Tue, 19 Feb 2008

Java Reflection

I'm only a few weeks into my Java class and I'm already annoyed at the language. I'm completely willing to ascribe this to newbieness, where I'm just not working with what the language gives me, but the metaobject stuff in Java seems a bit painful.

I'm working on a project for the class where I have to accept input in several different units of heat (BTUs, calories, and joules) and output the measurement in joules. I've made an abstract base class for the various units and created concrete classes for each unit the program has to read. I'd like to just have a list of available classes and have my program enumerate them automatically (rather than hardcoding the behavior for each), but the way I would normally think about doing this is painful in Java.

In Delphi, I'd do something like this:

type
  THeatUnits = class
   public
    constructor Create(Value : Real); virtual;
    class function GetUnitsName : String; virtual; abstract;
    function ConvertToJoules : Real; virtual; abstract;
  end;

  THeatUnitsClass = class of THeatUnits;

  TBTUs = class(THeatUnits);
  TCalories = class(THeatUnits);
  TJoules = class(THeatUnits);

const
  availableUnits : array [1..3] of THeatUnitsClass
                 = (TBTUs, TCalories, TJoules);

...

procedure DoStuff(Index : Integer; Value : Real);
  var
    Units : THeatUnits;
begin
  Units := availableUnits[Index].Create(Value);
  // Now do things polymorphically with Units.
end;

Delphi's class types often seem like a quick hack to me, but they beat what Java does in the same situation. For one thing, there doesn't seem to really be a class type in the same way that Delphi does it. There are instead objects of type Class. As far as I can tell, the best way to get one of those is to call Class.forName("ClassName"). But the painful part is that there's no specialization of class types at compile time, so the Java code equivalent to my Units := availableUnits[Index].Create(Value); above would be something like this:

static final AVAILABLE_UNITS = new String[] ("BTUs", "Calories", "Joules");

public void doStuff(int index, double value) {
  Class unitClass = Class.forName(AVAILABLE_UNITS[index]);
  Constructor unitConstructor = unitClass.getConstructor(new Class[] (double.class));
  HeatUnits units = (HeatUnits)unitConstructor.newInstance(new Object[] (new Double(value)));
  // now do things polymorphically with units.
}

(Common Lisp, of course, would be more succinct than Delphi, because everything is first-class; I would probably do something like this:

(defclass heat-units () ())

(defgeneric get-units-name (unit-class))
(defgeneric convert-to-joules (unit-class))

(defclass btus (heat-units) ())
(defclass calories (heat-units) ())
(defclass joules (heat-units) ())

(defparameter +available-units+ #(btus calories joules))

(defun do-stuff (index value)
  (let ((units (make-instance (svref +available-units+ index)
                              :value value)))
    ;; Now do things polymorphically with units.
    ))

)

As I said, though, I think this is just an artifact of not thinking in Java to the appropriate degree. Most of Java's reflection stuff seems set up to be useful at run-time while Delphi's run-time reflection is much uglier than Java's. And I think I'm going to approach my Java problem from a different direction, with an enum and a factory method. I was still struck by how annoyingly wordy (and not entirely typesafe) my first approach turned out to be in Java.


Phil! Gold