jupiter wrote:
Quote:
The Collections class is supposed to, among other things, return
type safe collections from existing collections with static methods
such as .checkedList().
>
My question is: What is so special about these static methods since
we can do the same thing by declaring the original list generically
with a type like <String>? I mean, once you create the list with
<String the compiler will no longer allow you to add, say,
Integer to the list. So what purpose does .checkedList() provide
in that context?
>
Maybe it's all about backward compatibility?
>
>
For the most part (where "most part" includes all of the collections
interface), all of the type constraints of generics are only checked at
compile time. The type-safe methods, e.g., checkedList, are guarantees
at /runtime/, something which generics can't do. Observe:
ArrayList<Stringfoo = new ArrayList();
List bar = foo;
bar.add(5);
String s = foo.get(0).substring(4);
This passes the compiler (although the compiler does give a warning), so
the code will be compiled to bytecode where it promptly emits a
ClassCastException. Passing the list through to checkedList still gives
an error, but it is emitted at the point of modification as opposed to
the point of access (which, in some cases, might not even give an error!).
In short, it is mostly a backwards-compatible feature, but it is
desirable in circumstances, so it is in no way an anachronism.
