[Philly ETE] Java Puzzlers NG

The more we work with Java 8, the more we go into the rabbit hole. Did they add all those streams, lambdas, monads, Optionals and CompletableFutures only to confuse us? It surely looks so! And Java 9 that heads our way brings even more of what we like the most, more puzzlers, of course! In this season we, as usual, have a great batch of the best Java WTF, great jokes to present them and great prizes for the winners!

Transcript

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2. JAVA PUZZLERS NG
   DOWN THE RABBIT HOLE
   http://www.davidfloresmedia.com/philly-rocky-statue/
   

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3. @gAmUssA @hazelcast #jbreak #hazelcastjet
   > whoami
   

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4. @gAmUssA @hazelcast #jbreak #hazelcastjet
   Solutions Architect
   > whoami
   

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5. @gAmUssA @hazelcast #jbreak #hazelcastjet
   Solutions Architect
   Developer Advocate
   > whoami
   

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6. @gAmUssA @hazelcast #jbreak #hazelcastjet
   Solutions Architect
   Developer Advocate
   @gamussa in internetz
   > whoami
   

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7. @gAmUssA @hazelcast #jbreak #hazelcastjet
   Solutions Architect
   Developer Advocate
   @gamussa in internetz
   Please, follow me on Twitter
   > whoami
   

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8. @gAmUssA @hazelcast #jbreak #hazelcastjet
   Solutions Architect
   Developer Advocate
   @gamussa in internetz
   Please, follow me on Twitter
   I’m very interesting ©
   > whoami
   

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11. Developer Advocate @JFrog
    on the internetz
    Developer Advocate @Hazelcast
    @gAmUssA on the internetz
    

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12. CLICK AND HACK
    THE TYPE-IT BROTHERS
    

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13. @tagir_valeev
    @gamussa #PhillyETE
    

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14. @gamussa #PhillyETE
    

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15. @gamussa #PhillyETE
    

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16. @gamussa #PhillyETE
    

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17. View Slide

18. 1. Two One entertaining guy on
    the stage
    

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19. 1. Two One entertaining guy on
    the stage
    2. Funny puzzling questions
    

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20. 1. Two One entertaining guy on
    the stage
    2. Funny puzzling questions
    3. You think and vote
    

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21. 1. Two One entertaining guy on
    the stage
    2. Funny puzzling questions
    3. You think and vote
    4.Official twitter hashtags
    #javapuzzlersng #PhillyETE
    

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22. FIRST RULE OF THE PUZZLERS:
    NO CHEATING!
    

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23. Which Java version are you on?
    @gamussa #PhillyETE
    

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24. Which Java version are you on?
    A. Java 7
    @gamussa #PhillyETE
    

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25. Which Java version are you on?
    A. Java 7
    B. Java 8
    @gamussa #PhillyETE
    

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26. Which Java version are you on?
    A. Java 7
    B. Java 8
    C. Java 9
    @gamussa #PhillyETE
    

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27. Which Java version are you on?
    A. Java 7
    B. Java 8
    C. Java 9
    D. Java 6
    @gamussa #PhillyETE
    

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28. Which Java version are you on?
    A. Java 7
    B. Java 8
    C. Java 9
    D. Java 6
    E. Java 5
    @gamussa #PhillyETE
    

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29. Which Java version are you on?
    A. Java 7
    B. Java 8
    C. Java 9
    D. Java 6
    E. Java 5
    F. Java 2
    @gamussa #PhillyETE
    

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30. Which Java version are you on?
    A. Java 7
    B. Java 8
    C. Java 9
    D. Java 6
    E. Java 5
    F. Java 2
    @gamussa #PhillyETE
    

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31. Watching the puzzlers like… #dafaq
    @gamussa #PhillyETE
    

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32. Watching the puzzlers like… #dafaq
    @gamussa #PhillyETE
    

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33. Everything works (or doesn't) 

    in the latest Java 8 and/or 9 update
    @gamussa #PhillyETE
    

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34. View Slide

35. RUSSIAN HACKERS
    HACKING THE BANK
    

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36. 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());

    }

    }

    

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37. 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
    

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38. 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
    

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39. 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
    

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40. 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
    

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41. 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
    

