[139] in java-interest
Overriding and an extention
daemon@ATHENA.MIT.EDU (Jeremy Fitzhardinge)
Mon May 29 17:28:08 1995
From: jeremy@sour.sw.oz.au (Jeremy Fitzhardinge)
To: java-interest@java.Eng.Sun.COM
Date: Mon, 29 May 1995 15:19:58 +1000 (EST)
1. If I have the following:
class A {
abstract void a();
}
interface I {
void a();
}
class B extends A implements I {
void a() {}
}
Is class B considered to have implemented both the interface and the
abstract method? Is there a way of having separate implementations,
so "B as an A" is different from "B as an I"?
2. Though the spec doesn't mention it, javac seems to do something
with "break" and "continue" statements in "synchronized" and "finally"
blocks. Do they really do something special, and if so, what?
3. An extention:
Add an "assert(arg)" "operator" which evaluates arg as a boolean
expression. If it's true, nothing happens. If not, and you compile
the class debug mode, it will throw an exception which would
generally not be caught. "abort()" could just throw the exception,
but that doesn't seem necessary. This would allow pre and post
condition checks similar to C's with a simple syntax (I noticed
that a lot of the javac code has things like if(!exp) { throw new
InternalError; }). The compiler could warn if there's any side-effects
in "arg", since the assertion expression need not be executed.
In addition, an assert() could be an optimisation hint to the
runtime. For example, if you have:
void x(int len, int[] a)
{
assert(len < a.length);
for(int i = 0; i < len; i++)
dosomething(a[i]);
}
If it's clever enough, the runtime could see the assertion, and
therefore know that i is always going to be less than the array
length, and need not generate bounds check code for each loop
iteration. Perhaps this isn't the best example, since the runtime
could do the same analysis with "for(int i = 0; i < a.length; i++)",
but an assertion may make it cheaper. The analysis would have to
be rigorous either way, but the assertion could say whether the
runtime should bother or not.
J
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