Purpose

This library defines some basic UOM (Unit Of Measure) types for OpenWMS.org. Beside the POJO types there are corresponding types for the persistence layer and an API that uses Jackson.

Motivation

The motivation to create an own library for Units Of Measures is because a UOM is not just a simple string, nor a numeric value it always consists of two parts: The absolute amount and the actual unit. To encapsulate these two values into one single new value for the use in OpenWMS.org this library exists.

Justification

Other Java libraries like the JSR-385 Unit Of Measurement target the same goal to implement units in Java and are very comprehensive. The JSR-385 Unit Of Measurement provides additional API and tools for different measurement systems. After analysis of the JSR-385 Unit Of Measurement we saw it as too complicated for our use case and started with an own and simple implementation.

Implementation

There is a general Measurable interface that represents an UOM and offers functionality to add and substract other Measurables. Arbitrary classes may implement this interface where those classes must take care of the generic type nature of this interface in order to not compare apples with pears.

Extendability

If someone wants to add additional units of measures, the Measurable interface needs to be implemented and registered in several converter classes, see the Piece class for an example.

Shortcomings

Even the Measurable is strictly typed in the type of the amount and the unit, the usage of this generic typed classes is cumbersome in Java. At least for the persistence layer with JPA this is awkward and annoying. So instead of using typed interfaces in JPA entity classes we often use the raw type and check the correct type instance afterward in code. This is a limitation down to the source code level that needs to be addressed.

Resources

$ mvn clean package -Drelease
$ mvn release:prepare
$ mvn release:perform