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42. @gamussa #PhillyETE
    

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43. @gamussa #PhillyETE
    

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44. @gamussa #PhillyETE
    

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45. 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());

    }

    }

    

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46. @gamussa #PhillyETE
    

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47. @gamussa #PhillyETE
    

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48. @gamussa #PhillyETE
    

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49. Available
    -42?!
    @gamussa #PhillyETE
    

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50. Available
    -42?!
    @gamussa #PhillyETE
    

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51. @gamussa #PhillyETE
    

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52. Collections.emptyList() == Collections.emptyList();

    Collections.emptyIterator() == Collections.emptyIterator();

    @gamussa #PhillyETE
    

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53. A. true/true
    Collections.emptyList() == Collections.emptyList();

    Collections.emptyIterator() == Collections.emptyIterator();

    @gamussa #PhillyETE
    

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54. A. true/true
    B. true/false
    Collections.emptyList() == Collections.emptyList();

    Collections.emptyIterator() == Collections.emptyIterator();

    @gamussa #PhillyETE
    

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55. A. true/true
    B. true/false
    C. false/true
    Collections.emptyList() == Collections.emptyList();

    Collections.emptyIterator() == Collections.emptyIterator();

    @gamussa #PhillyETE
    

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56. A. true/true
    B. true/false
    C. false/true
    D. false/false
    Collections.emptyList() == Collections.emptyList();

    Collections.emptyIterator() == Collections.emptyIterator();

    @gamussa #PhillyETE
    

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57. A. true/true
    B. true/false
    C. false/true
    D. false/false
    Collections.emptyList() == Collections.emptyList();

    Collections.emptyIterator() == Collections.emptyIterator();

    @gamussa #PhillyETE
    

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58. @gamussa #PhillyETE
    

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59. @gamussa #PhillyETE
    

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60. Spliterators.emptySpliterator() == Spliterators.emptySpliterator();

    Stream.empty() == Stream.empty();

    

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61. A. true/true
    Spliterators.emptySpliterator() == Spliterators.emptySpliterator();

    Stream.empty() == Stream.empty();

    

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62. A. true/true
    B. true/false
    Spliterators.emptySpliterator() == Spliterators.emptySpliterator();

    Stream.empty() == Stream.empty();

    

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63. A. true/true
    B. true/false
    C. false/true
    Spliterators.emptySpliterator() == Spliterators.emptySpliterator();

    Stream.empty() == Stream.empty();

    

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64. A. true/true
    B. true/false
    C. false/true
    D. false/false
    Spliterators.emptySpliterator() == Spliterators.emptySpliterator();

    Stream.empty() == Stream.empty();

    

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65. A. true/true
    B. true/false
    C. false/true
    D. false/false
    Spliterators.emptySpliterator() == Spliterators.emptySpliterator();

    Stream.empty() == Stream.empty();

    

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66. Singleton Strikes Back!
    @gamussa #PhillyETE
    

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67. Singleton Strikes Back!
    @gamussa #PhillyETE
    

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68. A. true/true
    B. true/false
    C. false/true
    D. false/false
    Spliterators.emptySpliterator() == Spliterators.emptySpliterator();

    Stream.empty() == Stream.empty();

    @gamussa #PhillyETE
    

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69. Even empty Stream has state!
    @gamussa #PhillyETE
    

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70. Even empty Stream has state!
    @gamussa #PhillyETE
    

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71. Even empty Stream has state!
    @gamussa #PhillyETE
    

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72. View Slide

73. ”Identical”
    1. Has the same state
    2. Not related to “equals and hashcode” contract
    3. Not related to references to objects in
    memory
    @gamussa #PhillyETE
    

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74. List[] twins = new List[2];

    Arrays.setAll(twins, ArrayList::new);

    @gamussa #PhillyETE
    

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75. List[] twins = new List[2];

    Arrays.setAll(twins, ArrayList::new);

    A. Absolutely identical empty lists
    @gamussa #PhillyETE
    

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76. List[] twins = new List[2];

    Arrays.setAll(twins, ArrayList::new);

    A. Absolutely identical empty lists
    B. Absolutely identical non-empty lists
    @gamussa #PhillyETE
    

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77. 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
    @gamussa #PhillyETE
    

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78. 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
    @gamussa #PhillyETE
    

