Preparing for the Java 25 Certification or Learning New Features

1. Preparing for the Java 25 cert + Learning New Features Workshop Jeanne Boyarsky 9/9/26 KCDC Lab: https://github.com/boyarsky/2026-kcdc https://linktr.ee/jeanneboyarsky 1
2. At end of session 1. https://speakerdeck.com/boyarsky 2. https://github.com/boyarsky/2026-kcdc-java25 solutions will be committed https://linktr.ee/jeanneboyarsky 2
3. Pause for a Commercial Java certs: 8/11/17/21 (on sale 25) Real World Java https://linktr.ee/jeanneboyarsky 3
4. Disclaimers I do not work for Oracle. I’m still not authorized to tell you any non public information of if I know any :) Some of the material in this presentation appears in our certification books. https://linktr.ee/jeanneboyarsky 4
5. Introductions https://linktr.ee/jeanneboyarsky 5
6. Agenda Module 1 2 3 4 Topics Intro, compact source files, instance main, IO class, multi file source code, module imports Flexible constructor bodies, unnamed variables, pattern matching changes Stream Gatherers, sequenced collections Virtual threads, scoped values, Bonus: JavaDoc, structured concurrency https://linktr.ee/jeanneboyarsky 6
7. Module Flow •Review lab from previous module •Lecture/review •Hands on exercises •10 minute break This means if a colleague needs to call you, the last 15-20 minutes of each hour is best. https://linktr.ee/jeanneboyarsky 7
8. Required Software for Lab Option 1 •Java 25+ - https://jdk.java.net/26/ •IDE of your choice or… https://linktr.ee/jeanneboyarsky 8
9. Required Software for Lab Option 2 •https://onecompiler.com/java for module 1 •https://dev.java/playground/ (for modules 2 and 3) https://linktr.ee/jeanneboyarsky 9
10. Features Feature Preview Final Compact Source FIles 21, 22, 23, 24 25 Instance Main 21, 22, 23, 24 25 IO class 25 Multi file source code 22 Module imports 23, 24 https://linktr.ee/jeanneboyarsky 25 10
11. Hello JavaOne public class Hello { public static void main( String[] args) { } } 25 void main() { IO.println( "Hello JavaOne!"); } System.out.println( "Hello JavaOne!"); • Compact class files • Instance main • IO class @jeanneboyarsky 11
12. Compact Source Files 25 • Default package • (package declaration not permitted) • Default module • Must declare main method Can have naming conflicts in different folders @jeanneboyarsky 12
13. Compact Source Files • Instance methods allowed? Yes • Static methods allowed? Yes • Constructors allowed? No @jeanneboyarsky 25 13
14. Compact Source Files 25 • Automatic: • import module java.base; • More than java.lang! More on module import @jeanneboyarsky 14
15. Instance main 25 void main() {} void main(String[] args) {} void main(String... args) {} static void main() {} public void main() {} public void main(String[] args) {} + more If two, calls one with args @jeanneboyarsky 15
16. What’s wrong? 25 public class Foo { private Foo() { } void main() { IO.println("Hello KCDC"); } No public no-args } constructor @jeanneboyarsky 16
17. IO Class IO 25 Wraps IO.print(obj); System.out.print(obj); IO.println(); System.out.println(); IO.println(obj); System.out.println(obj); IO.readln(); System.in.readlin(); IO.readln(prompt); System.in.readlin(prompt); @jeanneboyarsky 17
18. IO Class 25 Still need originals for • System.err • All other System.out methods such as printf, format • All other System.in methods such as read() • Ability to wrap in System.in in Scanner • Setting streams to different values @jeanneboyarsky 18
19. Multi-File Source-Code 22 java Conference.java public class Conference { public Conference(List<Speaker> speakers) { } } Speaker also autocompiled in memory @jeanneboyarsky 19
20. Multi-File Source-Code 22 javac Conference.java public class Conference { public Conference(List<Speaker> speakers) { } } Speaker also compiled on disk @jeanneboyarsky 20
21. Module Imports 25 // imports over 50 packages import module java.base; import java.util.List; vs import java.util.*; vs import module java.base; @jeanneboyarsky 21
22. Module Imports Shadowing Code import module.java.sql; Type 25 Imports Module imports import java.sql.*; 2 direct packages 3 transitive packages (via requires transitive) 1 service (Driver) Wildcard/package import 50+ classes import java.sql.Date; Import single class @jeanneboyarsky 1 class 22
23. Module 1 - Question 1 Which of these are valid main methods? A. public static void main(String[] args) {} B. void main(String args) {} C. final void main() { } D. private void main(String[] args) {} @jeanneboyarsky 23
24. Module 1 - Question 1 Which of these are valid main methods? A. public static void main(String[] args) {} A, C B. void main(String args) {} (B wrong type, D private) C. final void main() { } D. private void main(String[] args) {} @jeanneboyarsky 24
