In 1999, NASA lost the $125 million Mars Climate Orbiter as it went into orbital insertion. Due to a mismatch between US customary and SI units of measurements in one of the APIs, the spacecraft came to close to the planet, passed through the upper atmosphere and disintegrated. Sadly, this hasn’t been the only instance where a mismatch between units of measurements had catastrophic consequences, but it’s certainly one of the most spectacular and expensive ones. How could this happen? The bad news is: if you use primitive types to handle quantities in your code, due to that very same practice. At best, you’ve codified the unit in the name of the variable or the database field, e.g. calling it lengthInMetres. Otherwise, you’re only relying on convention, just like Lockheed Martin and NASA did. Join this talk to learn how JSR-385 can help you avoid $125 million mistakes, and discover the immeasurable world of dimensions, units and quantities.

1. How JSR 385 Could Have Saved
the Mars Climate Orbiter
Filip van Laenen, JSR 385 Contributor

2. Mars
Climate
Orbiter
● Launched on 11
December 1998
● 338 kilograms
● $125 million
● To study Martian
climate, atmosphere
and surface changes 2

3. 3

4. 4

5. 5
Mars
Climate
Orbiter
“The problem here was not the error; it was the
failure of NASA's systems engineering, and the
checks and balances in our processes, to detect
the error. That's why we lost the spacecraft.”
—Edward Weiler,
NASA associate administrator for space science,
IEEE Spectrum: Why the Mars Probe went off
course

6. Other Unit
Mishaps
6
● The 12 October 1492 “mishap”
● “Gimli Glider”, 1983
○ Air Canada Flight 143
○ Received less than half the fuel
needed
● Clarence the Tortoise, 2001
● A whole set of medication
dose errors

7. Problem Statement
7

8. What Is the Fundamental Problem?
8
Primitive (Java) types
are primitive types

9. What Is the Fundamental Problem?
9
● Primitive (Java) types are primitive types
○ As in: building blocks to build other types
● Examples fundamentally in conflict with OOP and DDD:
○ static final double C = 1079252849;
○ static final double SPEED_OF_LIGHT = 1079252849;
○ static final double SPEED_OF_LIGHT_IN_KM_PER_H = 1079252849;
● What it should be:
○ static final Quantity<Speed> SPEED_OF_LIGHT
= Quantities.getQuantity(1079252849, Units.KILOMETRE_PER_HOUR);

10. Rolling Your Own Library
10
● A lot of work
○ Development
○ Maintenance
○ Upgrades and extensions
● Error-prone
● Are you a units of measurement domain expert?
○ Even of the units of measurement system you’re used to?

11. What is JSR 385?
https://www.slideshare.net/filipvanlaenen
11

12. JSR 385: Units of Measurement API 2.0
12
Description:
This JSR is a major evolution of the Unit API 1.0 (JSR 363) specification.
Focused on the SI System redefinition, modularity and support for Java
SE 8/9 and above.
JSR 363: Units of Measurement API
This JSR specifies Java packages for modeling and working with
measurement values, quantities and their corresponding units.

13. JSR 385 Basic Concepts
13
● Dimensions
● Units
● Quantities

14. JSR 385 Basic Concepts
14
● Dimensions
● Units
● Quantities
● Prefixes
● Converters
● Formats
● System of units

15. Quantities
15

16. Definition of a Physical Quantity
16
“A physical quantity is a physical property of a phenomenon, body, or
substance, that can be quantified by measurement.”
Source: Wikipedia

17. Definition of a Physical Quantity
17
“A physical quantity is a physical property of a phenomenon, body, or
substance, that can be quantified by measurement.”
Source: Wikipedia
“A physical quantity can be expressed as the combination of a magnitude
expressed by a number – usually a real number – and a unit.”
Ibidem

18. 18
c = 299,792,458 m/s
Value Unit
Speed of Light

19. Definition of a Physical Quantity
19
“A physical quantity is a physical property of a phenomenon, body, or
substance, that can be quantified by measurement.”
Source: Wikipedia
“A physical quantity can be expressed as the combination of a magnitude
expressed by a number – usually a real number – and a unit.”
“The same physical quantity can be represented equivalently in many unit
systems.”
Ibidem

20. 20
c = 299,792,458 m/s
= 1,079,252,849 km/h
Speed of Light

21. Units
21

22. Base Units
22
● m
● s
● kg
● A
● K
● mol
● cd

23. Derived Units
23
● m/s
● m/s2
● rad (= m/m)
● Hz (= s-1
)
● N (= kg·m·s-2
)
● °C (= K - 273.15)

24. Multiples
24
● km
● ms
● Mm?
● ks?

25. Dimensions
25

26. Dimensions
26
● L: length
● T: time
● M: mass
● I: electric current
● Θ: thermodynamic temperature
● N: amount of substance
● J: luminous intensity
● (1)
Force: L·M·T-2

27. Doing the Maths
27

28. Equality and Equivalence
28
Units:
● kΩ ≠ mHz
● kΩ ≠ mΩ
● kΩ ≡ kΩ
● N ≅ kg·m·s-2
● kg·m-2
≅ mg·mm-2
Quantities:
● 1 kΩ ≠ 1 mHz
● 1 kΩ ≠ 1 mΩ
● 1 kΩ ≡ 1 kΩ
● 1 N ≅ 1 kg·m·s-2
● 1 kg·m-2
≅ 1 mg·mm-2
● 1 kΩ ≅ 1,000 Ω