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79. 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
    @gamussa #PhillyETE
    

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80. @gamussa #PhillyETE
    

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81. @gamussa #PhillyETE
    

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82. 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
    @gamussa #PhillyETE
    

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83. @gamussa #PhillyETE
    

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84. @gamussa #PhillyETE
    

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85. @gamussa #PhillyETE
    

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86. How single is a Single Abstract Method
    Interface?
    @gamussa #PhillyETE
    

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87. public interface Single {

    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 { }
    @gamussa #PhillyETE
    

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88. A. WTF?! ’Single’ means one, not three!
    public interface Single {

    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 { }
    @gamussa #PhillyETE
    

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89. A. WTF?! ’Single’ means one, not three!
    B. Problem is with partyHard(T), remove it and it will work
    public interface Single {

    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 { }
    @gamussa #PhillyETE
    

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90. 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
    public interface Single {

    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 { }
    @gamussa #PhillyETE
    

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91. 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 {

    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 { }
    @gamussa #PhillyETE
    

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92. 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 {

    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 { }
    @gamussa #PhillyETE
    

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93. View Slide

94. View Slide

95. 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 {

    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 { }
    @gamussa #PhillyETE
    

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96. @gamussa #PhillyETE
    

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97. RUSSIAN HACKERS
    STILL TRYING TO HACK THE BANK
    

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98. Hacking the bank
    ☑Bank software written in Java
    ☑Hack into it
    ☑Analyze the accounts
    @gamussa #PhillyETE
    

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99. Given the code above, which statement is wrong:
    Set accounts= new HashSet<>(Arrays.asList("Gates", "Buffett", "Bezos", "Zuckerberg"));

    System.out.println(”accounts= " + accounts);

    @gamussa #PhillyETE
    

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100. Given the code above, which statement is wrong:
     A. The Set is ordered by hashcode
     Set accounts= new HashSet<>(Arrays.asList("Gates", "Buffett", "Bezos", "Zuckerberg"));

     System.out.println(”accounts= " + accounts);

     @gamussa #PhillyETE
     

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101. 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
     Set accounts= new HashSet<>(Arrays.asList("Gates", "Buffett", "Bezos", "Zuckerberg"));

     System.out.println(”accounts= " + accounts);

     @gamussa #PhillyETE
     

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102. 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
     Set accounts= new HashSet<>(Arrays.asList("Gates", "Buffett", "Bezos", "Zuckerberg"));

     System.out.println(”accounts= " + accounts);

     @gamussa #PhillyETE
     

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103. 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 accounts= new HashSet<>(Arrays.asList("Gates", "Buffett", "Bezos", "Zuckerberg"));

     System.out.println(”accounts= " + accounts);

     @gamussa #PhillyETE
     

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104. 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 accounts= new HashSet<>(Arrays.asList("Gates", "Buffett", "Bezos", "Zuckerberg"));

     System.out.println(”accounts= " + accounts);

     @gamussa #PhillyETE
     

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105. View Slide

106. View Slide

107. Given the code above, which statement is wrong:
     A. The Set is ordered
     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 accounts= new HashSet<>(Arrays.asList("Gates", "Buffett", "Bezos", "Zuckerberg"));

     System.out.println(”accounts= " + accounts);

     @gamussa #PhillyETE
     

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108. public boolean add(E e) {

     return map.put(e, PRESENT)==null;

     }
     @gamussa #PhillyETE
     

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109. Your turn, FBI
     @gamussa #PhillyETE
     

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110. View Slide

111. Given the code above, which statement is wrong:
     Set accounts = Set.of("Gates", "Buffett", "Bezos", "Zuckerberg");

     System.out.println(”accounts= " + accounts);
     @gamussa #PhillyETE
     

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112. Given the code above, which statement is wrong:
     A. The Set is ordered
     Set accounts = Set.of("Gates", "Buffett", "Bezos", "Zuckerberg");

     System.out.println(”accounts= " + accounts);
     @gamussa #PhillyETE
     

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113. Given the code above, which statement is wrong:
     A. The Set is ordered
     B. The order is predictable across multiple runs of the JVM on the same machine
     Set accounts = Set.of("Gates", "Buffett", "Bezos", "Zuckerberg");