25. Module 1 - Question 2 Which of these are valid main methods? (yes more) A. main() {} B. public void main() {} C. void main(String... args) { } D. void final main(String[] args) {} @jeanneboyarsky 25
26. Module 1 - Question 2 Which of these are valid main methods? (yes more) A. main() {} B, C B. public void main() (A no{} void, D final order) C. void main(String... args) { } D. void final main(String[] args) {} @jeanneboyarsky 26
27. Module 1 - Question 3 What does this output? 1: private void print() { 2: IO.println("Ran!"); 3: } 4: void main() { 5: print(); 6: } A. Ran! B. Does not compile @jeanneboyarsky 27
28. Module 1 - Question 3 What does this output? 1: private void print() { 2: IO.println("Ran!"); 3: } 4: void main() { 5: print(); 6: } A A. Ran! B. Does not compile @jeanneboyarsky 28
29. Module 1 - Question 4 What does this output in a class named Foo.java? 1: private static void print() { 2: IO.println("Ran!"); 3: } 4: void main() { 5: Foo.print(); 6: } A. Ran! B. Does not compile @jeanneboyarsky 29
30. Module 1 - Question 4 What does this output in a class named Foo.java? 1: private static void print() { 2: IO.println("Ran!"); 3: } 4: void main() { (static) 5: Foo.print(); 6: } B A. Ran! B. Does not compile @jeanneboyarsky 30
31. Module 1 - Question 5 What does this output? record Foo() { void main() { IO.println("Ran!"); } } A. Ran! B. Does not compile @jeanneboyarsky 31
32. Module 1 - Question 5 What does this output? record Foo() { void main() { IO.println("Ran!"); (no public } constructor) } B A. Ran! B. Does not compile @jeanneboyarsky 32
33. Module 1 - Question 6 What does this output? record Foo() { public Foo() {} void main() { IO.println("Ran!"); } } A. Ran! B. Does not compile @jeanneboyarsky 33
34. Module 1 - Question 6 What does this output? record Foo() { public Foo() {} void main() { IO.println("Ran!"); } } A A. Ran! B. Does not compile @jeanneboyarsky 34
35. Module 1 - Question 7 Which of these are valid IO methods? A. IO.println("text"); B. IO.format("Weather: %s", "sunny"); C. IO.readln("Ok?"); D. IO.readPassword(); @jeanneboyarsky 35
36. Module 1 - Question 7 Which of these are valid IO methods? A. IO.println("text"); A, C B. IO.format("Weather: %s", "sunny"); C. IO.readln("Ok?"); D. IO.readPassword(); @jeanneboyarsky 36
37. Module 1 - Question 8 How many classes are created when running javac Container.java in a directory containing only Container.java and Contains.java? public Container { private Contains c; } A. 0 C. 2 B. 1 D. Error running javac @jeanneboyarsky 37
38. Module 1 - Question 8 How many classes are created when running javac Container.java in a directory containing only Container.java and Contains.java? public Container { private Contains c; } A. 0 C. 2 B. 1 D. Error running javac C @jeanneboyarsky 38
39. Module 1 - Question 9 How many classes are created when running java Container.java in a directory containing only Container.java and Contains.java? public Container { private Contains c; } A. 0 C. 2 B. 1 D. Error running javac @jeanneboyarsky 39
40. Module 1 - Question 9 How many classes are created when running java Container.java in a directory containing only Container.java and Contains.java? public Container { private Contains c; } A. 0 C. 2 B. 1 D. Error running javac A @jeanneboyarsky 40
41. Module 1 - Question 10 What are true of line 5? 1: import java.util.*; 2: import jeanne.List; 3: import module java.base; 4: 5: void main(List list) {} A. The import on line 1 is used for line 5 B. The import on line 2 is used for line 5 C. The import on line 3 is used for line 5 D. This program will run @jeanneboyarsky 41
42. Module 1 - Question 10 What are true of line 5? 1: import java.util.*; 2: import jeanne.List; 3: import module java.base; 4: 5: void main(List list) {} B A. The import on line 1 is used for line 5 B. The import on line 2 is used for line 5 C. The import on line 3 is used for line 5 D. This program will run @jeanneboyarsky 42
43. Lab & Break https://github.com/boyarsky/2026-kcdc-java25 https://linktr.ee/jeanneboyarsky 43
44. Agenda Module 1 2 3 4 Topics Intro, compact source files, instance main, IO class, multi file source code, module imports Flexible constructor bodies, unnamed variables, pattern matching changes Stream Gatherers, sequenced collections Virtual threads, scoped values, Bonus: JavaDoc, structured concurrency https://linktr.ee/jeanneboyarsky 44
45. Features Feature Preview Final Flexible Constructor Bodies 22, 23, 24 25 Unnamed Variables 21 22 Pattern Matching 17, 18,19, 20 21 https://linktr.ee/jeanneboyarsky 45
46. Flexible Constructor Bodies @jeanneboyarsky 25 46
47. Prologue Rules 25 • Can reference constructor params • Cannot reference instance methods • Cannot reference instance variables • Except for setting initial value of unitialized one @jeanneboyarsky 47
