Slides of talk given at Vilnius JUG meeting #9 (2012-11-14)

1. Java 8
Interesting parts
Vaidas Pilkauskas, Kasparas Rudokas
Vilnius JUG

2. Agenda
● Lambda
● Date & Time
● PermGen
● Annotations

3. Lambda
JSR 335

4. lambda vs. closure
A lambda is just an anonymous function.
myCollection.filter(e ‑> e.length() >= 5)
A closure is any function which closes over the environment in which it was
defined. This means that it can access variables not in its parameter list.
int minLength = 5;
myCollection.filter(new Predicate(){
boolean apply(MyElement e) {
return e != null ? e.length >= minLength : false;
}
})

5. Project Lambda (JSR 335)
● Language changes
○ Lambda Expression
○ Virtual Extension Method
● Library changes
○ Functional API
○ Parallel collections
○ Bulk Operations

6. Problem
String[] names = {"Alice", "Bob", "Charlie", Dave"};
Arrays.sort(names, new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
return o1.compareToIgnoreCase(o2);
}
});

7. "tiny" anonymous inner
class
String[] names = {"Alice", "Bob", "Charlie", Dave"};
Arrays.sort(names, new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
return o1.compareToIgnoreCase(o2);
}
});

8. "tiny" and useful
● callbacks
● runnables
● event handlers
● comparators
● ...and many other cases

9. All we need!
String[] names = {"Alice", "Bob", "Charlie", Dave"};
Arrays.sort(names, new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
return o1.compareToIgnoreCase(o2);
}
});

10. Solution - closure
String[] names = {"Alice", "Bob", "Charlie", Dave"};
Arrays.sort(names, (o1, o2) -> o1.compareToIgnoreCase(o2));

11. What do we mean by
closures?
● a unit of deferred execution
● code that can be passed as a value
● can be used in an assignment
statement
● capture of a surrounding environment

12. Lambda Expressions
s -> s.length()
(int x, int y) -> x+y
() -> 42
(x, y, z) -> {
if (x == y) return x;
else {
int result = y;
for (int i = 1; i < z; i++)
result *= i;
return result;
}
}

13. Lambda Syntax
principally because something similar
has been generally well-received in
other Java-like languages (C# and
Scala), and a clearly "better" alternative
did not present itself

14. Lambda
● Effectively final variables

15. Lambda
● Effectively final variables
● Shadowing

16. Lambda
● Effectively final variables
● Shadowing
● break, continue must be local

17. Lambda
● Effectively final variables
● Shadowing
● break, continue must be local
● this - lambda is not an instance
method. (this == surrounding context
object)

18. Lambda
● Effectively final variables
● Shadowing
● break, continue must be local
● this - lambda is not an instance
method. (this == surrounding context
object)
● throw completes method abruptly

19. Functional Interfaces
A functional interface is an interface
that has just one abstract method, and
thus represents a single function
contract.

20. Functional Interfaces
SAM - Single Abstract Method interfaces
one method excluding Object methods
interface Runnable { void run(); } // Functional
interface Foo { boolean equals(Object obj); }
// Not functional; equals is already an implicit member
interface Comparator<T> {
boolean equals(Object obj);
int compare(T o1, T o2);
}
// Functional; Comparator has one abstract non-Object method

21. Method References
Method reference is a shorthand for a lambda invoking just
that method
System::getProperty
"abc"::length
String::length
super::toString
ArrayList::new
Arrays.sort(ints, Integer::compareTo);

22. Method References
Static methods simply translate like lambda with same
arguments and return type
class Math {
public static int max(int a, int b) {...}
}
interface Operator<T> {
T eval(T left, T right);
}
Operator<Integer> lambda = (a, b) -> Math.max(a, b);
Operator<Integer> methodRef = Math::max;

23. Method References
Non static method reference of type T translates like
lambda with an additional argument of type T
Comparator<String> c = (s1, s2) -> s1.compareToIgnoreCase
(s2);
//translates to:
Comparator<String> c = String::compareToIgnoreCase;

24. Method References
Instance method reference translates like lambda with same
arguments and return type (and implicit receiver)
Callable<Integer> l = () -> "boo".length();
//translates to:
Callable<Integer> c = "boo"::length;

25. Method References
String[] names = {"Alice", "Bob", "Charlie", Dave"};
Arrays.sort(names,
(o1, o2) -> o1.compareToIgnoreCase(o2));
//translates to
String[] names = {"Alice", "Bob", "Charlie", Dave"};
Arrays.sort(names, String::compareToIgnoreCase);

26. Compatibility
Cannot add new interface methods
(without forcing current interface users to implement them)
But we need new lambda enabled methods
(on Java core libraries!)