     System.out.println(”accounts= " + accounts);
     @gamussa #PhillyETE
     

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114. Given the code above, which statement is wrong:
     A. The Set is ordered
     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
     Set accounts = Set.of("Gates", "Buffett", "Bezos", "Zuckerberg");

     System.out.println(”accounts= " + accounts);
     @gamussa #PhillyETE
     

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115. Given the code above, which statement is wrong:
     A. The Set is ordered
     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 accounts = Set.of("Gates", "Buffett", "Bezos", "Zuckerberg");

     System.out.println(”accounts= " + accounts);
     @gamussa #PhillyETE
     

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116. Given the code above, which statement is wrong:
     A. The Set is ordered
     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 accounts = Set.of("Gates", "Buffett", "Bezos", "Zuckerberg");

     System.out.println(”accounts= " + accounts);
     @gamussa #PhillyETE
     

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117. @gamussa #PhillyETE
     

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118. @gamussa #PhillyETE
     

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119. @gamussa #PhillyETE
     

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120. Given the code above, which statement is wrong:
     A. The Set is ordered
     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 accounts = Set.of("Gates", "Buffett", "Bezos", "Zuckerberg");

     System.out.println(”accounts= " + accounts);
     @gamussa #PhillyETE
     

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121. 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;

     }

     }

     }
     @gamussa #PhillyETE
     

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122. Juggling Accident
     

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123. Juggling Accident
     

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124. What’s correct?
     @gamussa #PhillyETE
     

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125. What’s correct?
     A. If you convert your application to module, classpath
     dependencies will still be resolved correctly
     @gamussa #PhillyETE
     

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126. 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
     @gamussa #PhillyETE
     

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127. 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
     @gamussa #PhillyETE
     

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128. 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
     @gamussa #PhillyETE
     

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129. 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
     @gamussa #PhillyETE
     

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130. View Slide

131. View Slide

132. 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
     @gamussa #PhillyETE
     

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133. @gamussa #PhillyETE
     

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134. @gamussa #PhillyETE
     

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135. View Slide

136. static void killThemAll(Collection expendables) {

     Iterator heroes = expendables.iterator();

     heroes.forEachRemaining(e -> {

     if (heroes.hasNext()) {

     heroes.next();

     heroes.remove();

     }

     });

     System.out.println(expendables);

     }

     

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137. A. You killed them all
     static void killThemAll(Collection expendables) {

     Iterator heroes = expendables.iterator();

     heroes.forEachRemaining(e -> {

     if (heroes.hasNext()) {

     heroes.next();

     heroes.remove();

     }

     });

     System.out.println(expendables);

     }

     

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138. A. You killed them all
     B. You killed only even ones
     static void killThemAll(Collection expendables) {

     Iterator heroes = expendables.iterator();

     heroes.forEachRemaining(e -> {

     if (heroes.hasNext()) {

     heroes.next();

     heroes.remove();

     }

     });

     System.out.println(expendables);

     }

     

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139. A. You killed them all
     B. You killed only even ones
     C. They all survived
     static void killThemAll(Collection expendables) {

     Iterator heroes = expendables.iterator();

     heroes.forEachRemaining(e -> {

     if (heroes.hasNext()) {

     heroes.next();

     heroes.remove();

     }

     });

     System.out.println(expendables);

     }

     

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140. A. You killed them all
     B. You killed only even ones
     C. They all survived
     D. You killed only odd ones
     static void killThemAll(Collection expendables) {

     Iterator heroes = expendables.iterator();

     heroes.forEachRemaining(e -> {

     if (heroes.hasNext()) {

     heroes.next();

     heroes.remove();

     }

     });

     System.out.println(expendables);

     }

     

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141. 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 expendables) {

     Iterator heroes = expendables.iterator();

     heroes.forEachRemaining(e -> {

     if (heroes.hasNext()) {

     heroes.next();

     heroes.remove();

     }

     });

     System.out.println(expendables);

     }

     

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142. 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 expendables) {

     Iterator heroes = expendables.iterator();

     heroes.forEachRemaining(e -> {

     if (heroes.hasNext()) {

     heroes.next();

     heroes.remove();

     }

     });

     System.out.println(expendables);

     }

     

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143. @gamussa #PhillyETE
     

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144. @gamussa #PhillyETE
     

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145. 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 expendables) {

