The document discusses how the JSR 385 Units of Measurement API could have prevented errors in NASA's Mars Climate Orbiter mission. It provides background on the mission, noting that it failed due to a discrepancy between the units used by different teams. The document then explains key concepts in the JSR 385 specification like dimensions, units, quantities, and systems of units. It argues that using this standard avoids errors prone to occurring when developing proprietary unit libraries.

1. How JSR 385
Could Have Saved
the Mars Climate Orbiter
Werner Keil & Thodoris Bais

3. Thodoris Bais
JUG Founder & Leader
Agile Coach
Trainer
@thodorisbais

4. Mars
Climate
Orbiter
● Launched on 11
December 1998
● 338 kilograms
● $125 million
● To study Martian
climate, atmosphere
and surface changes
4

5. 5

6. 6

7. 7
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

8. Other Unit
Mishaps
8
● 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

9. Problem Statement
9

10. What Is the Fundamental Problem?
10
Primitive (Java) types
are primitive types

11. What Is the Fundamental Problem?
11
● 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);

12. Rolling Your Own Library
12
● 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?

13. What is JSR 385?
13

14. JSR 385: Units of Measurement API 2.0
14
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.

15. JSR 385 Basic Concepts
15
● Dimensions
● Units
● Quantities

16. JSR 385 Basic Concepts
16
● Dimensions
● Units
● Quantities
● Prefixes
● Converters
● Formats
● System of units

17. Quantities
17

18. Definition of a Physical Quantity
18
“A physical quantity is a physical property of a phenomenon, body, or
substance, that can be quantified by measurement.”
Source: Wikipedia

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.”
Ibidem

20. 20
c = 299,792,458 m/s
Value Unit
Speed of Light

21. Definition of a Physical Quantity
21
“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.”

22. 22
c = 299,792,458 m/s
= 1,079,252,849 km/h
Speed of Light

23. Units
23

24. Base Units
24
● m
● s
● kg
● A
● K
● mol
● cd

25. Derived Units
25
● m/s
● m/s2
● rad (= m/m)
● Hz (= s-1)
● N (= kg·m·s-2)
● °C (= K - 273.15)

26. Multiples
26
● km
● ms
● Mm?
● ks?

27. Dimensions
27

28. Dimensions
28
● 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

29. Doing the Maths
29

30. Equality and Equivalence
30
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

31. Quantity Addition
● 1 Ω + 1 Ω = 2 Ω
● 1 Ω + 1 mΩ = 1.001 Ω
● 1 mΩ + 1 Ω = 1,001 mΩ
● 1 Ω + 1 m “does not compute”
31

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

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

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

35. Question: How Much is 0 °C + 0 °C?
● 0 °C?
● 0 °C?
● 273.15 °C?
35

36. Question: How Much is 0 °C + 0 °C?
● 0 °C
● 0 °C
● 273.15 °C
36

37. 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 37

38. 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 38

39. 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 39

40. 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)
40

41. Multiplication
41
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

42. Division
42
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

43. Powers
43
Units:
● (m)2 = m2
● (km)2 = km2
● (m2)2 = m4
Quantities:
● (3 m)2 = 9 m2

44. Celsius Revisited
44
Units:
● (K)2 = K2
● (°C)2 ≟ (K - 273.15)2
● m × °C ≟ m·(K - 273.15)
Quantities:
● 0 °C2 ≟ 74,610.9225 K2
● 1 m°C ≟ 1 mK or 273.151K

45. Parsing Units and
Quantities
45

46. Speed of Light
46
● 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

47. Systems of Unit
47

48. Systems of Unit
48
● Metric system
○ Metre, kilogram, second, ampere, …
● Imperial system
○ Foot, ounce, pound, gallon, Fahrenheit, horsepower, …
● United States customary units
○ Foot, ounce, pound, gallon, Fahrenheit, gallon, …
● Roman units of measurement
○ Pes, uncia, libra, …
● Norwegian units of measurement
○ Fot, favn, mål, tønne, snes, …

49. Demo Time!
49

50. What Was the Fundamental Problem Again?
50
Primitive (Java) types
are primitive types

51. JSR 385 Status
51

52. JSR 385
52
● JCP page
○ https://www.jcp.org/en/jsr/detail?id=385
● GitHub repositories
○ https://github.com/unitsofmeasurement/
● Current status:
○ Public Review (PR)
○ Preparing for Final Release

53. A Small Word about JCP
and JSRs…
53

54. The Java Community Process
54
“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

55. Becoming a JCP Member
55
“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

56. 56

57. Thank you!
57