27. Default methods
(also known as)
● virtual extension methods
● defender methods

28. Default methods
interface NormalInterface {
void myNormalMethod();
void myDefaultMethod () default {
System.out.println("-> myDefaultMethod");
}
}
class NormalInterfaceImpl implements NormalInterface {
@Override
public void myNormalMethod() {
System.out.println("-> myNormalMethod");
}
}

29. Functional Collections
Major changes in collection API:
● Internal iteration support with Iterable/Stream interfaces
(forEach, filter, map, etc)

30. Functional Collections
Major changes in collection API:
● Internal iteration support with Iterable/Stream interfaces
(forEach, filter, map, etc)
● Explicit parallel APIs for greater parallelism support.
These can be combined with Fork/Join to divide the
tasks

31. Functional Collections
Major changes in collection API:
● Internal iteration support with Iterable/Stream interfaces
(forEach, filter, map, etc)
● Explicit parallel APIs for greater parallelism support.
These can be combined with Fork/Join to divide the
tasks
● Greater stress on immutability and avoiding in-place
mutation which was done in the conventional for-each
loops

32. Example
String[] names = {"Alice", "Bob", "Charlie", "Dave"};
List<String> filteredNames = Streams.stream(names)
.filter(e -> e.length() >= 4)
.into(new ArrayList<>());
filteredNames.stream().forEach(System.out::println);

33. Streams
● No storage - nothing is stored in stream

34. Streams
● No storage - nothing is stored in stream
● Functional in nature - new values are produced

35. Streams
● No storage - nothing is stored in stream
● Functional in nature - new values are produced
● Laziness-seeking. Many operations can be
implemented lazily

36. Streams
● No storage - nothing is stored in stream
● Functional in nature - new values are produced
● Laziness-seeking. Many operations can be
implemented lazily
● Bounds optional. Think of infinite stream

37. Functional interfaces
java.util.functions
● Predicate - a property of the object passed as
argument
● Block - an action to be performed with the object
passed as argument
● Mapper - transform a T to a U
● ...

38. Parallelism
//sequential
int sum = myCollection.stream()
.filter(b -> b.getColor() == BLUE)
.map(b -> b.getWeight())
.sum();
//parallel
int sum = myCollection.parallel()
.filter(b -> b.getColor() == BLUE)
.map(b -> b.getWeight())
.sum();

39. Advantages
● Elements may be computed lazily. If we apply a Mapper
to a collection of a thousand elements but only iterate
over the first three, the remaining elements will never be
mapped.

40. Advantages
● Elements may be computed lazily. If we apply a Mapper
to a collection of a thousand elements but only iterate
over the first three, the remaining elements will never be
mapped.
● Method chaining is encouraged. Hence there's no need
to store intermediate results in their own collections.

41. Advantages
● Elements may be computed lazily. If we apply a Mapper
to a collection of a thousand elements but only iterate
over the first three, the remaining elements will never be
mapped.
● Method chaining is encouraged. Hence there's no need
to store intermediate results in their own collections.
● Internal iteration hides implementation decisions. For
example, we could parallelize a map() operation just by
writing myCollection.parallel().map(e ‑> e.length()).