48. Constructor Flow - What prints? public Bridge(int num) { IO.print("n"); } public class Bridge { { IO.print("i"); } public Bridge() { IO.print("p"); this(1); IO.print("e"); } 25 } public void main() { new Bridge(); } pinepine @jeanneboyarsky 48
49. Unnamed Variables 22 • _ (single underscore) is valid variable name • Restrictions • Cannot read • Cannot write • Can define multiple in same scope • Useful for: catch, lambda, pattern matching, etc @jeanneboyarsky 49
50. Unnamed Variables 22 // good String _ = "init"; String _ = "again"; // no good _ = "write"; IO.println(_); String _; @jeanneboyarsky 50
51. Using Pattern Matching 21 null allowed guard clause return switch (obj) { case null -> ""; case Integer i when i >= 21 -> "can gamble"; case Integer i -> "too young to gamble"; case String s when "poker".equals(s) -> "table game"; case String s when "craps".equals(s) -> "table game"; case String s -> "other game"; default -> throw new IllegalArgumentException("unexpected type"); }; pattern variable Still Null Pointer if don’t specify case null @jeanneboyarsky 51
52. Order matters 21 return switch (obj) { case null -> ""; case Integer i -> "too young to gamble"; case Integer i when i >= 21 -> "can gamble”; // DOES NOT COMPILE case String s -> "other game"; default -> throw new IllegalArgumentException("unexpected type"); }; Order like catching exceptions! @jeanneboyarsky 52
53. Enums 21 enum Suit { HEART, DIAMOND, CLUB, SPADE; } String symbol(Suit suit) { } return switch (suit) { case HEART -> "♥"; case Suit.DIAMOND -> "♦"; case CLUB -> "♣"; case Suit.SPADE -> "♠"; }; Fails compilation if miss one since no default @jeanneboyarsky 53
54. Record Patterns 21 enum Suit { HEART, DIAMOND, CLUB, SPADE; } enum Rank { NUM_2, NUM_3, NUM_4, NUM_5, NUM_6, NUM_7 NUM_8, NUM_9, NUM_10, JACK, QUEEN, KING, ACE; } record Card(Suit suit, Rank rank) { } if (card instanceof Card c) { IO.println(c.suit()); Deconstructed IO.println(c.rank()); } if (card instanceof Card(Suit suit, Rank rank)) { IO.println(suit); IO.println(rank); } https://linktr.ee/jeanneboyarsky 54
55. Nested Record Patterns record MultiDeck(int deckNum, Card card) { 21 } if (multi instanceof MultiDeck(int deckNum, Card(Suit suit, Rank rank))) { System.out.println(deckNum); System.out.println(suit); System.out.println(rank); } https://linktr.ee/jeanneboyarsky 55
56. Switch pattern matching 21 switch(card) { case Card(Suit suit, Rank rank) when suit == Suit.HEART -> System.out.println("Heart"); case Card c -> System.out.println("other"); } https://linktr.ee/jeanneboyarsky 56
57. Switch pattern matching 21 String str = ""; switch (str) { case "heart" -> System.out.println("heart"); } Card card = new Card(Suit.HEART, Rank.NUM_2); switch (card) { case Card(Suit suit, Rank rank) when suit == Suit.HEART -> System.out.println("Heart"); case Card c -> System.out.println("other"); } Must be exhaustive if using “when” https://linktr.ee/jeanneboyarsky 57
58. What can’t you do? 21 • In switch(x), x still can’t be • boolean • float • double • long • Expanding non-records (coming ?) https://linktr.ee/jeanneboyarsky 58
59. Module 2 - Question 1 What is the output? Blue(String s) { IO.print(s); super(); } private class Blue { { IO.print("i"); } private Blue() { IO.print("c"); this("l"); } void main() { new Blue(); } } A. cli B. clicli C. Does not compile D. Does not run @jeanneboyarsky 59
60. Module 2 - Question 1 What is the output? private class Blue { { IO.print("i"); } private Blue() { IO.print("c"); this("l"); } B Blue(String s) { IO.print(s); super(); } void main() { new Blue(); } } A. cli B. clicli C. Does not compile D. Does not run @jeanneboyarsky 60
61. Module 2 - Question 2 Which lines could independently be replaced by IO.println(shade); 1: public class Blue { 2: private String shade = "sky blue"; 3: 4: Blue() { 5: 6: super(); 7: 8: } 9: A. 3} C. 6 B. 5 D. 7 @jeanneboyarsky 61
62. Module 2 - Question 2 Which lines could independently be replaced by IO.println(shade); 1: public class Blue { 2: private String shade = "sky blue"; 3: 4: Blue() { 5: 6: super(); 7: 8: } 9: A. 3} C. 6 C, D B. 5 D. 7 @jeanneboyarsky 62
63. Module 2 - Question 3 Which lines could be independently replaced by shade = "sky blue"; 1: public class Blue { 2: private String shade; 3: 4: Blue() { 5: 6: super(); 7: 8: } C. 7 9: } A. 5 B. 6 D. line 5 can have the replacement applied twice @jeanneboyarsky 63
64. Module 2 - Question 3 Which lines could be independently replaced by shade = "sky blue"; 1: public class Blue { 2: private String shade; 3: 4: Blue() { 5: 6: super(); 7: 8: } C. 7 9: } B, C A. 5 B. 6 D. line 5 can have the replacement applied twice @jeanneboyarsky 64
65. Module 2 - Question 4 Which lines could be independently replaced by shade = "sky blue"; 1: public class Blue { 2: private String shade = "sky blue"; 3: 4: Blue() { 5: 6: super(); 7: 8: } C. 7 9: } A. 5 B. 6 D. line 5 can have the replacement applied twice @jeanneboyarsky 65
