errata for Java in a Nutshell

From Java In a Nutshell, 5th Edition, section 4.1.3: "Second, consider List methods such as add() that are declared to accept an argument whose type is specified by the type parameter. This is the more surprising case: when the type parameter is unknown, the compiler does not let you invoke any methods that have a parameter of the unknown type because it cannot check that you are passing an appropriate value. A List<?> is effectively read-only since the compiler does not allow us to invoke methods like add( ), set(), and addAll( )." This is incorrect and misleading. First, the compiler *does* allow you to invoke the add() method in this example -- it simply requires that a suitable value (a subtype of the capture variable) be provided. The null reference is such a value, and so it is always possible to write "l.add(null)" when l has type List<?>. Second, there are other ways to mutate a List<?> in addition to directly invoking add(), set(), or addAll(). The clear() method, for example, mutates a collection without making reference to the class's type parameter. And methods like Collections.sort() allow a name to be inferred for the unknown type, which then permits values of inferred type T to be returned by get(), etc., and then passed to add(), etc. It's dangerous to suggest that a read-only interface to a collection can be had by simply weakening its type argument. Instead, programmers should be encouraged to use Collections.unmodifiableList(), etc., and, for better compile-time enforcement, consider weakening the type to Iterable<T> (this interface still doesn't guarantee read-only access statically, though, because the Iterator<T> interface has a remove() method). —Dan