42. Try it out today!
The prototype compiler is being implemented in OpenJDK.
● Source code is available at http://hg.openjdk.java.
net/lambda/lambda
● Binary snapshots of the lambda-enabled JDK prototype
are available athttp://jdk8.java.net/lambda
IDE support:
● Netbeans 8 Nightly Builds with experimental Lambda
support
● IDEA 12 EAP with experimental Lambda support

43. Date & Time API
JSR 310

44. Oldies: Date
● Mutable

45. Oldies: Date
● Mutable
● Years being indexed from 1900

46. Oldies: Date
● Mutable
● Years being indexed from 1900
● Months being indexed from 0

47. Oldies: Date
● Mutable
● Years being indexed from 1900
● Months being indexed from 0
Ex.:
Date d = new Date(1L);
System.out.println(d.toString());

48. Oldies: Date
● Mutable
● Years being indexed from 1900
● Months being indexed from 0
Ex.:
Date d = new Date(1L);
System.out.println(d.toString());
//Thu Jan 01 02:00:00 EET 1970

49. Oldies: Calendar
● Mutable

50. Oldies: Calendar
● Mutable
● Not very convenient (lack of simple field
methods)

51. Oldies: Calendar
● Mutable
● Not very convenient (lack of simple field
methods)
● Very difficult to extend (add new calendars)

52. Oldies: Calendar
● Mutable
● Not very convenient (lack of simple field
methods)
● Very difficult to extend (add new calendars)
Ex.:
Calendar c = Calendar.getInstance();
int weekday = c.get(DAY_OF_WEEK);

53. Oldies: Calendar
● Mutable
● Not very convenient (lack of simple field
methods)
● Very difficult to extend (add new calendars)
Ex.:
Calendar c = Calendar.getInstance();
int weekday = c.get(DAY_OF_WEEK);
Date d = c.getTime();

54. Under pressure...

55. JSR 310
● Immutable

56. JSR 310
● Immutable
● Defines consistent language for domain
○ Offset from UTC vs TimeZone
○ Machine vs Human
○ ISO 8601

57. JSR 310
● Immutable
● Defines consistent language for domain
○ Offset from UTC vs TimeZone
○ Machine vs Human
○ ISO 8601
● No old Date/Calendar usage (clean)

58. JSR 310
● Immutable
● Defines consistent language for domain
○ Offset from UTC vs TimeZone
○ Machine vs Human
○ ISO 8601
● No old Date/Calendar usage (clean)
● Extensible

59. JSR 310
● Immutable
● Defines consistent language for domain
○ Offset from UTC vs TimeZone
○ Machine vs Human
○ ISO 8601
● No old Date/Calendar usage (clean)
● Extensible
● Led by JodaTime creator (S. Colebourne)

60. Example: JSR 310
Clock clock = Clock.systemUTC();
LocalDate localDate = LocalDate.now(clock);
//2012-11-13

61. Example: JSR 310
Clock clock = Clock.systemUTC();
LocalDate localDate = LocalDate.now(clock);
//2012-11-13
MonthDay brasilBday = MonthDay.of(JUNE, 21);

62. Example: JSR 310
Clock clock = Clock.systemUTC();
LocalDate localDate = LocalDate.now(clock);
//2012-11-13
MonthDay brasilBday = MonthDay.of(JUNE, 21);
ZoneId zoneId = ZoneId.of("America/New_York");
Clock clock = Clock.system(zoneId);
ZonedDateTime zonedDateTimeUS = ZonedDateTime.now(clock);
//2012-11-11T04:17:58.693-05:00[America/New_York]

63. Example: JSR 310
import static javax.time.calendrical.LocalPeriodUnit.
HOURS;
Period p = Period.of(5, HOURS);
LocalTime time = LocalTime.now();
LocalTime newTime;
newTime = time.plus(5, HOURS);
// or
newTime = time.plusHours(5);
// or
newTime = time.plus(p);

64. Why not Joda Time?
● Machine timelines
● Pluggable chronology
● Nulls
● Internal implementation

65. VM/GC
JEP 122 & JEP 156

66. PermGen removal
● Java Heap vs PermGen

67. PermGen removal
● Java Heap vs PermGen

68. PermGen removal
● Java Heap vs PermGen
● Why it was required anyway?

69. PermGen removal
● Java Heap vs PermGen
● Why it was required anyway?
● So what?

70. Annotations
JSR 308 & JEP 120

71. Type Annotations
● In Java 7 we have annotations on
declarations
Ex.:
class SomeClass { … }
class ListElement<E> { … }
public void v() (int I) {
long l;
{

