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A difficult OOD question...

Huang Teng Shiu

Joined: Aug 11, 2002
Posts: 5
Hi all:

I am a reader of 'Thinking in Java' and now had an OO design question...
The question is a little difficult....
(I am from Taiwan , I can only use easier English)
From 'Thinking in Java 2nd edition' page 360 ,
Bruce introduced a design trick , that uses Month2 for each Month's type.
I want to rewrite this code and implement 'Comparable' interface , as follows :

After this , I want to introduce a 'Season' interface as following:

This interface is very simple , just wrap a 'feeling' to each MyMonth
In Northern Hemisphere ,
For example : March,April,and May are 'warm' ,
so , getFeeling() should return 'warm'
and June,July,Auguse are 'hot' , so , getFeeling() return 'hot' ...etc

To accomplish this , I extend MyMonth and implements SeasonIF,
The new class is named "" (Northern Hemisphere)
The code is as follows :

OK, here is the problem , in fact , the code above is incorrect.
Because 'getIndex()' is from 'MyMonth' and it looks up the index from 'MyMonthList',
It will never find getIndex(NorthMonth) from 'MyMonthList',
therefore , the return value will always be '-1'.

But I don't know how to overcome this , let's skip this...
And if I write a client to test my NorthMonth , the result will be :

The program runs with the error:
ClassCastException: test.MyMonth

I know the error's origin is '(NorthMonth)(MyMonth.APR)'
but I don't know how go fix it....

I've tried many ways to rewrite my client code (
and , but I find no way to solve the problem 'elegantly' ...

The only solution I found is very DIRTY :

I have to re-code almost all '' to ''
including initializing twelve static NorthMonth (from JAN to DEC) ,
private static NorthMonth[] NorthMonthArray ,
private static List NorthMonthList = Arrays.asList(NorthMonthArray) ,
public int getIndex(NorthMonth nm) ,
public int compareTo()...

By this way , it really works , but it is too dirty ....
It lacks the 'elegance' , 'elasticity' and 'code-reusability' of Object-oriented Java

If there are some presuppositions:
1. Do not initialize 12 static NorthMonth in
2. Do not re-code getIndex() and compareTo() , which should be handled by
3. Only implement getFeeling()
4. In , each month is 'MyMonth' , not integer-based
5. MyMonth not abstract .

Do you have better idea how to improve the design ?
Thank you very much.

Best regards.
[ edited to preserve formatting using the [code] and [/code] UBB tags -ds ]
[ August 11, 2002: Message edited by: Dirk Schreckmann ]
Dirk Schreckmann

Joined: Dec 10, 2001
Posts: 7023
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Barkat Mardhani
Ranch Hand

Joined: Aug 05, 2002
Posts: 787
Hello there:

Without going in detail in your code, I wanted to
pointout that in the first line of above code you
are trying to take the static data member APR
from MyMonth class and cast it to NorthMonth
type. Well there is no data member called APR in
this class. You would either have to create an
object of MyMonth with name "APR" and caste it or
create a static data member called APR in this
class. Hope this gives you some clue...

[ August 11, 2002: Message edited by: Barkat Mardhani ]
[ edited to break apart super long lines of code -ds ]
[ August 12, 2002: Message edited by: Dirk Schreckmann ]
Ilja Preuss

Joined: Jul 11, 2001
Posts: 14112
Hi smallufo,
I think your problem is mainly caused by using inheritance - isn't it rather that a month is *associated* to a season? How are these classes used, btw?

The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
Robert Paris
Ranch Hand

Joined: Jul 28, 2002
Posts: 585
Your problem is that you're casting backwards. Look at this:
OK, MyMonth is an Object, correct? So I can do:
Object obj = (Object) new MyMonth("Jan");
HOWEVER, Object is NOT a MyMonth. So this is WRONG:
MyMonth month = (MyMonth) new Object("Jan");
So your class NorthMonth can be cast to a MyMonth:
MyMonth month = (MyMonth) new NorthMonth("Jan");
But since NorthMonth inherits from MyMonth, there's no need to create a MyMonth first, simply do this in main:
NorthMonth northApril = new NorthMonth("April");
and do smth like this in NorthMonth:

(By the way, that will also take care of your other problem)
I agree. Here's the link:
subject: A difficult OOD question...
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