     Iterator heroes = expendables.iterator();

     heroes.forEachRemaining(e -> {

     if (heroes.hasNext()) {

     heroes.next();

     heroes.remove();

     }

     });

     System.out.println(expendables);

     }

     

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146. Don’t do that. Really, don’t.
     @gamussa #PhillyETE
     

     View Slide

147. Don’t do that. Really, don’t.
     killThemAll(new ArrayList(Arrays.asList("N","S","W","S","L","S","L","V")));

     []
     @gamussa #PhillyETE
     

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148. Don’t do that. Really, don’t.
     killThemAll(new ArrayList(Arrays.asList("N","S","W","S","L","S","L","V")));

     []
     killThemAll(new LinkedList(Arrays.asList("N","S","W","S","L","S","L","V")));
     @gamussa #PhillyETE
     

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149. Don’t do that. Really, don’t.
     killThemAll(new ArrayList(Arrays.asList("N","S","W","S","L","S","L","V")));

     []
     killThemAll(new LinkedList(Arrays.asList("N","S","W","S","L","S","L","V")));
     [S,S,S,V]
     @gamussa #PhillyETE
     

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150. Don’t do that. Really, don’t.
     killThemAll(new ArrayList(Arrays.asList("N","S","W","S","L","S","L","V")));

     []
     killThemAll(new LinkedList(Arrays.asList("N","S","W","S","L","S","L","V")));
     [S,S,S,V]
     killThemAll(new ArrayDeque(Arrays.asList("N","S","W","S","L","S","L","V")));

     [N,S,W,S,L,S,L,V]
     @gamussa #PhillyETE
     

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151. Don’t do that. Really, don’t.
     killThemAll(new ArrayList(Arrays.asList("N","S","W","S","L","S","L","V")));

     []
     killThemAll(new LinkedList(Arrays.asList("N","S","W","S","L","S","L","V")));
     [S,S,S,V]
     killThemAll(new ArrayDeque(Arrays.asList("N","S","W","S","L","S","L","V")));

     [N,S,W,S,L,S,L,V]
     @gamussa #PhillyETE
     

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152. Don’t do that. Really, don’t.
     killThemAll(new ArrayList(Arrays.asList("N","S","W","S","L","S","L","V")));

     []
     killThemAll(new LinkedList(Arrays.asList("N","S","W","S","L","S","L","V")));
     [S,S,S,V]
     killThemAll(new ArrayDeque(Arrays.asList("N","S","W","S","L","S","L","V")));

     [N,S,W,S,L,S,L,V]
     killThemAll(new TreeSet(Arrays.asList("N","S","W","S","L","S","L","V")));
     @gamussa #PhillyETE
     

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153. Don’t do that. Really, don’t.
     killThemAll(new ArrayList(Arrays.asList("N","S","W","S","L","S","L","V")));

     []
     killThemAll(new LinkedList(Arrays.asList("N","S","W","S","L","S","L","V")));
     [S,S,S,V]
     killThemAll(new ArrayDeque(Arrays.asList("N","S","W","S","L","S","L","V")));

     [N,S,W,S,L,S,L,V]
     killThemAll(new TreeSet(Arrays.asList("N","S","W","S","L","S","L","V")));
     [N,W,L,L]
     @gamussa #PhillyETE
     

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154. Subtle Difference
     @gamussa #PhillyETE
     

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155. @FunctionalInterface

     public interface OriginalPredicate {

     boolean test(T t);

     }
     OriginalPredicate lambda = (Object obj) -> "adidas".equals(obj);

     OriginalPredicate methodRef = "adidas"::equals;
     

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156. A. Both work just fine
     @FunctionalInterface

     public interface OriginalPredicate {

     boolean test(T t);

     }
     OriginalPredicate lambda = (Object obj) -> "adidas".equals(obj);

     OriginalPredicate methodRef = "adidas"::equals;
     

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157. A. Both work just fine
     B. Lambda works, method ref fails
     @FunctionalInterface

     public interface OriginalPredicate {

     boolean test(T t);

     }
     OriginalPredicate lambda = (Object obj) -> "adidas".equals(obj);

     OriginalPredicate methodRef = "adidas"::equals;
     

