Inadequate models of physical measurements can lead to significant programmatic errors. In particular, the practice of modeling a measure as a simple number with no regard to the units it represents creates fragile code. Another developer or another part of the code might misinterpret the number as representing a different unit of measurement. For example, it might be unclear whether a person's weight is expressed in pounds, kilograms, or stone. The issue is difficult to solve through testing and has already cost millions of dollars to society worldwide. (NASA lost a $125 million Mars orbiter because a Lockheed Martin engineering team used English units of measurement while the agency's team used the more conventional metric system for a key spacecraft operation.)
JSR-275 introduces a new package, javax.measure, with an interface, Measurable, and a class, Measure.
Measurable represents an attribute which could be measured which could be measured (e.g. Altitude, Height), and for which a value can be retrieved, given a compatible unit:
Measurable<length> height = person.getHeight();
long inches = height.longValue(NonSI.INCH);
By using generics, Measurable is typesafe; a compile-time error would be generated when passing Measurable<mass> to a function that requires Measurable<length>:
Measurable<mass> weight = person.getWeight();
person.setHeight( weight ); // error!
Measure represents a particular measurement, the combination of a numeric value and a particular unit. A particular measurable value (e.g. geoffreyWiseman.getHeight()) could be represented by different measures (e.g. Measure.valueOf(73,NonSI.INCH), Measure.valueOf(1.8542,SI.METRE)).
The API also contains quantities (e.g. Mass, Height, Power, Pressure), which are used to parameterize the generic measures and measurements, and Units (INCH, METRE), which are held within systems of units. SI represents the metric system, both with prefixes and units (METER, KILO(METER), etc.), and NonSI holds common units that aren't part of the metric system (DAY, FOOT, KNOT, etc.).
It's possible to convert between units of the same dimension using a UnitConverter. Parsing and formatting are accounted for, including compound units for combinations like feet and inches; hours, minutes and seconds. The API is designed for extension, supporting the creation of new units, systems of units, quantities.
There has been some discussion of using JSR-275 to support JSR-310, the date/time API, but there would need to be further discussion. Stephen Colebourne points out in JSR-275: Suitable for Java 7? that date/time units can't be treated like consistent scientific units for conversion:
Er, how exactly did we manage to convert between months and days? What does that really mean? Well, assuming I've understood everything, what it does is use a definition of a year as 365 days, 5 hours, 49 minutes, and 12 seconds, and a month being one twelth of that. It then goes ahead and does the conversion from months to days using that data.
To learn more, investigate the specification or the reference implementation.