I would like to describe such cases when we create problems for "future us" just by an accident. I will show how different Java data types can ease or increase the pain in supporting the application later. Most common pitfals and tricky corner cases you probably have never thought about.

1. Data
Wars
The Bloody Enterprise
strikes back

2. Victor
Polischuk
@alkovictor

3. It is GOOD
when we
have a lot of
data
when we
have data
several years
old, the older
the better
It is
BAD
when we
have to
remove
historical
data

4. It is BAD
when we
have a lot of
code
when we
have code
several years
old, the older
the worse
It is
GOOD
when we
have to
remove
historical
code

5. Money,
numbers, and
arithmetic
Identities
Text data and
Strings
Date and time

6. Please
Participate

7. Money

8. Money
Float & Double
Convert to Integer

9. Money Float &
Double
Problem
Developers usually have no idea how it is represented:
• 𝑒 = 2.718281828459045
• π = 3.141592653589793
• tan
π
2
= 1.633123935319537𝑬16

10. Money Float &
Double
Quiz #1
• = ?
Float: 0.6 + 0.1
• = ?
Double: 0.6 + 0.1

11. Money Float &
Double
Quiz #1
• = 0.70000005
Float: 0.6 + 0.1
• = 0.7
Double: 0.6 + 0.1

12. Money: Float & Double: stackoverflow.com

13. Money Float &
Double
Quiz #2
• = ?
Float: 0.2 + 0.1
• = ?
Double: 0.2 + 0.1

14. Money Float &
Double
Quiz #2
• = 0.3
Float: 0.2 + 0.1
• = 0.30000000000000004
Double: 0.2 + 0.1

15. Money Float &
Double
Drill
Down
• Binary representation: [sign] [exponent] [mantissa]
• Float: 1 bit, 8 bits, 23 bits
• Double: 1 bit, 11 bits, 52 bits
• Value: (2 ∗ 1 − 𝑠 − 1) ∗ 2 𝑒−2 𝐸−1−1 ∗ (1 + 𝑚/2 𝑀)
0.1f = 0-01111011-10011001100110011001101
0.1f = + 2-127+123 * (2-1 + 2-4 + 2-5 + 2-8 + 2-9 + 2-12 + 2-13 + 2-16 + 2-17 …)
0.1f = 2-4 * (1 + 5033165 / 223) = 0.100000001490116119384765625

16. Money Float &
Double
Example

17. Money Float &
Double
Quiz #3
+0.0f = 0-00000000-00000000000000000000000
-0.0f = 1-00000000-00000000000000000000000
+0.0f == -0.0f?

18. Money Float &
Double
Drill
Down
/**
* Get or create float value for the given float.
*
* @param d the float
* @return the value
*/
public static ValueFloat get(float d) {
if (d == 1.0F) {
return ONE;
} else if (d == 0.0F) {
// -0.0 == 0.0, and we want to return 0.0 for both
return ZERO;
}
return (ValueFloat) Value.cache(new ValueFloat(d));
}

19. Money Float &
Double
Summary
Just never use it
Forget it exists
Unless you are working on a video codec

20. Money Convert
to Integer
Multiply decimals up to integers
• (as a constant probably)
Keep the “scale” somewhere else

21. Money Convert
to Integer
Quiz #1
•= ?
10 * 230 + 5

22. Money Convert
to Integer
Quiz #1
•= 28, where 10 is 10%
10 * 230 + 5

23. Money Convert
to Integer
Drill
Down
@Embeddable
public class Amount implements Serializable {
private int rate;
@Transient
private final int scale;
public Amount() {
scale = 6;
}
public Amount(int rate, int scale) {
this.scale = scale;
setRate(rate);
}
…

24. Money Convert
to Integer
Summary
• It is better to keep precision closer to the number
• It is better when arithmetic just works
• It is better when equals and compareTo work
• int <*/+> int can exceed int (same with long)
• Consistency is almost always above performance

25. Money Solution BigDecimal
Precision and accuracy are known and adjustable
Arithmetic is included
Supported by JDK, JDBC, and etc
Performance is quite nice

26. identity

27. Identity
Unexpected Overflow
UUID

28. Identity Overflow
Integer and Long are finite types
Sometimes they can overflow
Moreover they usually twice smaller than you think

29. Identity Overflow Example

30. Identity Overflow Example

31. Identity Overflow
Sucker
Punch
• Also, “some languages” cannot work with 53+ bits integer types
• In addition, “some languages” work with custom 32-bit integer types

32. Identity Overflow Summary
There is a difference between DB and API identity
• Always use integer types as identity for DB
• Always use text types as identity for API
• Avoid using 32- bit types as identity at all
Unless you are 99.9% sure

