Java Puzzlers NG S02: Down the Rabbit Hole as presented at Devoxx US 2017

3. Developer Advocate @JFrog @jbaruch on the internetz Developer Advocate @Hazelcast @gAmUssA on the internetz

5. @tagir_valeev @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

6. @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

9. 1. Two entertaining guys on the stage 2. Funny puzzling questions 3. You think and vote 4.T-shirts are airborne 5. Official twitter hashtags: #javapuzzlersng #DevoxxUS

11. Which Java version are you on? A. Java 7 B. Java 8 C. Java 9 D. Java 6 E. Java 5 F. Java 2 @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

12. Watching the puzzlers like… #dafaq @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

13. Everything works (or doesn't) in the latest Java 8 and/or 9 update @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

16. public class PerfectRobbery { private Semaphore bankAccount = new Semaphore(-42); public static void main(String[] args) { PerfectRobbery perfectRobbery = new PerfectRobbery(); perfectRobbery.takeAllMoney(); perfectRobbery.checkBalance(); } public void takeAllMoney(){ bankAccount.drainPermits(); } public void checkBalance(){ System.out.println(bankAccount.availablePermits()); } } A. IllegalArgumentException –can’t create semaphore with negative B. UnsupportedOperationException –can’t drain when negative C. 0 D. -42

18. A. IllegalArgumentException –can’t create semaphore with negative B. UnsupportedOperationException –can’t drain when negative C. 0 D. -42 public class PerfectRobbery { private Semaphore bankAccount = new Semaphore(-42); public static void main(String[] args) { PerfectRobbery perfectRobbery = new PerfectRobbery(); perfectRobbery.takeAllMoney(); perfectRobbery.checkBalance(); } public void takeAllMoney(){ bankAccount.drainPermits(); } public void checkBalance(){ System.out.println(bankAccount.availablePermits()); } }

19. Available -42?! @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

20. Available -42?! @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

22. A. true/true B. true/false C. false/true D. false/false Collections.emptyList() == Collections.emptyList(); Collections.emptyIterator() == Collections.emptyIterator(); @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

23. A. true/true B. true/false C. false/true D. false/false Collections.emptyList() == Collections.emptyList(); Collections.emptyIterator() == Collections.emptyIterator(); @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

25. A. true/true B. true/false C. false/true D. false/false Spliterators.emptySpliterator() == Spliterators.emptySpliterator(); Stream.empty() == Stream.empty();

26. Singleton Strikes Back! @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

27. A. true/true B. true/false C. false/true D. false/false Spliterators.emptySpliterator() == Spliterators.emptySpliterator(); Stream.empty() == Stream.empty(); @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

28. Even empty Stream has state! @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

30. ”Identical” 1. Has the same state 2. Not related to “equals and hashcode”contract 3. Not related to references to objects in memory @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

31. List[] twins = new List[2]; Arrays.setAll(twins, ArrayList::new); A. Absolutely identical empty lists B. Absolutely identical non-empty lists C. Non-identical empty lists D. Non-identical non-empty lists @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

33. List[] twins = new List[2]; Arrays.setAll(twins, ArrayList::new); A. Absolutely identical empty lists B. Absolutely identical non-empty lists C. Non-identical empty lists D. Non-identical non-empty lists @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

35. How single is a Single Abstract Method Interface? @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

36. A. WTF?! ’Single’means one,not three! B. Problem is with partyHard(T),remove it and it will work C. Problem is the drinkIn methods,removing one of them and it will work D. It will work fine! Both partyHard() and drinkIn() are merged in SingleAndHappy,leaving one abstract method public interface Single<T> { default void partyHard(String songName) { System.out.println(songName); } void partyHard(T songName); void drinkIn(T drinkName); void drinkIn(String dringName); } @FunctionalInterface public interface SingleAndHappy extends Single<String> { } @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

38. A. WTF?! ’Single’means one,not three! B. Problem is with partyHard(T),remove it and it will work C. Problem are the drinkIn methods,removing it will leave one abstract method D. Yes! Both partyHard() and drinkIn() are merged in SingleAndHappy, leaving one abstract method public interface Single<T> { default void partyHard(String songName) { System.out.println(songName); } void partyHard(T songName); void drinkIn(T drinkName); void drinkIn(String dringName); } @FunctionalInterface public interface SingleAndHappy extends Single<String> { } @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

41. Hacking the bank ☑Bank software written in Java ☑Hack into it ☑Analyze the accounts @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

