Re: Revisit: List list = new ArrayList();

From:

Lew <lew@lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 07 Nov 2007 21:21:25 -0500

Message-ID:

<ctGdnfICrNE776_anZ2dnUVZ_gWdnZ2d@comcast.com>

Knute Johnson 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.

It depends. I've done that, but not always. The best practice is to declare
a variable to be the most general type suitable to its purpose. Writer might
lack a method that, say, PrintWriter has, and thus you'd not declare a
variable Writer if you needed that extra method.

If you don't need the specific behavior, then you absolutely should declare
the variable to be of the more general type.

And why would you not;

Collection c = new ArrayList();

You might need "c" to be a list - ordered, permissive of duplicates and
index-accessible.

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?

Upcast. The technical name is a "widening reference conversion".
<http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.1.5>

Joshua Bloch has a tip about this in his seminal /Effective Java/.

--
Lew