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158. A. Both work just fine
     B. Lambda works, method ref fails
     C. Method ref works, lambda fails
     @FunctionalInterface

     public interface OriginalPredicate {

     boolean test(T t);

     }
     OriginalPredicate lambda = (Object obj) -> "adidas".equals(obj);

     OriginalPredicate methodRef = "adidas"::equals;
     

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159. 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 {

     boolean test(T t);

     }
     OriginalPredicate lambda = (Object obj) -> "adidas".equals(obj);

     OriginalPredicate methodRef = "adidas"::equals;
     

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160. 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 {

     boolean test(T t);

     }
     OriginalPredicate lambda = (Object obj) -> "adidas".equals(obj);

     OriginalPredicate methodRef = "adidas"::equals;
     

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161. View Slide

162. View Slide

163. 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 {

     boolean test(T t);

     }
     CopyCatPredicate lambda = (Object obj) -> "adadas".equals(obj);

     CopyCatPredicate methodRef = "adadas"::equals;

     

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164. 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 {

     boolean test(T t);

     }
     CopyCatPredicate lambda = (Object obj) -> "adadas".equals(obj);

     CopyCatPredicate methodRef = "adadas"::equals;

     

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165. View Slide

166. View Slide

167. View Slide

168. 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 {

     boolean test(T t);

     }
     CopyCatPredicate lambda = (Object obj) -> "adadas".equals(obj);

     CopyCatPredicate methodRef = "adadas"::equals;

     

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169. @gamussa #PhillyETE
     

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170. 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.
     “
     @gamussa #PhillyETE
     

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171. View Slide

172. List list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk",
     "Spock").sequential()

     .filter(new TreeSet<>()::add).collect(Collectors.toList());

     System.out.println(list);

     @gamussa #PhillyETE
     

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173. A.[Data, Kirk, Spock]
     List list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk",
     "Spock").sequential()

     .filter(new TreeSet<>()::add).collect(Collectors.toList());

     System.out.println(list);

     @gamussa #PhillyETE
     

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174. A.[Data, Kirk, Spock]
     B.[Spock, Kirk, Data, Data, Kirk, Spock]
     List list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk",
     "Spock").sequential()

     .filter(new TreeSet<>()::add).collect(Collectors.toList());

     System.out.println(list);

     @gamussa #PhillyETE
     

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175. A.[Data, Kirk, Spock]
     B.[Spock, Kirk, Data, Data, Kirk, Spock]
     C.[Spock, Kirk, Data]
     List list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk",
     "Spock").sequential()

     .filter(new TreeSet<>()::add).collect(Collectors.toList());

     System.out.println(list);

     @gamussa #PhillyETE
     

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176. A.[Data, Kirk, Spock]
     B.[Spock, Kirk, Data, Data, Kirk, Spock]
     C.[Spock, Kirk, Data]
     D.[Data, Data, Kirk, Kirk, Spock, Spock]
     List list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk",
     "Spock").sequential()

     .filter(new TreeSet<>()::add).collect(Collectors.toList());

     System.out.println(list);

     @gamussa #PhillyETE
     

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177. 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 list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk",
     "Spock").sequential()

     .filter(new TreeSet<>()::add).collect(Collectors.toList());

     System.out.println(list);

     @gamussa #PhillyETE
     

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178. View Slide

179. View Slide

180. 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 list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk",
     "Spock").sequential()

     .filter(new TreeSet<>()::add).collect(Collectors.toList());

     System.out.println(list);

     @gamussa #PhillyETE
     

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181. 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 list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk",
     "Spock").sequential()

     .filter(new TreeSet<>()::add).collect(Collectors.toList());

     System.out.println(list);

     @gamussa #PhillyETE
     

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182. View Slide

183. View Slide

184. 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 list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk",
     "Spock").sequential()

     .filter(new TreeSet<>()::add).collect(Collectors.toList());

     System.out.println(list);

     @gamussa #PhillyETE
     

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185. @gamussa #PhillyETE
     

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186. filter(new TreeSet<>()::add)
     @gamussa #PhillyETE
     

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187. filter(new TreeSet<>()::add) filter(i -> new TreeSet<>().add(i))
     @gamussa #PhillyETE
     