42. Given the code above,which statement is wrong: A. The Set is ordered by hashcode B. The order is predictable across multiple runs of the JVM on the same machine C. The order of elements in Set is not predictable D. Statements A & B are correct Set<String> accounts= new HashSet<>(Arrays.asList("Gates", "Buffett", "Bezos", "Zuckerberg")); System.out.println(”accounts= " + accounts); @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

44. Given the code above,which statement is wrong: A. The Set is ordered by hashcode B. The order is predictable across multiple runs of the JVM on the same machine C. The order of elements in Set is not predictable D. Statements A & B are correct Set<String> accounts= new HashSet<>(Arrays.asList("Gates", "Buffett", "Bezos", "Zuckerberg")); System.out.println(”accounts= " + accounts); @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

45. public boolean add(E e) { return map.put(e, PRESENT)==null; } @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

46. Your turn,FBI @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

48. Given the code above,which statement is wrong: A. The Set is ordered by hashcode B. The order is predictable across multiple runs of the JVM on the same machine C. The order of elements in Set is not predictable D. Statements A & B are correct Set<String> accounts = Set.of("Gates", "Buffett", "Bezos", "Zuckerberg"); System.out.println(”accounts= " + accounts); @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

51. Given the code above,which statement is wrong: A. The Set is ordered by hashcode B. The order is predictable across multiple runs of the JVM on the same machine C. The order of elements in Set is not predictable D. Statements A & B are correct Set<String> accounts = Set.of("Gates", "Buffett", "Bezos", "Zuckerberg"); System.out.println(”accounts= " + accounts); @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

52. private int probe(Object pe) { int idx = Math.floorMod(pe.hashCode() ^ SALT, elements.length); while (true) { E ee = elements[idx]; if (ee == null) { return -idx - 1; } else if (pe.equals(ee)) { return idx; } else if (++idx == elements.length) { idx = 0; } } } @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

53. Juggling Accident

54. What’s correct? A. If you convert your application to module,classpath dependencies will still be resolved correctly B. If one of the dependencies was converted to a module,you have to declare it in module-info in order to use C. Once you added the module-info to your project you have to declare the dependencies twice,in classpath and in module- info D. None of the above @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

56. What’s correct? A. If you convert your application to module,classpath dependencies will still be resolved correctly B. If one of the dependencies was converted to a module,you have to declare it in module-info in order to use C. Once you added the module-info to your project you have to declare the dependencies twice,in classpath and in module- info D. None of the above @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

59. A. You killed them all B. You killed only even ones C. They all survived D. You killed only odd ones E. All answers are correct static void killThemAll(Collection<Hero> expendables) { Iterator<Hero> heroes = expendables.iterator(); heroes.forEachRemaining(e -> { if (heroes.hasNext()) { heroes.next(); heroes.remove(); } }); System.out.println(expendables); }

61. A. You killed them all B. You killed only even ones C. They all survived D. You killed only odd ones E. All answers are correct static void killThemAll(Collection<Hero> expendables) { Iterator<Hero> heroes = expendables.iterator(); heroes.forEachRemaining(e -> { if (heroes.hasNext()) { heroes.next(); heroes.remove(); } }); System.out.println(expendables); }

62. Don’t do that.Really,don’t. killThemAll(new ArrayList<String>(Arrays.asList("N","S","W","S","L","S","L","V"))); [] killThemAll(new LinkedList<String>(Arrays.asList("N","S","W","S","L","S","L","V"))); [S,S,S,V] killThemAll(new ArrayDeque<String>(Arrays.asList("N","S","W","S","L","S","L","V"))); [N,S,W,S,L,S,L,V] killThemAll(new TreeSet<String>(Arrays.asList("N","S","W","S","L","S","L","V"))); [N,W,L,L] @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

63. Subtle Difference @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

64. A. Both work just fine B. Lambda works,method ref fails C. Method ref works,lambda fails D. Won’t compile @FunctionalInterface public interface OriginalPredicate<T> { boolean test(T t); } OriginalPredicate<Object> lambda = (Object obj) -> ”adidas".equals(obj); OriginalPredicate<Object> methodRef = ”adidas"::equals;

65. A. Both work just fine B. Lambda works,method ref fails C. Method ref works,lambda fails D. Won’t compile @FunctionalInterface public interface OriginalPredicate<T> { boolean test(T t); } OriginalPredicate<Object> lambda = (Object obj) -> ”adidas".equals(obj); OriginalPredicate<Object> methodRef = ”adidas"::equals;

67. A. Both work just fine B. Lambda works,method ref fails C. Method ref works,lambda fails D. Not a functional interface,will fail on annotation processing @FunctionalInterface Public interface CopyCatPredicate { <T> boolean test(T t); } CopyCatPredicate lambda = (Object obj) -> " adadas".equals(obj); CopyCatPredicate methodRef = " adadas"::equals;

