Monday, August 10, 2009

Measures of confusion with JSR-275

I love the idea behind JSR-275, a units of measurement API for Java (see here for a background article by Jean-Marie Dautelle, the lead author) but I wish there were more examples out there about how to use it properly.

I'm presently writing some code for a spatial simulation. I want the user to be able to specify constraints on spatial entities such as their maximum area, width, movement over time and so forth. Bringing JSR-275 to bear on this means that I can use a consistent set of units within the code while leaving the user free to express their input data in whatever units are convenient for them.

The theory is beautiful but my, alas, my code is ugly.

Take this snippet for example...
// get a constraint from the user
Constraint constraint = ...

// get the unit used internally for this constraint (e.g. "m")
Unit<?> stdUnit = ...

// the value for the constraint (in the user's units)
double value = constraint.getValue();

// get the label for the user's units (e.g. "ft")
String unitName = constraint.getUnitLabel();

// match the label to a jsr-275 class
Unit<?> unit = Unit.valueOf(unitName);

if ( !unit.isCompatible(stdUnit) ) {
// e.g. mixing up length and area units
// throw an exception
}

if (!unit.equals(stdUnit)) {
/*
* The user is exercising their freedom to use a different unit
* to us (curse them) so we convert their value to one
* expressed in our units.
*
* (This is where it gets ugly)
*/
UnitConverter converter = null;

// is it an area constraint ?
if (unit.isCompatible(Area.UNIT)) {
converter = unit.asType(Area.class).getConverterTo(
stdUnit.asType(Area.class) );

// is it a length constraint ?
} else if (unit.isCompatible(Length.UNIT)) {
converter = unit.asType(Length.class).getConverterTo(
stdUnit.asType(Length.class) );

// bummer - it was something else I forgot to allow for
} else {
throw new RuntimeException("Constraint system doesn't allow for "
+ unit.toString());
}

value = converter.convert(value);


Now I'd like to avoid the if-else block that says Is it an area ? Is it a length ? and just say...
   converter = unit.getConverterTo(stdUnit);

...but the compiler isn't happy with this. It wants the Unit references to be fully parameterized.

If I was only working with length, for instance, things would be simple...
   Unit<Length> stdUnit = ... // our standard unit (e.g. m)
Unit<Length> unit = ... // the user's unit (e.g. cubits)
UnitConverter converter = unit.getConverterTo( stdUnit );


That's concise and elegant. But since I want to allow constraints to be applied to different quantities (length and area at the moment; later possibly others) I seem stuck with the if-else alternative parameterizations above.

I'm sure I must be missing something here. So, dear reader, if you are a JSR 275 guru please enlighten me !