[15] in java-interest
Re: more queries: exceptions, new and package
daemon@ATHENA.MIT.EDU (Arthur van Hoff)
Mon Apr 24 16:24:28 1995
Date: Mon, 24 Apr 1995 12:05:17 -0700
From: Arthur.Vanhoff@Eng.Sun.COM (Arthur van Hoff)
To: java-interest@java.Eng.Sun.COM
> From majordom@java.Sun.COM Mon Apr 24 09:13 PDT 1995
> From: jeremy@sour.sw.oz.au (Jeremy Fitzhardinge)
Hi Jeremy,
> Most of these questions are things which arn't addressed explicitly rather
> than real ambiguities.
>
> The new operator:
>
> What is its precidence? I've made it about the same as unary ops,
> so it isn't confused with normal array operations (eg, so you have
> to use (new int[10])[4] to index a newly created array).
I think that is correct.
> Can simple types have initializers? That is, new int(12) makes a
> reference to an int initialized with 12.
No. Integers are not objects so you can't new them. If you want a
reference to an java.lang.Integer you should use new Integer(12).
> Do constructors for complex types have to have parens? Is (new
> Thing) meaningful on its own?
They must have parens.
> I'm confused about references. Are all variables references to
> their instances, or are simple objects really there and complex
> ones references?
Simple objects are "value" objects. They are copied in an assignment.
Arrays and instances are copied by reference.
> For example, what is the type of the expression
> "new int;"? Is it just an int, in which case the new does nothing?
> What if I really want an int reference?
You can't have an int reference. We have added a set of utility
classes that allow you to wrap an object around an integer. Check out:
java.lang.Integer and others.
> Exceptions:
>
> If you have something like
> try {
> foo();
> } catch(specific_thing a) {
> ...
> } finally {
> tidy();
> }
You can't have a catch AND a finally in the same statement.
try-finally and try-catch are really different statements.
You should write:
try {
try {
foo();
} catch (Exception e) {
...
}
} finally {
toidy();
}
> and foo() throws something which specific_thing doesn't match, does
> the finally block get run, as the exception passes through, or
> would you need to catch and rethrow Object to get that effect?
The finally methods is ALWAYS executed. That is the whole point.
You can even put a return in the try clause and it would still
execute the finally branch BEFORE actually returning.
> The syntax I'm using for try/catch/finally blocks is:
> tryblock: "try" block
> ("catch" "(" arg ")" block)*
> {"finally" block}
> ;
> That is, catch and finally blocks have to be directly adjacent to
> try blocks, and it's possible to have a try block with no catch or
> finally. Is this OK?
It is more like (the {}'s mean one or more):
'try' <stat> { 'catch' '(' <decl-expr> ')' <stat> }
or
'try' <stat> 'finally' <stat>
> Also, is the argument to a catch block allowed to shadow another
> local variable? Is it just like a local variable declaration with
> a scope of the catch block?
Shadowing is discussed in the language spec. It is allowed but our
compiler emits a warning.
> The "package" statement:
>
> The spec says in 6.1 that a package declaration must be the first
> non-comment thing in a file. However, I've found that a number of
> files in the Java Alpha2 distribution (for example,
> classsrc/net/InetAddress.java) have import statements before their
> package statement, which javac accepts. Is it wrong, or is the
> spec wrong?
That is wrong. It should be the first thing in the file. Thanks
for pointing it out.
> Casts again:
>
> I've changed my parsing to match javac; that is, a (simple_type
> ("[" "]")*) is always a cast. I also made it that (e1) op e2 is
> always a cast if op can only be a unary operator, but is a binary
> expression if op could also be binary.
I think that is right.
How is your compiler coming along? Are you writing it in Ansi-C?
I would be interested to take a look at it some time...
Have fun,
Arthur van Hoff (avh@eng.sun.com)
http://java.sun.com/people/avh/
Sun Microsystems Inc, M/S UPAL02-301,
100 Hamilton Avenue, Palo Alto CA 94301, USA
Tel: +1 415 473 7242, Fax: +1 415 473 7104
-
Note to Sun employees: this is an EXTERNAL mailing list!
Info: send 'help' to java-interest-request@java.sun.com
