Re: Revisit: List list = new ArrayList();
On Nov 7, 6:09 pm, Knute Johnson <nos...@rabbitbrush.frazmtn.com>
wrote:
I saw some code go by today that was;
Writer fw = new FileWriter(--- // since ... confuses some people
I know we had a discussion the other day about;
List list = new ArrayList();
and why you would do that. I'm curious as to how many people would do
the above with FileWriter, BufferedOutputStream, or FileOutputStream.
And why would you not;
Collection c = new ArrayList();
The other question I had is what do you call that? Is it a cast? A
sort of implicit cast across assignment? Is it mentioned in the JLS
someplace?
It's an implicit widening reference conversion (5.1.5):
--\--
A widening reference conversion exists from any type S to any type T,
provided S is a subtype (=A74.10) of T.
Widening reference conversions never require a special action at run
time and therefore never throw an exception at run time. They consist
simply in regarding a reference as having some other type in a manner
that can be proved correct at compile time.
See =A78 for the detailed specifications for classes, =A79 for interfaces,
and =A710 for arrays.
--/--
The type of the variable carries around some documentation of what
promises the code expects from the referenced object. In the case of
List<T>, I try to use the most general interface I can; if I need
get(int) but don't care much about the run time, then obviously it's
got to be a List<T> since indexed get is not provided by
Collection<T>, but if all I'm doing is iterating over the contents
then Iterable<T> might suffice (being slightly more general than
Collection<T>). If the algorithm I'm writing relies on O(1) seek
time, then I'd declare it as ArrayList (in the absence of a
"ConstantTimeSeekingList" interface :-).
With the stream decorator patterns I categorize things into sources
(which I pass around as Reader or InputStream instances) and
processors (which I pass around very little, and declare as their
concrete type). Source streams provide the basic stream, and should
be pretty much interchanable, but processors include things like
BufferedReader that provide features not available from the basic
Stream/Reader interfaces.
-O