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188. filter(new TreeSet<>()::add) filter(i -> new TreeSet<>().add(i))
     !=
     @gamussa #PhillyETE
     

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189. filter(new TreeSet<>()::add) filter(i -> new TreeSet<>().add(i))
     !=
     New instance is
     created every time!
     @gamussa #PhillyETE
     

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190. filter(new TreeSet<>()::add) filter(i -> new TreeSet<>().add(i))
     !=
     New instance is
     created every time!
     Instance method is
     created once!
     @gamussa #PhillyETE
     

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191. @gamussa #PhillyETE
     

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192. Optional.of("obvious").orElseGet(null);

     Optional.empty().map(null).orElse("obvious");
     @gamussa #PhillyETE
     

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193. A. obvious / obvious
     Optional.of("obvious").orElseGet(null);

     Optional.empty().map(null).orElse("obvious");
     @gamussa #PhillyETE
     

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194. A. obvious / obvious
     B. obvious / NullPointerException
     Optional.of("obvious").orElseGet(null);

     Optional.empty().map(null).orElse("obvious");
     @gamussa #PhillyETE
     

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195. A. obvious / obvious
     B. obvious / NullPointerException
     C. NullPointerException / obvious
     Optional.of("obvious").orElseGet(null);

     Optional.empty().map(null).orElse("obvious");
     @gamussa #PhillyETE
     

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196. A. obvious / obvious
     B. obvious / NullPointerException
     C. NullPointerException / obvious
     D. NullPointerException / NullPointerException
     Optional.of("obvious").orElseGet(null);

     Optional.empty().map(null).orElse("obvious");
     @gamussa #PhillyETE
     

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197. @gamussa #PhillyETE
     

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198. @gamussa #PhillyETE
     

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199. A. obvious / obvious
     B. obvious / NullPointerException
     C. NullPointerException / obvious
     D. NullPointerException / NullPointerException
     Optional.of("obvious").orElseGet(null);

     Optional.empty().map(null).orElse("obvious");
     @gamussa #PhillyETE
     

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200. 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
     @gamussa #PhillyETE
     

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201. 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
     @gamussa #PhillyETE
     

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202. 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
     @gamussa #PhillyETE
     

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203. @gamussa #PhillyETE
     

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204. @gamussa #PhillyETE
     

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205. A. obvious / obvious
     B. obvious / NullPointerException
     C. NullPointerException / obvious
     D. NullPointerException / NullPointerException
     Optional.of("obvious").orElseGet(null);

     Optional.empty().map(null).orElse("obvious");
     @gamussa #PhillyETE
     

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206. @gamussa #PhillyETE
     

     View Slide

207. @gamussa #PhillyETE
     

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208. @gamussa #PhillyETE
     

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209. @gamussa #PhillyETE
     

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210. Identical?
     @gamussa #PhillyETE
     

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211. When agentA == agentB?
     

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212. 1. Consumer agentA = s -> System.out.println(s);

     Consumer agentB = s -> System.out.println(s);
     When agentA == agentB?
     

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213. 1. Consumer agentA = s -> System.out.println(s);

     Consumer agentB = s -> System.out.println(s);
     2. Consumer agentA = System.out::println;

     Consumer agentB = System.out::println;
     When agentA == agentB?
     

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214. 1. Consumer agentA = s -> System.out.println(s);

     Consumer agentB = s -> System.out.println(s);
     2. Consumer agentA = System.out::println;

     Consumer agentB = System.out::println;
     3. Supplier> supplier = () -> s -> System.out.println(s);

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     When agentA == agentB?
     

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215. 1. Consumer agentA = s -> System.out.println(s);

     Consumer agentB = s -> System.out.println(s);
     2. Consumer agentA = System.out::println;

     Consumer agentB = System.out::println;
     3. Supplier> supplier = () -> s -> System.out.println(s);

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     4. Supplier> supplier = () -> System.out::println;

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     When agentA == agentB?
     

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216. A.All
     1. Consumer agentA = s -> System.out.println(s);

     Consumer agentB = s -> System.out.println(s);
     2. Consumer agentA = System.out::println;

     Consumer agentB = System.out::println;
     3. Supplier> supplier = () -> s -> System.out.println(s);

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     4. Supplier> supplier = () -> System.out::println;

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     When agentA == agentB?
     