66. Module 2 - Question 4 Which lines could be independently replaced by shade = "sky blue"; 1: public class Blue { 2: private String shade = "sky blue"; 3: 4: Blue() { 5: 6: super(); 7: 8: } C. 7 9: } C, D A. 5 B. 6 D. line 5 can have the replacement applied twice @jeanneboyarsky 66
67. Module 2 - Question 5 Which lines could independently be replaced by shade = "sky blue"; 1: public class Blue { 2: private final String shade; 3: 4: Blue() { 5: 6: super(); 7: 8: } C. 7 9: } D. line 5 can have A. 5 B. 6 the replacement applied twice @jeanneboyarsky 67
68. Module 2 - Question 5 Which lines could independently be replaced by shade = "sky blue"; 1: public class Blue { 2: private final String shade; 3: 4: Blue() { 5: 6: super(); 7: 8: } C. 7 9: } D. line 5 can have A, B, C A. 5 B. 6 the replacement applied twice @jeanneboyarsky 68
69. Module 2 - Question 6 Which lines have compiler errors? 1: void main(String[] _) { 2: double _ = 0.00; 3: String _ = null; 4: IO.println(_); 5: boolean isNull = _ == null; 6: } A. Line 1 B. Line 2 C. Line 3 D. Line 4 E. Line 5 @jeanneboyarsky 69
70. Module 2 - Question 6 Which lines have compiler errors? 1: void main(String[] _) { 2: double _ = 0.00; 3: String _ = null; 4: IO.println(_); 5: boolean isNull = _ == null; 6: } A, D, E A. Line 1 B. Line 2 C. Line 3 D. Line 4 E. Line 5 @jeanneboyarsky 70
71. Module 2 - Question 7 Which variables can be replaced by _? void main(String[] args) { enum Suit { HEART, DIAMOND, SPADE, CLUB }; enum Symbol { ACE, JACK, QUEENS, KING}; record Card(Suit suit, Symbol symbol) {} var card = new Card(Suit.HEART, Symbol.ACE); try { if (card instanceof Card(Suit suit, Symbol symbol)) IO.println(suit); } catch (NullPointerException e) { IO.print("party!"); } } A. args B. card C. symbol D. e @jeanneboyarsky 71
72. Module 2 - Question 7 Which variables can be replaced by _? void main(String[] args) { enum Suit { HEART, DIAMOND, SPADE, CLUB }; enum Symbol { ACE, JACK, QUEENS, KING}; record Card(Suit suit, Symbol symbol) {} var card = new Card(Suit.HEART, Symbol.ACE); try { if (card instanceof Card(Suit suit, Symbol symbol)) IO.println(suit); } catch (NullPointerException e) { IO.print("party!"); } } C, D A. args B. card C. symbol D. e @jeanneboyarsky 72
73. Module 2 - Question 8 How many lines do you need to remove to make this code compile? double odds(Object obj) { return switch (obj) { case String s -> throw new IllegalArgumentException("unknown game"); case String s when "blackjack".equals(s) -> .05; case String s when "baccarat".equals(s) -> .12; case Object o -> throw new IllegalArgumentException("unknown game"); default -> throw new IllegalArgumentException("known game"); }; } A. 0 C. 2 B. 1 D. 3 https://linktr.ee/jeanneboyarsky 73
74. Module 2 - Question 8 How many lines do you need to remove to make this code compile? double odds(Object obj) { return switch (obj) { case String s -> throw new IllegalArgumentException("unknown game"); case String s when "blackjack".equals(s) -> .05; case String s when "baccarat".equals(s) -> .12; case Object o -> throw new IllegalArgumentException("unknown game"); String s & Object default -> throw new IllegalArgumentException("known game"); o or default }; } C A. 0 C. 2 B. 1 D. 3 https://linktr.ee/jeanneboyarsky 74
75. Module 2 - Question 9 How many variables can you replace with _? double odds(Object obj) { return switch (obj) { case String s when "blackjack".equals(s) -> .05; case String s -> throw new IllegalArgumentException("unknown game"); case Object o -> throw new IllegalArgumentException("unknown game"); }; } A. 0 C. 2 B. 1 D. 3 https://linktr.ee/jeanneboyarsky 75
76. Module 2 - Question 9 How many variables can you replace with _? double odds(Object obj) { return switch (obj) { case String s when "blackjack".equals(s) -> .05; case String s -> throw new IllegalArgumentException("unknown game"); case Object o -> throw new IllegalArgumentException("unknown game"); }; } C A. 0 C. 2 B. 1 D. 3 https://linktr.ee/jeanneboyarsky 76
77. Module 2 - Question 10 Which of these variables can be replaced by _? private int unused; void transform(List<String> list) { if (list instanceof ArrayList arr) try { list.removeIf(x -> true); } catch (NullPointerException e) { } } A. unused B. arr C. x D. e https://linktr.ee/jeanneboyarsky 77
78. Module 2 - Question 10 Which of these variables can be replaced by _? private int unused; void transform(List<String> list) { if (list instanceof ArrayList arr) try { list.removeIf(x -> true); } catch (NullPointerException e) { } } B, C, D A. unused B. arr C. x D. e https://linktr.ee/jeanneboyarsky 78
79. Lab & Break https://github.com/boyarsky/2026-kcdc-java25 https://linktr.ee/jeanneboyarsky 79
80. Agenda Module 1 2 3 4 Topics Intro, compact source files, instance main, IO class, multi file source code, module imports Flexible constructor bodies, unnamed variables, pattern matching changes Stream Gatherers, sequenced collections Virtual threads, scoped values, Bonus: JavaDoc, structured concurrency https://linktr.ee/jeanneboyarsky 80
