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The Unit of Measurement library provides a set of Java language programming interfaces for handling units and quantities. The interfaces provide a layer which separates client code, which would call the API, from library code, which implements the API. Various groups have implemented the API, see the list of known implementations for details.
The Unit of Measurement library is free software. It may be used, modified and redistributed by anyone for any purpose requiring only maintaining the copyright and license terms on the source code and derivative files. See the license page for details.
The Unified Code for Units of Measure is a code system intended to include all units of measures being contemporarily used in international science, engineering, and business. The purpose is to facilitate unambiguous electronic communication of quantities together with their units. The focus is on electronic communication, as opposed to communication between humans. A typical application of The Unified Code for Units of Measure are electronic data interchange (EDI) protocols, but there is nothing that prevents it from being used in other types of machine communication.
The Unified Code for Units of Measure is inspired by and heavily based on ISO 2955-1983, ANSI X3.50-1986, and HL7's extensions called "ISO+". The respective ISO and ANSI standards are both entitled Representation of [...] units in systems with limited character sets where ISO 2955 refers to SI and other units provided by ISO 1000-1981, while ANSI X3.50 extends ISO 2955 to include U.S. customary units. Because these standards carry the restriction of "limited character sets" in their names they seem to be of less value today where graphical user interface and laser printers are in wide-spread use, which is why the European standard ENV 12435 in its clause 7.3 declares ISO 2955 obsolete.
ENV 12435 is dedicated exclusively to the communication of measurements between humans in display and print, and does not provide codes that can be used in communication between systems. It does not even provide a specification that would allow communication of units from one system to the screen or printer of another system. The issue about displaying units in the common style defined by the 9th Conférence Générale des Poids et Mesures (CGPM) in 1947 is not just the character set. Although The Unicode Standard and its predecessor ISO/IEC 10646 is the richest character set ever it is still not enough to specify the presentation of units because there are important typographical details such as superscripts, subscripts, roman and italics.