A difficult OOD question...

[Huang Teng Shiu]

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 , Month2.java
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 :

 
package test;
 
import java.util.*;
 
public class MyMonth implements Comparable
{
  private String Name;
  
  public static MyMonth JAN = new MyMonth("January");
  public static MyMonth FEB = new MyMonth("February");
  public static MyMonth MAR = new MyMonth("March");
  public static MyMonth APR = new MyMonth("April");
  public static MyMonth MAY = new MyMonth("May");
  public static MyMonth JUN = new MyMonth("June");
  public static MyMonth JUL = new MyMonth("July");
  public static MyMonth AUG = new MyMonth("August");
  public static MyMonth SEP = new MyMonth("September");
  public static MyMonth OCT = new MyMonth("Octber");
  public static MyMonth NOV = new MyMonth("November");
  public static MyMonth DEC = new MyMonth("December");
  
  private static MyMonth[] MyMonthArray = 
    { JAN , FEB , MAR , APR , MAY , JUN ,
      JUL , AUG , SEP , OCT , SEP , DEC };
  private static List MyMonthList = Arrays.asList(MyMonthArray);
  
  protected MyMonth() {}
  
  protected MyMonth(String name) {
    this.Name = name;
  }
  
  public String toString() {
    return this.Name;
  }
 
  public static int getIndex(MyMonth m) {
    return MyMonthList.indexOf(m);
  }
  
  /**
   * implements Comparable 
   */
  public int compareTo(Object o)
  {
    MyMonth m = (MyMonth) o;
    if (getIndex(this) < MyMonth.getIndex(m) )
      return -1;
    else if (getIndex(this) == MyMonth.getIndex(m) )
      return 0;
    else
      return 1;
  }
}

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

public interface SeasonIF {
  public String getFeeling();
}

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 "NorthMonth.java" (Northern Hemisphere)
The code is as follows :

 
package test;
 
/**
 * Northern Hemisphere Month
 */
public class NorthMonth extends MyMonth implements SeasonIF
{
  /**
   * implements SeasonIF
   */
  public String getFeeling()
  {
    if (getIndex(this) == 2 || getIndex(this) == 3 || getIndex(this) == 4 )
      return ("warm");
    else
    if (getIndex(this) == 5 || getIndex(this) == 6 || getIndex(this) == 7 )
      return ("hot");
    else
    if (getIndex(this) == 8 || getIndex(this) == 10 || getIndex(this) == 11 )
      return ("cool");
    else
      return ("cold");
  }
}

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 :

package test;
 
public class testNorthMonth
{
 public static void main(String[] args)
 {
    NorthMonth northApril = (NorthMonth)(MyMonth.APR); //ERROR HERE !
    System.out.println( "In northern hemisphere , April is :" + northApril.getFeeling() );
 }
}

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 (testNorthMonth.java)
and NorthMonth.java , 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 'MyMonth.java' to 'NorthMonth.java'
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 NorthMonth.java
2. Do not re-code getIndex() and compareTo() , which should be handled by MyMonth.java
3. Only implement getFeeling()
4. In MyMonth.java , 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]

smallufo,
Welcome to JavaRanch!
We ain't got many rules 'round these parts, but we do got one. Please change your display name to comply with The JavaRanch Naming Policy.
Thanks Pardner! Hope to see you 'round the Ranch!

[Barkat Mardhani]

Hello there:

 
public class testNorthMonth{
  public static void main(String[] args) {
    NorthMonth northApril = (NorthMonth)(MyMonth.APR); //ERROR HERE !
    System.out.println( 
      "In northern hemisphere , April is :" + northApril.getFeeling() ); 
  }
}

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...

Thanks.
Barkat
[ 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]

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?

[Robert Paris]

Your problem is that you're casting backwards. Look at this:
Object
--->MyMonth
-------->NorthMonth
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:
//WRONG CODE
MyMonth month = (MyMonth) new Object("Jan");
So your class NorthMonth can be cast to a MyMonth:
//OK CODE
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:

public NorthMonth(String name)
{
    super( name );
}

(By the way, that will also take care of your other problem)