29. Quantity Addition
● 1 Ω + 1 Ω = 2 Ω
● 1 Ω + 1 mΩ = 1.001 Ω
● 1 Ω + 1 kΩ = 1,001 Ω
● 1 Ω + 1 m “does not compute”
29

30. Question: How Much is 0 °C + 0 °C?
30

31. Question: How Much is 0 °C + 0 °C?
● 0 °C?
31

32. Question: How Much is 0 °C + 0 °C?
● 0 °C?
● 0 °C?
32

33. Question: How Much is 0 °C + 0 °C?
● 0 °C?
● 0 °C?
● 273.15 °C?
33

34. Question: How Much is 0 °C + 0 °C?
● 0 °C
● 0 °C
● 273.15 °C
34

35. Question: How Much is 0 °C + 0 °C?
● 0 °C
● 0 °C
● 273.15 °C
Two absolute temperatures:
0 °C + 0 °C = 273.15 K + 273.15 K
= 546.30 K
= 273.15 °C
35

36. Question: How Much is 0 °C + 0 °C?
● 0 °C
● 0 °C
● 273.15 °C
An absolute temperature and a temperature change:
0 °C + 0 °C = 273.15 K + 0 K
= 273.15 K
= 0 °C
36

37. Question: How Much is 0 °C + 0 °C?
● 0 °C
● 0 °C
● 273.15 °C
Two temperature changes:
0 °C + 0 °C = 0 K + 0 K
= 0 K
= 0 °C
37

38. Question: How Much is 0 °C + 0 °C?
273.15 °C (546.30 K)
0 °C + 0 °C = 0 °C (273.15 K)
0 °C (0 K)
38

39. Multiplication
39
Units:
● kg × m = kg·m
● kg × mm = kg·mm
● g × km = g·km ≟ kg·m
● m × m = m2
● km × km = km2
● mm × km = mm·km ≟ m2
● µm × m = µm·m ≟ mm2
Quantities:
● 2 kg × 3 m = 6 kg·m

40. Division
40
Units:
● kg / m = kg·m-1
● kg / mm = kg·mm-1
● g / mm = g·mm-1
≟ kg·m-1
● m2
/ m = m
● m / m = 1
● km / km = 1
● km / m = km/m ≟ 1k
Quantities:
● 3 kg / 2 m = 1.5 kg·m-1

41. Powers
41
Units:
● (m)2
= m2
● (km)2
= km2
● (m2
)2
= m4
Quantities:
● (3 m)2
= 9 m2

42. Celsius Revisited
42
Units:
● (K)2
= K2
● (°C)2
≟ (K - 273.15)2
● m × °C ≟ m·(K - 273.15)
Quantities:
● 0 °C2
≟ 74,610.9225 K2

43. Parsing Units and
Quantities
43

44. Speed of Light
44
● 299,792,458 m/s
● 299792458 m/s
● 299792.458 km/s
● 299,792.458 km/s
● 299,792.458 km*s-1
● 299,792.458 km×s-1
● 299,792.458 km·s-1
● 299,792.458 km·s⁻¹
● 299,792.458 km*s^-1
● 1,079,252,849 km/h
● 1,079,252,849 km/t
● 670,616,629 mph

45. Systems of
Measurement
45

46. Systems of Measurement
46
● Metric system
○ Metre, kilogram, second, ampere, …
● Imperial system
○ Foot, ounce, pound, gallon, Fahrenheit, horsepower, …
● United States customary units
○ Foot, ounce, pound, gallon, Fahrenheit, horsepower, …
● Roman units of measurement
○ Pes, uncia, libra, …
● Norwegian units of measurement
○ Fot, favn, mål, tønne, snes, …

47. What Was the Fundamental Problem Again?
47
Primitive (Java) types
are primitive types

48. JSR 385 Status
48

49. JSR 385
49
● JCP page
○ https://www.jcp.org/en/jsr/detail?id=385
● GitHub repositories
○ https://github.com/unitsofmeasurement/
● Current status:
○ Early Draft Review (EDR)
○ Preparing for Public Review and Public Review Ballot

50. A Small Word about
JCP and JSRs…
50

51. The Java Community Process
51
“The JCP gives you a chance to have your own work become an official
component of the Java platform and to offer suggestions for improving
and growing the technology. Either way, everyone in the Java community
benefits from your participation. That's one of the reasons the JCP is open
to everyone.”
Source: https://www.jcp.org/en/participation/overview

52. Becoming a JCP Member
52
“Membership in the JCP offers you a chance to become a permanent part
of the Java platform's history by contributing your work and
recommendations to the various standard specifications, and/or a chance
of serving on the Executive Committee.”
Source: https://www.jcp.org/en/participation/membership

53. 53

54. 54
What I
used to
be

55. 55
Computas about
to sign the JSPA

56. Questions?
Comments?
https://www.slideshare.net/filipvanlaenen
@filipvanlaenen
56