Re: ordinal() returns inconsistent values?

From:
Lew <noone@lewscanon.com>
Newsgroups:
comp.lang.java.programmer
Date:
Fri, 09 Apr 2010 17:26:16 -0400
Message-ID:
<hpo61p$e77$1@news.albasani.net>
Roedy Green wrote:

It is dangerous to write Java code that depends on the ordinal
remaining invariant. Invent some invariant field on each enum
constant which might just be a single letter or a short word for use
in databases to represent the value.


Amen to that!

More generally one nearly always faces the task of mapping an enum to some
untyped representation and back. Of course, enums build that facility in
already with 'name()' and 'valueOf()'.

<http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.9>
"... the automatically generated methods (values() and valueOf(String)) or
methods that override the final methods in Enum: (equals(Object), hashCode(),
clone(), compareTo(Object), name(), ordinal(), and getDeclaringClass())."

<http://java.sun.com/javase/6/docs/api/java/lang/Enum.html#name()>
"Most programmers should use the toString() method in preference to this one,
as the toString method may return a more user-friendly name." (Original is in
boldface.)

Huh? The very API Javadocs tell you strongly not to use 'name()', which
corresponds to the static 'valueOf(String)'. But what's the static method
that balances 'toString()'? I call mine 'fromString()'.

There's a Hibernate enum converter that takes as parameters the names of the
to- and fro- methods, defaulting to 'name()' and 'valueOf()'.

One's 'fromString()' can be as forgiving or strict as one cares to make it. I
like mine to be case sensitive and to fall back to 'valueOf()' if nothing
better fits. 'fromString( "foo" )' and 'fromString( "FOO" )' would both
return the 'FOO' enum constant in my example below.

template:

/** ${name}.
  */
public enum ${name}
{
   // CONSTANTS GO HERE, e.g.,
   // FOO( "foo" ),
   ;

   private final String repr;

   /**
    * Constructor.
    * @param rep String representation of enum value.
    */
   ${name}( String rep )
   {
     this.repr = rep;
   }

   @Override
   public final String toString()
   {
     return this.repr;
   }

   /**
    * Look up enum constant from String representation.
    * @param rep String representation of enum value.
    * @return ${name} constant matching the representation.
    */
   public static ${name} fromString( final String rep )
   {
     if ( rep == null )
     {
       return null;
     }
     for ( ${name} val : values() )
     {
       if ( rep.equals( val.toString() ) )
       {
         return val;
       }
     }
     return valueOf( rep );
   }
}

--
Lew

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