Re: Question about Typesafe Enums


"Hendrik Maryns" <hendrik_maryns@xxxxxxxxxxxxx> wrote in message
news:dnph8d$r85$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Rhino schreef:
> I'm trying to incorporate some of the new features of Java 1.5 into some
> of my existing classes but I'm somewhat confused about how to do something
> with specific with typesafe enums.
>
> The main article in the SDK documentation is
> http://java.sun.com/j2se/1.5.0/docs/guide/language/enums.html. Based on
> that article, I'd like to go a beyond something they show in the Planets
> example. I need the guidance of those of you who understand this better to
> tell me if I can do what I want to do. I don't have the terminology
> straight in my mind yet so forgive me if I don't articulate this
> correctly.
>
> In the Planet enum that is shown in the article, they have a list of
> planets and each planet has two specific values, a mass and a radius. The
> enum provides methods mass() and radius() to get those values when they
> are needed. Therefore, if I want to get the mass of the Earth, my class
> can get it via Planet.EARTH.mass(), e.g.
>
> System.out.println("The mass of the Earth is " + Planet.EARTH.mass());
>
> So far so good.
Indeed.

Now, would it be possible to set up other enums to
> correspond to the values associated with the planets and use them to
> reference the values? For example, could I create an enum called Mass that
> was associated with the mass of each planet, then refer to the mass of a
> specific planet via the new enum, something like this:
>
> System.out.println("The mass of the Earth is " + Planet.EARTH.Mass);
>
> I'm not wedded to the syntax in that last example; in fact, it looks a bit
> nasty to me. I'm just trying to indicate that it would use the new Mass
> enum instead of the mass() method to refer to the first value associated
> with the given planet.

I don´t think you can do this sort of hard-linking two enums together,
but you could define mass() to return something of type Mass, and in the
initialisation of Earth tell it to return Mass.EARTH_MASS.

> It would be neat to be able to avoid writing methods like mass() and just
> use additional enums like Mass to refer to the specific facts that I want.
> Basically, I'd like to be able to use meaningful words like Mass, rather
> than numbers, as subscripts to the implied array that is in the Planet
> enum.

Why would you want to have anything to do with those numbers? Although
they are used by the JVM internally, there is absolutely no to access
them. And how do you see this connected to Mass?

> How could I do what I want to do?

What DO you want to do?

And, maybe more importantly, would it be a
> good idea or would it cause problems?

Doesn´t seem like a good idea, but you are really unclear.

H.

=================================================
I was going to intersperse my followups within your post, just like you did
to mine, but my newsreader isn't co-operating by putting >> in front of my
original words and > in front of your reply....

I'm sorry to be so unclear; like I said, I'm still strugging with the
terminology. Let me try explaining it a bit differently.

Okay, let's say we have an array called Planet and it looks like this (I'll
just use the inner planets to minimize typing):
Object[][] Planet = {
{"Mercury", 3.303e+23, 2.4397e6, 0},
{"Venus", 4.869e+24, 6.0518e6, 0},
{"Earth", 5.976e+24, 6.37814e6, 1},
{"Mars", 6.421e+23, 3.3972e6, 1}
};

In this array, the first column contains a planet name, the second column
contains the mass of the planet, the third column contains the radius of the
planet, and the fourth column contains the number of natural satellites that
the planet has.

If I want to know the number of satellites for the planet Mars, I can get it
via this: Planet[3][3]. But the meanings of the '3's in the subscripts are
not terribly obvious to someone looking at the code; no one would have any
way of knowing the _meaning_ of the fourth row or the fourth column of the
array.

I'd much rather be able to reference the number of satellies for a planet
with syntax that demonstrates a knowledge of the nature of the data, i.e.
the first column contains the planet's name, the fourth column is the number
of satellites, etc. To my way of thinking, this array has (implied) column
names and those column names comprise an enum consisting of the values NAME,
MASS, RADIUS, and SATELLITES. Also, the individual rows of the array can be
uniquely identified by the planet name which is therefore an implied enum
too, consisting of the values Mercury, Venus, Earth and Mars.

I'm not sure how the syntax should look but I'm picturing something that has
words from enums in place of numeric subscripts, something like this:
Planet["Mars"][SATELLITES], which means 'the number-of-satellites value from
the row of the array Planet whose key (first column value) is "Mars"'.

Again, this syntax is probably not entirely appropriate for maintaining
consistentcy with other syntax in Java but it gives the flavour I am
looking for.

I would also like to be able to use variables so that it would be possible
to loop through the array and list the number of satellites for each planet
without using a numeric subscript. Something like:

for (String name, int satellites : Planet) {
System.out.println("Planet " + name + " has " + Planet[NAME][SATELLITES]
+ " satellites.");
}

Sorry, I know that won't compile as it is. I'm not trying to make a firm
proposal on what the syntax should be, I'm just trying to convey the flavour
of what I'd like to do.

It seems to me that Enums are the vital ingredient in making this work but I
may be perverting them beyond their intended use. Or maybe there is a much
better way to do this without using Enums at all.

I'm just looking for some feedback on whether the concept I'm exploring even
makes sense and, if it does, how could I accomplish it with typesafe enums?

Rhino


.

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