69. A. Both work just fine B. Lambda works,method ref fails C. Method ref works,lambda fails D. Not a functional interface,will fail on annotation processing @FunctionalInterface Public interface CopyCatPredicate { <T> boolean test(T t); } CopyCatPredicate lambda = (Object obj) -> " adadas".equals(obj); CopyCatPredicate methodRef = " adadas"::equals;

71. A generic function type for a functional interface may be implemented by a method reference expression (§15.13), but not by a lambda expression (§15.27) as there is no syntax for generic lambda expressions. “ @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

73. A.[Data, Kirk, Spock] B.[Spock, Kirk, Data, Data, Kirk, Spock] C.[Spock, Kirk, Data] D.[Data, Data, Kirk, Kirk, Spock, Spock] E.Are you nuts? Won’t compile! Data with Kirk?! List<String> list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk", "Spock").sequential() .filter(new TreeSet<>()::add).collect(Collectors.toList()); System.out.println(list); @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

78. filter(new TreeSet<>()::add) filter(i -> new TreeSet<>().add(i))!= New instance is created every time! Instance method is created once! @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

80. A. obvious / obvious B. obvious / NullPointerException C. NullPointerException / obvious D. NullPointerException / NullPointerException Optional.of("obvious").orElseGet(null); Optional.empty().map(null).orElse("obvious"); @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

82. A. obvious / obvious B. obvious / NullPointerException C. NullPointerException / obvious D. NullPointerException / NullPointerException Optional.of("obvious").orElseGet(null); Optional.empty().map(null).orElse("obvious"); Will never happen Will never happen @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

84. A. obvious / obvious B. obvious / NullPointerException C. NullPointerException / obvious D. NullPointerException / NullPointerException Optional.of("obvious").orElseGet(null); Optional.empty().map(null).orElse("obvious"); @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

86. Identical? @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

87. A. All B. 3 and 4 C. Only 3 D. Other 1. Consumer<String> agentA = s -> System.out.println(s); Consumer<String> agentB = s -> System.out.println(s); 2. Consumer<String> agentA = System.out::println; Consumer<String> agentB = System.out::println; 3. Supplier<Consumer<String>> supplier = () -> s -> System.out.println(s); Consumer<String> agentA = supplier.get(); Consumer<String> agentB = supplier.get(); 4. Supplier<Consumer<String>> supplier = () -> System.out::println; Consumer<String> agentA = supplier.get(); Consumer<String> agentB = supplier.get(); When agentA == agentB?

89. A. All B. 3 and 4 C. Only 3 D. Other 1. Consumer<String> agentA = s -> System.out.println(s); Consumer<String> agentB = s -> System.out.println(s); 2. Consumer<String> agentA = System.out::println; Consumer<String> agentB = System.out::println; 3. Supplier<Consumer<String>> supplier = () -> s -> System.out.println(s); Consumer<String> agentA = supplier.get(); Consumer<String> agentB = supplier.get(); 4. Supplier<Consumer<String>> supplier = () -> System.out::println; Consumer<String> agentA = supplier.get(); Consumer<String> agentB = supplier.get(); When agentA == agentB?

90. Reuse is only possible for pure functions Consumers accept parameters == have state Supplier in 4 has state –the resolved method reference @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

91. Conclusions @jbaruch @gamussa #DevoxxUS http://jfrog.com/shownotes

92. -Write readable code! -Comment all the tricks -Sometimes it’s just a bug -Static code analysis FTW - IntelliJ IDEA! -RTFM! -Don’t abuse lambdas and streams!

93. -Trust us, we have much more where those came from. -Puzzlers? Gotchas? Fetal position inducing behavior? -puzzlers@jfrog.com

94. -Shownotes! -http://jfrog.com/shownotes -Slides -Video in 24 hours -Links -Ratings -Raffle! (come early)

95. -Did you like it? -Praise us on twitter! -#javapuzzlersng #DevoxxUS -@gamussa -@jbaruch -Didn’t like it? -/dev/null

Java Puzzlers NG S02: Down the Rabbit Hole as presented at Devoxx US 2017

More Related Content

What's hot (7)

Viewers also liked (20)

Similar to Java Puzzlers NG S02: Down the Rabbit Hole as presented at Devoxx US 2017 (20)

More from Baruch Sadogursky (20)

Recently uploaded (20)

Java Puzzlers NG S02: Down the Rabbit Hole as presented at Devoxx US 2017