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217. A.All
     B.3 and 4
     1. Consumer agentA = s -> System.out.println(s);

     Consumer agentB = s -> System.out.println(s);
     2. Consumer agentA = System.out::println;

     Consumer agentB = System.out::println;
     3. Supplier> supplier = () -> s -> System.out.println(s);

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     4. Supplier> supplier = () -> System.out::println;

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     When agentA == agentB?
     

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218. A.All
     B.3 and 4
     C.Only 3
     1. Consumer agentA = s -> System.out.println(s);

     Consumer agentB = s -> System.out.println(s);
     2. Consumer agentA = System.out::println;

     Consumer agentB = System.out::println;
     3. Supplier> supplier = () -> s -> System.out.println(s);

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     4. Supplier> supplier = () -> System.out::println;

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     When agentA == agentB?
     

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219. A.All
     B.3 and 4
     C.Only 3
     D.Other
     1. Consumer agentA = s -> System.out.println(s);

     Consumer agentB = s -> System.out.println(s);
     2. Consumer agentA = System.out::println;

     Consumer agentB = System.out::println;
     3. Supplier> supplier = () -> s -> System.out.println(s);

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     4. Supplier> supplier = () -> System.out::println;

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     When agentA == agentB?
     

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220. A.All
     B.3 and 4
     C.Only 3
     D.Other
     1. Consumer agentA = s -> System.out.println(s);

     Consumer agentB = s -> System.out.println(s);
     2. Consumer agentA = System.out::println;

     Consumer agentB = System.out::println;
     3. Supplier> supplier = () -> s -> System.out.println(s);

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     4. Supplier> supplier = () -> System.out::println;

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     When agentA == agentB?
     

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221. @gamussa #PhillyETE
     

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222. @gamussa #PhillyETE
     

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223. A.All
     B.3 and 4
     C.Only 3
     D.Other
     1. Consumer agentA = s -> System.out.println(s);

     Consumer agentB = s -> System.out.println(s);
     2. Consumer agentA = System.out::println;

     Consumer agentB = System.out::println;
     3. Supplier> supplier = () -> s -> System.out.println(s);

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     4. Supplier> supplier = () -> System.out::println;

     Consumer agentA = supplier.get();

     Consumer agentB = supplier.get();
     When agentA == agentB?
     

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224. Reuse is only possible for pure functions
     @gamussa #PhillyETE
     

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225. Reuse is only possible for pure functions
     Consumers accept parameters ==
     have state
     @gamussa #PhillyETE
     

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226. Reuse is only possible for pure functions
     Consumers accept parameters ==
     have state
     Supplier in 4 has state – the
     resolved method reference
     @gamussa #PhillyETE
     

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227. Conclusions
     @gamussa #PhillyETE
     

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228. View Slide

229. -Write readable code!
     

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230. -Write readable code!
     -Comment all the tricks
     

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231. -Write readable code!
     -Comment all the tricks
     -Sometimes it’s just a bug
     

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232. -Write readable code!
     -Comment all the tricks
     -Sometimes it’s just a bug
     -Static code analysis FTW -
     IntelliJ IDEA!
     

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233. -Write readable code!
     -Comment all the tricks
     -Sometimes it’s just a bug
     -Static code analysis FTW -
     IntelliJ IDEA!
     -RTFM!
     

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234. -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!
     

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235. View Slide

236. -Trust us, we have much
     more where those came
     from.
     

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237. -Trust us, we have much
     more where those came
     from.
     -Puzzlers? Gotchas? Fetal
     position inducing behavior?
     

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

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239. View Slide

240. -Did you like it?
     

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241. -Did you like it?
     -Praise me on twitter!
     

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242. -Did you like it?
     -Praise me on twitter!
     -#javapuzzlersng #PhillyETE
     

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243. -Did you like it?
     -Praise me on twitter!
     -#javapuzzlersng #PhillyETE
     -@gamussa
     

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244. -Did you like it?
     -Praise me on twitter!
     -#javapuzzlersng #PhillyETE
     -@gamussa
     -Didn’t like it?
     

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245. -Did you like it?
     -Praise me on twitter!
     -#javapuzzlersng #PhillyETE
     -@gamussa
     -Didn’t like it?
     -/dev/null
     

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