81. Features Feature Preview Final Stream Gatherers 22, 23 24 Sequenced Collections https://linktr.ee/jeanneboyarsky 21 81
82. New Intermediate Operation 24 public Stream<R> gather( Gatherer<? super T, ? , R> gatherer) Predefined or custom https://linktr.ee/jeanneboyarsky 82
83. Predefined Method 24 Description Gatherers.windowFixed() Groups of size x with possibly smaller last one Gatherers.windowSliding() Groups of size x differing by 1 Gatherers.fold() Combine elements into value Gatherers.scan() Accumulates storing intermediate Gatherers.mapConcurrent() Map with parallelism https://linktr.ee/jeanneboyarsky 83
84. Window Fixed 24 Stream.of(1, 2, 3, 4, 5) .gather(Gatherers.windowFixed(3)) .forEach(IO::println); [1, 2, 3] [4, 5] https://linktr.ee/jeanneboyarsky 84
85. Window Sliding 24 Stream.of(1, 2, 3, 4, 5) .gather(Gatherers.windowSliding(3)) .forEach(IO::println); [1, 2, 3] [2, 3, 4] [3, 4, 5] https://linktr.ee/jeanneboyarsky 85
86. Fold 24 Stream.of("w", "o", "l", "f") .gather(Gatherers.fold(() -> “", (s, c) -> s + c)) .forEach(IO::println); wolf https://linktr.ee/jeanneboyarsky 86
87. Scan 24 Stream.of(10, 15, 13) .gather(Gatherers.scan( () -> 0, (t, a) -> t + a)) .forEach(IO::println); 10 25 38 https://linktr.ee/jeanneboyarsky 87
88. Map Concurrent 24 Stream.of("a", "b", "c") .gather(Gatherers.mapConcurrent( 2, s -> s.toUpperCase())) .forEach(IO::println); A B C https://linktr.ee/jeanneboyarsky 88
89. Custom Gatherer parts 24 Function Description Initializer New state. Can be mutable Integrator Integrates new element Combiner Combines two states Finisher One final action https://linktr.ee/jeanneboyarsky 89
90. Parallel Builders 24 • Gatherer.of(integrator) • Gatherer.of(integrator, finisher) • Gatherer.of(intitializer, integrator, combiner, finisher) @jeanneboyarsky 90
91. Sequential Builders 24 • Gatherer.ofSequential(integrator) • Gatherer.ofSequential(initializer, integrator) • Gatherer.ofSequential(integrator, finisher) • Gatherer.ofSequential(initializer, integrator, finisher) No combiner as don’t need to merge intermediate states when sequential @jeanneboyarsky 91
92. Customer Gatherer: Initializer 24 // create empty set Supplier<Set<Character>> initializer = TreeSet::new; @jeanneboyarsky 92
93. Customer Gatherer: Integrator 24 // add to intermediate set if letter // stop stream processing when not a letter Gatherer.Integrator<Set<Character>, Character, String> integrator = (set, ch, downstream) -> { if (Character.isLetter(ch)) { set.add(ch); return true; } return false; }; @jeanneboyarsky 93
94. Customer Gatherer: Finisher 24 // convert into a string BiConsumer<Set<Character>, Gatherer.Downstream<? super String>> finisher = (set, downstream) -> downstream.push( set.stream() .map(String::valueOf) .collect(Collectors.joining(""))); } @jeanneboyarsky 94
95. Customer Gatherer: Using Sequential 24 var gatherer = Gatherer.ofSequential( initializer, integrator, finisher); Stream.of('J', 'a', 'v', 'a', ' ‘, '2', '5', 'r', 'o', 'c', 'k', 's') .gather(gatherer) .forEach(IO::println); Jav @jeanneboyarsky 95
96. Customer Gatherer: Combiner 24 // combine the two sets BinaryOperator<Set<Character>> combiner = (set1, set2) -> { set1.addAll(set2); return set1; }; @jeanneboyarsky 96
97. Customer Gatherer: Using Parallel 24 var gatherer = Gatherer.of( initializer, integrator, combiner, finisher); Stream.of('J', 'a', 'v', 'a', ' ‘, '2', '5', 'r', 'o', 'c', 'k', 's') .gather(gatherer) .forEach(IO::println); Jav @jeanneboyarsky 97
98. Java 17 ArrayList, LinkedList, Deque, TreeSet, TreeMap became sequenced collections LinkedHashSet and LinkedHashMap too https://linktr.ee/jeanneboyarsky 98
99. What does this output? 21 HashSet<String> set = new HashSet<>(); set.add("Donald"); set.add("Mickey"); set.add("Minnie"); System.out.println(set.iterator().next()); Encounter order undefined(but Mickey on my https://linktr.ee/jeanneboyarsky 99
100. Java 21 https://linktr.ee/jeanneboyarsky 21 100
101. New APIs SequencedCollection SequencedSet SequencedCollection <E> reversed(); void addFirst(E); void addLast(E); E getFirst(); E getLast(); E removeFirst(); E removeLast(); SequencedSet<E> reversed(); SequencedMap SequencedMap<K,V> reversed(); SequencedSet<K> sequencedKeySet(); SequencedCollection<V> sequencedValues(); SequencedSet<Entry<K,V>> sequencedEntrySet(); V putFirst(K, V); V putLast(K, V); Entry<K, V> rstEntry(); Entry<K, V> lastEntry(); Entry<K, V> pollFirstEntry(); Entry<K, V> pollLastEntry(); https://linktr.ee/jeanneboyarsky fi 21 101
102. Now defined! 21 SequencedSet<String> set = new LinkedHashSet<>(); set.add("Donald"); set.add("Mickey"); set.add("Minnie"); System.out.println(set.getFirst()); Donald Note: LInkedHashSet not new https://linktr.ee/jeanneboyarsky 102
