For each of these dimensions we can define a primary unit:

The dimension...unit... construct covers these base dimensions:
dimension time - unit second;
dimension distance - unit meter;
dimension mass - unit kilogram;
dimension temperature - unit Kelvin;
dimension electricCharge - unit Coulomb;

Internal machine representations of dimensioned variables are relative to the base units.

Derived Dimensions
Using the base dimensions, others may be expressed through multiplication and division, e.g.,

The dimension...m - =... construct covers these derived dimensions:
dimension speed - = distance - / time;
dimension acceleration - = speed - / time;
dimension impulse - = mass - * speed;
dimension force - = mass - * acceleration;

Derived Units
Units are final variables of dimension types. The ones that are not defined with the base dimensions are derived. Their definition may suggest internal representation like integers, floats or doubles, but the actual representation is adapted at compile time to the highest precision applicable in the context.

The final keyword is not needed:
mass - gram - = kilogram / 1000;
distance - kilometer - = 1000 * meter;
time - minute - = 60 * seconds;
time - hour - = 60 * minute;
time - day - = 24 * hour;

Dimensioned Numbers
The actual usage of dimensions is in dimensioned numbers. These have a type made up of a numeric type (int, float, ...) and a dimension. Actual values are made up of a combination of numeric values (0, 1, 0.1, ...) and units. As with dimensions and units, multiplication and division on dimensioned numbers yield dimensioned numbers of possibly different dimensions. The examples in Listing 1 would be accepted by the compiler.

These examples would produce a compile error, as seen in Listing 2.

Physics Package
The sample package in Listing 3 illustrates how a physics package might look once the dimension and unit keywords are added to Java.

As shown in the physics package, dimensions can be derived from other dimensions and constants could be provided for common physics values.

Finance Package
A finance package would also be an interesting way to take advantage of dimensioned number support in Java. See Listing 4 for an example.

The sample finance package shown brings up an interesting aspect of dimensioned number support in a programming language: that of non-constant units that are derived from variable dimensioned numbers.

Computing Packages
A computing package might also be of value. Such a package could greatly simplify the process of building and debugging applications which require reliable calculation of memory or disk space quantities or conversions between computer-oriented units of measure. Listing 5 shows such a package.

Other dimensions and units would also apply in the computing package, such as CPU clock speed, system and network traffic load, or end-user frustration levels, as they wait for Java applets to execute within Netscape Navigator 2.0.

Implementation
Java compilers may adopt the proposed extension without major difficulty (if you disagree with this proposition, send me e-mail). However, full support will require a modification of the .class file format for the dimensioned numeric data types. This would call for a synchronized operation by all Java compiler builders and users. That can only be justified if the proposed extension has proved to be worthwhile to enough Java developers (and if Sun agrees).

Therefore experiments are needed; the proof of the pudding is in the eating. We need to try out the extension, using preliminary support by compilers and preprocessors, that does not require changes to the class format. This support would be possible in the following way:

An example for new physics.java is shown in Listing 6.

Who is going to implement these ideas? At least for the time being, not me. I am extremely busy with other projects, e.g., see http://www.delftware.nl/scriptic/. So if you are a compiler builder, or if you want to contribute to a GNU Java compiler, please go ahead and let me know.

Emulating Units in Java Today
While no compiler support is available today for dimensions and units in Java, you may use the previous physics.java file to deal with physical units in your code now. Just forget about the double*length syntax etc, but write things like the following:

double len = 1.5*kilometer;
double t = 8.3*minute;
System.out.println("Speed: " + ( (len/t) / (meter/second) ) +
" m/s");

This working style doesn't give you the full benefits of dimensional checking, but at least it will make your code more readable, and you will have less worry about units.