72. Type Annotations
● JSR-308 brings annotations on Type use
● Are an enabler for the checkers framework
Ex.:
new @Interned MyObject();
myString = (@NonNull String) myObject;
void monitorTemperature() throws
@Critical TemperatureException { ... }

73. Repeating Annotations
● Before
@Schedules ({
@Schedule(dayOfMonth="Last"),
@Schedule(dayOfWeek="Fri", hour="23")
({
public void doPeriodicCleanup() { ... }

74. Repeating Annotations
● Before
@Schedules ({
@Schedule(dayOfMonth="Last"),
@Schedule(dayOfWeek="Fri", hour="23")
({
public void doPeriodicCleanup() { ... }
● After
@Schedule(dayOfMonth="Last”)
@Schedule(dayOfWeek="Fri", hour="23")
public void doPeriodicCleanup() { ... }

75. Q&A

76. References
* Lambda
http://stackoverflow.com/questions/220658/what-is-the-difference-between-a-closure-and-a-lambda
http://openjdk.java.net/projects/lambda/
http://jcp.org/aboutJava/communityprocess/edr/jsr335/index2.html
http://vimeo.com/48577033 (slides: http://www.slideshare.net/tkowalcz/java-gets-a-closure)
http://datumedge.blogspot.co.uk/2012/06/java-8-lambdas.html
http://www.theserverside.com/news/thread.tss?thread_id=68718
http://medianetwork.oracle.com/video/player/1785479333001
https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=6080
https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=5089
http://www.lektorium.tv/lecture/?id=14048
http://www.lektorium.tv/lecture/?id=14049
http://blog.xebia.com/2012/11/05/report-will-java-8s-lambda-change-the-face-of-the-world/
http://www.slideshare.net/fsarradin/java-8-lambda
http://programmers.stackexchange.com/questions/173441/what-triggered-the-popularity-of-lambda-functions-in-modern-mainstream-programmi?newsletter=1&nlcode=29983%
7c903a
http://www.slideshare.net/bje/java-closures
* Collections
http://www.javabeat.net/2012/05/enhanced-collections-api-in-java-8-supports-lambda-expressions/
http://cr.openjdk.java.net/~briangoetz/lambda/collections-overview.html
http://architects.dzone.com/articles/java-collections-api

77. References
* Remove the Permanent Generation
http://www.cubrid.org/blog/dev-platform/understanding-jvm-internals/
http://javaeesupportpatterns.blogspot.com/2011/10/java-7-features-permgen-removal.html
http://java.dzone.com/articles/busting-permgen-myths
https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=5135
* JSR 310: Date and Time API
http://java.dzone.com/articles/introducing-new-date-and-time
http://sourceforge.net/apps/mediawiki/threeten/index.php?title=ThreeTen
http://www.infoq.com/news/2010/03/jsr-310
https://docs.google.com/document/pub?id=1rd8yplQZIRz3LxMzpVLuskr1b0HwBmK9PXpdgBYojSw
http://sourceforge.net/apps/mediawiki/threeten/index.php?title=User_Guide
https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=4350
* General Java8
http://openjdk.java.net/projects/jdk8/features
http://www.pcadvisor.co.uk/news/software/3401314/oracle-java-upgrades-still-worthwhile-despite-postponed-features/
http://dhruba.name/2011/07/06/oracle-discusses-java-7-8-new-features-on-video/
http://channel9.msdn.com/Events/Lang-NEXT/Lang-NEXT-2012/Java-8
http://www.parleys.com/#st=5&id=2850&sl=1
http://www.parleys.com/#st=5&id=2847
https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=2872
https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=10458

78. References
*Annotations
https://oracleus.activeevents.com/connect/sessionDetail.ww?SESSION_ID=4469
https://blogs.oracle.com/abuckley/entry/jsr_308_moves_forward
http://jcp.org/en/jsr/detail?id=308
http://openjdk.java.net/jeps/120
http://types.cs.washington.edu/checker-framework/