Re: HashTable

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 19 Jul 2007 12:13:50 -0700

Message-ID:

<f7od7p$1il0$1@ihnp4.ucsd.edu>

Twisted wrote:

On Jul 19, 3:59 am, Piotr Kobzda <pi...@gazeta.pl> wrote:

Patricia Shanahan wrote:

[...]

Just a small comment to your nice demo:

if (obj == null || !(obj instanceof HasEquals)) {

Since null is never an instance of any class, enough is to say:

if (!(obj instanceof HasEquals)) {

Does this definitely work? Any language lawyers around? I can see
three sensible possibilities:

null instanceof HasEquals == true (since HasEquals foo = null; is
legal)
null instanceof HasEquals == false (since no constructed HasEquals
instance is null)
null instanceof HasEquals throws NullPointerException (since the
reference is null)

and I don't see that it's in any way "obvious" which of the three the
Java folks would have chosen. All make their own kind of sense.

Fortunately, the JLS resolves the issue: "At run time, the result of the
instanceof operator is true if the value of the RelationalExpression is
not null and the reference could be cast (?15.16) to the ReferenceType
without raising a ClassCastException. Otherwise the result is false."

http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.20.2

Piotr is indeed right, and the expression could be simplified. However,
I will probably still go on coding the explicit null check in cases in
which it is not necessary, because I think it makes the correctness of
the code more obvious.

Patricia