103. Module 3 - Question 1 Which part of a stream gatherer sets the value if the stream is empty? A. Constructor C. Initializer B. Integrator D. Instantiator https://linktr.ee/jeanneboyarsky 103
104. Module 3 - Question 1 Which part of a stream gatherer sets the value if the stream is empty? C A. Constructor C. Initializer B. Integrator D. Instantiator https://linktr.ee/jeanneboyarsky 104
105. Module 3 - Question 2 Which part of a stream gatherer processes a single value from the stream? A. Combiner C. Integrator B. Consolidator D. Initializer https://linktr.ee/jeanneboyarsky 105
106. Module 3 - Question 2 Which part of a stream gatherer processes a single value from the stream? C A. Combiner C. Integrator B. Consolidator D. Initializer https://linktr.ee/jeanneboyarsky 106
107. Module 3 - Question 3 Which part of a stream gatherer is only used for a parallel stream? A. Combiner C. Integrator B. Consolidator D. Merger https://linktr.ee/jeanneboyarsky 107
108. Module 3 - Question 3 Which part of a stream gatherer is only used for a parallel stream? A A. Combiner C. Integrator B. Consolidator D. Merger https://linktr.ee/jeanneboyarsky 108
109. Module 3 - Question 4 Which part of a stream gatherer returns the value for the next step in the pipeline at the end of the gatherer A. Ender C. StreamOperation B. Finisher D. Terminator https://linktr.ee/jeanneboyarsky 109
110. Module 3 - Question 4 Which part of a stream gatherer returns the value for the next step in the pipeline at the end of the gatherer B A. Ender C. StreamOperation B. Finisher D. Terminator https://linktr.ee/jeanneboyarsky 110
111. Module 3 - Question 5 If you want to return multiple elements to the stream what parts can push to the downstream? A. Initializer C. Combiner B. Integrator D. Finisher https://linktr.ee/jeanneboyarsky 111
112. Module 3 - Question 5 If you want to return multiple elements to the stream what parts can push to the downstream? B, D A. Initializer C. Combiner B. Integrator D. Finisher https://linktr.ee/jeanneboyarsky 112
113. Module 3 - Question 6 What is the output of the following? var list = Stream.of("kansas", "missouri") .gather(Gatherers.scan( () -> "", (s1, s2) -> s1 + " " + s2)) .toList(); IO.println(list); A. [kansas missouri] B. [ kansas missouri] C. [kansas, kansas missouri] D. [ kansas, kansas missouri] https://linktr.ee/jeanneboyarsky 113
114. Module 3 - Question 6 What is the output of the following? var list = Stream.of("kansas", "missouri") .gather(Gatherers.scan( () -> "", (s1, s2) -> s1 + " " + s2)) .toList(); IO.println(list); D A. [kansas, missouri] B. [ kansas, missouri] C. [kansas, kansas missouri] D. [ kansas, kansas missouri] https://linktr.ee/jeanneboyarsky 114
115. Module 3 - Question 7 What is the output of the following? var list = Stream.of("kansas", "missouri") .gather(Gatherers.fold( () -> "", (s1, s2) -> s1 + " " + s2)) .toList(); IO.println(list); A. [kansas missouri] B. [ kansas missouri] C. [kansas, kansas missouri] D. [ kansas, kansas missouri] https://linktr.ee/jeanneboyarsky 115
116. Module 3 - Question 7 What is the output of the following? var list = Stream.of("kansas", "missouri") .gather(Gatherers.fold( () -> "", (s1, s2) -> s1 + " " + s2)) .toList(); IO.println(list); B A. [kansas missouri] B. [ kansas missouri] C. [kansas, kansas missouri] D. [ kansas, kansas missouri] https://linktr.ee/jeanneboyarsky 116
117. Module 3 - Question 8 What is the output of the following? var list = Stream.iterate(1, i -> i + 1) .limit(5) .gather(Gatherers.windowFixed(4)) .toList(); IO.println(list); } A. [[1, 2, 3, 4]] B. [[1, 2, 3, 4], [5]] C. [[1, 2, 3, 4], [2, 3, 4, 5]] D. None of the above https://linktr.ee/jeanneboyarsky 117
118. Module 3 - Question 8 What is the output of the following? var list = Stream.iterate(1, i -> i + 1) .limit(5) .gather(Gatherers.windowFixed(4)) .toList(); IO.println(list); } B A. [[1, 2, 3, 4]] B. [[1, 2, 3, 4], [5]] C. [[1, 2, 3, 4], [2, 3, 4, 5]] D. None of the above https://linktr.ee/jeanneboyarsky 118
119. Module 3 - Question 9 What is the output of the following? var list = Stream.iterate(1, i -> i + 1) .limit(5) .gather(Gatherers.windowSliding(4)) .toList(); IO.println(list); } A. [[1, 2, 3, 4]] B. [[1, 2, 3, 4], [5]] C. [[1, 2, 3, 4], [2, 3, 4, 5]] D. None of the above https://linktr.ee/jeanneboyarsky 119
120. Module 3 - Question 9 What is the output of the following? var list = Stream.iterate(1, i -> i + 1) .limit(5) .gather(Gatherers.windowSliding(4)) .toList(); IO.println(list); } C A. [[1, 2, 3, 4]] B. [[1, 2, 3, 4], [5]] C. [[1, 2, 3, 4], [2, 3, 4, 5]] D. None of the above https://linktr.ee/jeanneboyarsky 120
