OK, here's a problem which many people must have encountered, and there
must be a 'best practice' solution.
I have a thing which is configurable by plugging other things into it.
Obviously the other things I plug in must conform to specific interfaces,
but what thing I actually plug in is determined at run-time by reading
the name of the plugin class from a configuration file. An example (in
Java 1.4) is as follows:
1 String v = config.getValueAsString( "authenticator_class");
2
3 if (v != null) {
4 Class authenticatorClass;
5
6 try {
7 authenticatorClass = Class.forName( v);
8 } catch (ClassNotFoundException c) {
9 throw new InitialisationException(
10 "Could not find class [" + v
11 + "]", c);
12 }
13
14 try {
15 authenticator =
16 (Authenticator) authenticatorClass
17 .newInstance();
18 } catch (ClassCastException e) {
19 throw new InitialisationException(
20 "Not a valid authenticator class", e);
21 } catch (InstantiationException f) {
12 throw new InitialisationException(
23 "Could not instantiate authenticator", f);
24 }
25 }
Obviously one can vacuously 'bring this up to date' by changing line 4 to
4 Class<?> authenticatorClass;
but I feel that the right thing to do must surely be to use
4 Class<Authenticator> authenticatorClass;
5
6 try {
7 authenticatorClass =
(Class<Authenticator>) Class.forName( v);
8 } catch (Exception e) {
then if the class specified did not inherit from Authenticator a
ClassCastException would be caught at line 8, and the second try/catch
block might become redundant. However, if I do that, Java 1.6 gives me a
warning at line 7:
'Type safety: Unchecked cast from Class<capture#1-of ?> to
Class<Authenticator>'
Eclipse offers to fix this by adding an @SuppressWarnings clause, but I'm
not sure I want to suppress warnings...
What is the preferred pattern in Java 1.5/1.6, and why?