33. Identity UUID
• Are not guaranteed to be globally unique
• Not K-ordered
• In most of the cases are excessively big (128 bit)
• Can be the reason of a serious performance degradation
• Have different versions which may suite better or worse
• Strangely enough RDBMS rarely supports UUID/GUID data types
• Weird:
• Time based on 100-nanosecond intervals since 15th of October 1582
• Were invented/published around 1999

34. Identity UUID
Store as
String
• 16 bytes
UUID is 128-bit value
• 36 symbols – which is more than 2 times bigger
A96A0D4C-49D0-4431-B126-4C66688ADEF3

35. Identity UUID
Drill
Down
High long
32
time_low
16
time_mid
4
version
12
time_hi
Low long
4
variant
12
clock_seq
48
node

36. Identity UUID Example
$ uuidgen
0c8aa0f6-9f6f-4fad-9662-1b683f2f4a0d
$ uuidgen
1ee09695-3a04-4e7a-8bab-e67dabc4b5a2
$ uuidgen -t
3770f4d0-88b3-11e6-bba6-005056bb68cb
$ uuidgen -t
3b14dd54-88b3-11e6-8c53-005056bb68cb

37. Identity Solution
• Use text representation for public identities (API)
• Database Sequences (Long)
• UUID + Database Sequences (Long)
• UUID (BigInteger/Binary)
• Twitter Snowflake (Long) – outdated
• UUID (String)
• *Flake (128 bit)

38. String

39. String
Java and encoding
JDBC drivers and DB types

40. String Encoding
Java uses UTF-16 for String encoding
UTF-16 has symbol range: 0x0000..0x10FFFF
String uses char[] (byte[] in JDK9)
Char has range: 0x0000..0xFFFF

41. String Encoding Quiz #1
How to represent
range
0x100000..0x10FFFF
using char?

42. String Encoding Quiz #1
• Define surrogate range: 0xD800..0xDFFF (0x800 characters)
• Split it equally to “High”: 0xD800..0xDBFF and “Low”:
0xDC00..0xDFFF
• Combine “High”-to-“Low” to get 0x400 * 0x400 = 0x100000 symbols
• Profit???
• Profit!!!

43. String Encoding Quiz #2
String x = new String(new char[]{
'z',0xD801,0xDC37,'a','b','c'
});
System.out.println(x);
System.out.println(x.substring(0,2));
System.out.println(x.substring(2));

44. String Encoding Quiz #2
z𐐷abc
z?
?abc

45. String Encoding Solution
No solution, just be aware
Yet, it might be more sophisticated soon

46. String JDBC vs
DB
Mapping DB specific types to JDBC
Some DB or driver exceptional cases
BLOB vs CLOB
Narrower DB encoding

47. String JDBC vs
DB
Mapping
public enum JDBCType implements SQLType {
CHAR(Types.CHAR),
VARCHAR(Types.VARCHAR),
LONGVARCHAR(Types.LONGVARCHAR),
...
BLOB(Types.BLOB),
CLOB(Types.CLOB),
...
NCHAR(Types.NCHAR),
NVARCHAR(Types.NVARCHAR),
LONGNVARCHAR(Types.LONGNVARCHAR),
NCLOB(Types.NCLOB),

48. String JDBC vs
DB
Mapping
• CHARACTER [(len)] or CHAR [(len)]
• VARCHAR (len)
• BOOLEAN
• SMALLINT
• INTEGER or INT
• DECIMAL [(p[,s])] or DEC [(p[,s])]
• NUMERIC [(p[,s])]
• REAL
• FLOAT(p)
• DOUBLE PRECISION
• DATE
• TIME
• TIMESTAMP
• CLOB [(len)] or CHARACTER LARGE OBJECT [(len)] or CHAR LARGE OBJECT [(len)]
• BLOB [(len)] or BINARY LARGE OBJECT [(len)]

49. String JDBC vs
DB
Quiz #1
What is JDBC
type
LONGVARCHAR?

50. String JDBC vs
DB
Drill
Down
setStringInternal(int var1,String var2) throws SQLException {
...
int var6 = var2 != null?var2.length():0;
...
if(var6 <= this.maxVcsCharsSql) {
this.basicBindString(var1, var2);
} else if(var6 <= this.maxStreamNCharsSql) {
this.setStringForClobCritical(var1, var2);
} else {
this.setStringForClobCritical(var1, var2);
}

51. String JDBC vs
DB
CHAR &
VARCHAR

52. String JDBC vs
DB
BLOB &
CLOB
• just bytes
Binary Large OBject
• just characters in your DB encoding
Character Large OBject

53. String JDBC vs
DB
Char &
NChar
• uses your DB encoding or no encoding
Char, Varchar, CLOB…
• uses specified encoding
NChar, NVarchar, NCLOB…
• does not have NBLOB
BLOB