121. Module 4 - Question 10 How many of these lines compile? SequencedCollection<Integer> seq1 = new HashSet<>(); SequencedSet<Integer> seq2 = new HashSet<>(); SequencedMap<Integer> seq3 = new HashSet<>(); SequencedCollection<Integer> seq4 = new TreeSet<>(); SequencedSet<Integer> seq5 = new TreeSet<>(); SequencedMap<Integer> seq6 = new TreeSet<>(); A. One C. Three B. Two D. Four https://linktr.ee/jeanneboyarsky 121
122. Module 4 - Question 10 How many of these lines compile? SequencedCollection<Integer> seq1 = new HashSet<>(); SequencedSet<Integer> seq2 = new HashSet<>(); SequencedMap<Integer> seq3 = new HashSet<>(); SequencedCollection<Integer> seq4 = new TreeSet<>(); (seq4=and 5) SequencedSet<Integer> seq5 newseq TreeSet<>(); SequencedMap<Integer> seq6 = new TreeSet<>(); B A. One C. Three B. Two D. Four https://linktr.ee/jeanneboyarsky 122
123. Lab & Break https://github.com/boyarsky/2026-kcdc-java25 https://linktr.ee/jeanneboyarsky 123
124. Agenda Module 1 2 3 4 Topics Intro, compact source files, instance main, IO class, multi file source code, module imports Flexible constructor bodies, unnamed variables, pattern matching changes Stream Gatherers, sequenced collections Virtual threads, scoped values, Bonus: JavaDoc, structured concurrency https://linktr.ee/jeanneboyarsky 124
125. Features Feature Preview Final Virtual Threads 19, 20 21 Scoped Values 21, 22, 23, 24 25 JavaDoc 18, 23 Structured Concurrency 21, 22, 23, 24, 25, 26, 27 https://linktr.ee/jeanneboyarsky TBD 125
126. Leaps in Concurrency + Executors Threads 21 + Virtual Threads Plus many APIs… https://linktr.ee/jeanneboyarsky 126
127. Why we need threads Program I’m bored… Do calculation and call REST API CPU Do calculation and call REST API Network https://linktr.ee/jeanneboyarsky I’m waiting for a reply 127
128. Why we need virtual threads Program Platform Thread 1 Platform Thread n Still bored… Send to threads Do calculation and call REST API Do calculation and call REST API Proceed Network https://linktr.ee/jeanneboyarsky CPU Still waiting for a reply 128
129. With virtual threads Program Send to threads Proceed Platform Thread 1 Platform Thread n Virtual Thread 1 Virtual Thread t+1 Do calculation and call REST API Do calculation and call REST API Virtual Thread t Virtual Thread x Do calculation and call REST API Do calculation and call REST API Thanks for the work! CPU Me too! Network https://linktr.ee/jeanneboyarsky 129
130. When most helpful? 21 • Thousands of threads • Not CPU bound https://linktr.ee/jeanneboyarsky 130
131. Comparing 21 Platform (Traditional) Thread Heavyweight Virtual Thread Tied to OS threads Runs on OS thread but doesn’t monopolize Often shortlived Often pooled Lightweight Instance of java.lang.Thread Instance of java.lang.Thread https://linktr.ee/jeanneboyarsky 131
132. Fill in the blank 21 try (ExecutorService service = Executors._________) { service.submit(() -> doStuff()); } Examples: newSingleThreadExecutor() newFixedThreadPool(5) newCachedThreadPool() newVirtualThreadPerTaskExecutor() https://linktr.ee/jeanneboyarsky 132
133. Autocloseable 19 Method Java 21 shutdown() shutdownNow() No more tasks, but finish ones have Stop tasks in progress close() Calls shutdown by default https://linktr.ee/jeanneboyarsky 133
134. If need directly Platform Thread Virtual Thread Thread.ofPlatform() Thread.ofVirtual() new Thread(…) Not via constructor https://linktr.ee/jeanneboyarsky 21 134
135. Scoped Values 25 static final ScopedValue<List<String>> SESSIONS = ScopedValue.newInstance(); void main() { var sessions = List.of("Java 25", "AI"); ScopedValue.where(SESSIONS, sessions).run( () -> { attend(); }); } void attend() { immutable IO.println(SESSIONS.get()); scoped data } @jeanneboyarsky 135
136. Scoped Values 25 static final ScopedValue<List<String>> SESSIONS = ScopedValue.newInstance(); void main() { IO.print(SESSIONS.isBound()); IO.print(SESSIONS.orElse("Not found"); SESSIONS.get(); } false Not found NoSuchElementException @jeanneboyarsky 136
137. Structured Concurrency ? Incubated Java 19 Sixth preview in Java 27 @jeanneboyarsky 137
138. Structured Concurrency Preview 25 static final ScopedValue<List<String>> SESSIONS = ScopedValue.newInstance(); void main() { ScopedValue.where(SESSIONS, List.of()) .run(() -> { scope.fork(() -> attend()); scope.fork(() -> attend()); }); } @jeanneboyarsky 138
139. Bonus: JavaDoc 23 /// While /** */ uses HTML, /// triple slashes use **markdown** /// /// commonmark.org format /// - on odd days, … /// - on even days, … /// /// @return an important number @jeanneboyarsky 139
140. Module 4 - Question 1 Which is used to create/with a virtual thread? (more than 1 is correct) A. Executors.newVirtualThread() B. Executors.newVirtualThreadExecutor() C. Executors.newVirtualThreadPerTaskExecutor() D. new VirtualThread() E. Thread.ofVirtual() F. Thread.ofVirtualThread() https://linktr.ee/jeanneboyarsky 140
141. Module 4 - Question 1 Which is used to create/with a virtual thread? (more than 1 is correct) A. Executors.newVirtualThread() B. Executors.newVirtualThreadExecutor() C, E C. Executors.newVirtualThreadPerTaskExecutor() D. new VirtualThread() E. Thread.ofVirtual() F. Thread.ofVirtualThread() https://linktr.ee/jeanneboyarsky 141
142. Module 4 - Question 2 Which is used to create/with a platform thread? (more than 1 is correct) A. Executors.newCachedThreadPool() B. Executors.newPlatformThreadPool() C. Executors.newPlatformThreadPerTaskExecutor() D. new Thread() E. new PlatformThread() F. Thread.ofPlatform() https://linktr.ee/jeanneboyarsky 142
143. Module 4 - Question 2 Which is used to create/with a platform thread? (more than 1 is correct) A. Executors.newCachedThreadPool() B. Executors.newPlatformThreadPool() A, D, F C. Executors.newPlatformThreadPerTaskExecutor() D. new Thread() E. new PlatformThread() F. Thread.ofPlatform() https://linktr.ee/jeanneboyarsky 143
144. Module 4 - Question 3 What is true doStuff() at line v1? try (var service = Executors.newVirtualThreadPerTaskExecutor()) { service.submit(() -> doStuff()); } // line v1 A. doStuff() may be running C. doStuff() completed B. doStuff() was killed D. None of the above https://linktr.ee/jeanneboyarsky 144
145. Module 4 - Question 3 What is true doStuff() at line v1? try (var service = Executors.newVirtualThreadPerTaskExecutor()) { service.submit(() -> doStuff()); } C // line v1 A. doStuff() may be running C. doStuff() completed B. doStuff() was killed D. None of the above https://linktr.ee/jeanneboyarsky 145
146. Module 4 - Question 4 Which types are are immutable in SV? static final ScopedValue<XXX>> SV = ScopedValue.newInstance(); A. List B. List<String> C. LocalDateTime D. String @jeanneboyarsky 146
147. Module 4 - Question 4 Which types are are immutable in SV? static final ScopedValue<XXX>> SV = ScopedValue.newInstance(); C, D A. List B. List<String> C. LocalDateTime D. String @jeanneboyarsky 147
148. Module 4 - Question 5 What is the output of the following? static final ScopedValue<Integer> LENGTH = ScopedValue.newInstance(); void timing() { IO.print(LENGTH.get()); } void main() { timing(); ScopedValue.where(ID, 10) .run(this::timing); } A. 0 B. 10 C. Code does not compile D. Throws an exception @jeanneboyarsky 148
149. Module 4 - Question 5 What is the output of the following? static final ScopedValue<Integer> LENGTH = ScopedValue.newInstance(); void timing() { IO.print(LENGTH.get()); } void main() { timing(); ScopedValue.where(ID, 10) .run(this::timing); } D A. 0 B. 10 C. Code does not compile D. Throws an exception @jeanneboyarsky 149
150. Module 4 - Question 6 What is the output of the following? static final ScopedValue<Integer> LENGTH = ScopedValue.newInstance(); void timing() { IO.print(LENGTH.get()); } void main() { ScopedValue.where(ID, 10) .run(this::timing); } A. 0 B. 10 C. Code does not compile D. Throws an exception @jeanneboyarsky 150
151. Module 4 - Question 6 What is the output of the following? static final ScopedValue<Integer> LENGTH = ScopedValue.newInstance(); void timing() { IO.print(LENGTH.get()); } void main() { ScopedValue.where(ID, 10) .run(this::timing); } B A. 0 B. 10 C. Code does not compile D. Throws an exception @jeanneboyarsky 151
152. Module 4 - Question 7 What is the output of the following? static final ScopedValue<Integer> LENGTH = ScopedValue.newInstance(); void timing() { IO.print(LENGTH.orElse(2)); } void main() { timing(); } A. 0 B. 2 C. Code does not compile D. Throws an exception @jeanneboyarsky 152
153. Module 4 - Question 7 What is the output of the following? static final ScopedValue<Integer> LENGTH = ScopedValue.newInstance(); B void timing() { IO.print(LENGTH.orElse(2)); } void main() { timing(); } A. 0 B. 2 C. Code does not compile D. Throws an exception @jeanneboyarsky 153
154. Module 4 - Question 8 What is the output of the following? static final ScopedValue<Integer> LENGTH = ScopedValue.newInstance(); void timing() { IO.print(LENGTH.isBound()); } void main() { timing(); ScopedValue.where(ID, 10) .run(this::timing); } A. falsefalse B. falsetrue C. truefalse D. truetrue @jeanneboyarsky 154
155. Module 4 - Question 8 What is the output of the following? static final ScopedValue<Integer> LENGTH = ScopedValue.newInstance(); void timing() { IO.print(LENGTH.isBound()); } void main() { timing(); ScopedValue.where(ID, 10) .run(this::timing); } B A. falsefalse B. falsetrue C. truefalse D. truetrue @jeanneboyarsky 155
156. Module 4 - Question 9 True or false: you can fork a ScopedValue? A. true B. false @jeanneboyarsky 156
157. Module 4 - Question 9 True or false: you can fork a ScopedValue? A. true B. false Depends on Java version @jeanneboyarsky 157
158. Module 4 - Question 10 I learned a lot today and I’m ready to code A. True B. False @jeanneboyarsky 158
159. Lab & Giveaway https://github.com/boyarsky/2026-kcdc-java25 https://linktr.ee/jeanneboyarsky 159