54. String JDBC vs
DB
Cp1251 vs
Cp1252
Sometimes encoding does not matter much
Unless too smart drivers spoil it
Unless they are not compatible

55. String JDBC vs
DB
Solution
Check your DB encoding upfront
If needed use N* DB types and N* JDBC types as well

56. String JDBC vs
DB
Solution
Losing data because of encoding is lame
If you expect some strange strings coming use N* types
Never forget that symbol is not a char/byte it may save you one day
Your JDBC driver can screw you

57. Date

58. Date
Time zones
DST and leap miracles

59. Date Time
Zone
Does DB and App time zone match?
What can go wrong if they don’t?

60. Date Time
Zone
Quiz #1
• Database: Oracle 11g
• Database time zone: CET/CEST (+01:00/+02:00)
• Application: Java 8
• Application time zone: EET/EEST (+02:00/+03:00)
• setTimestamp(‘2016-10-14 15:35:01’)?
• getTimestamp()?

61. Date Time
Zone
Quiz #1
Hint
final int oracleYear(int var1) {
int var2 = ((this.rowSpaceByte[0 + var1] & 255) - 100) * 100
+ (this.rowSpaceByte[1 + var1] & 255) - 100;
return var2 <= 0?var2 + 1:var2;
}
final int oracleMonth(int var1) { return this.rowSpaceByte[2 + var1] - 1; }
final int oracleDay(int var1) { return this.rowSpaceByte[3 + var1]; }
final int oracleHour(int var1) { return this.rowSpaceByte[4 + var1] - 1; }
final int oracleMin(int var1) { return this.rowSpaceByte[5 + var1] - 1; }
final int oracleSec(int var1) { return this.rowSpaceByte[6 + var1] - 1; }
final int oracleTZ1(int var1) { return this.rowSpaceByte[11 + var1]; }
final int oracleTZ2(int var1) { return this.rowSpaceByte[12 + var1]; }

62. Date Time
Zone
Quiz #1
• 2016-10-14 15:35:01
Database
• 2016-10-14 15:35:01
Application

63. Date Time
Zone
Quiz #1
• 2016-10-14 15:35:01
Database
• 2016-10-14 15:35:01
Application with UTC time zone

64. Date Time
Zone
Quiz #2
JavaScript client: time zone unknown
Java server: EET time zone
How to pass dates?

65. Date Time
Zone
Quiz #2
• Date… ehmm… JSON does not know what it is…
• Long is a bit of a problem for 53+ impotent integer types (now 41,
~140,000 years and we will cross the border)
• String as ISO 8601 is a lesser evil

66. Date Time
Zone
Solution
Use the same App/DB time zone
Check your DB driver to ensure conversion safety
Store timestamps as long: DB and API
Store timestamps as String: API

67. Date DST &
Magic
Missing and extra hours
Leap seconds

68. Date DST &
Magic
Calculations
24 hours in a day
60 minutes in an hour
60 seconds in a minute
FTW: 24 * 60 * 60 * 1000

69. Date DST &
Magic
Quiz #1
27.03.2016 00:00:00 - 28.03.2016 00:00:00 (EET/EEST)

70. Date DST &
Magic
Quiz #1
26.03.2016 22:00:00 - 27.03.2016 21:00:00 (UTC) – 23h

71. Date DST &
Magic
Quiz #2
31.12.2016 00:00:00 – 01.01.2017 00:00:00 (EET/EEST)
01.01.2017 00:00:00 – 02.01.2017 00:00:00 (EET/EEST)

72. Date DST &
Magic
Quiz #2
30.12.2016 22:00:00 – 31.01.2017 22:00:00 (UTC) – 24h
31.12.2017 22:00:00 – 01.01.2017 22:00:00 (UTC) – 24h+1s

73. Date DST &
Magic
Quiz #2
It will happen: 31.12.2016 23:59:60 (UTC)
It had happened: 30.06.2015 23:59:60 (UTC)
Blame the Earth, and Moon, and Sun
Blame software developers

74. Date One Last
Thing
Date vs
Interval
Date is a tuple of year, month, day, hour, and etc.
Instant is a precise point on the timeline

75. Date One Last
Thing
Date vs
Interval
Date can be converted to Instant
Instant can be converted to Date
• even within a Chronology
However, “conversion rate” is not constant

76. Date Summary
• Use UTC as much as possible
• Keep in mind the difference between Date and Instant
• Think of Date/Instant interoperation as it was designed/used by
idiots
• 24*60*60*1000 is, basically, simplification. Quite harmful at times.
• Use proper date libraries – you wouldn’t want to reinvent it again.
• GMT is not yet another name for UTC, beware!

77. Thank
you