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

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@gAmUssA @hazelcast #jbreak #hazelcastjet > whoami

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

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

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

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

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

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CLICK AND HACK THE TYPE-IT BROTHERS

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@tagir_valeev @gamussa #PhillyETE

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@gamussa #PhillyETE

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@gamussa #PhillyETE

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@gamussa #PhillyETE

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1. Two One entertaining guy on the stage

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

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

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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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FIRST RULE OF THE PUZZLERS: NO CHEATING!

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Which Java version are you on? @gamussa #PhillyETE

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

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

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

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

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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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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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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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Watching the puzzlers like… #dafaq @gamussa #PhillyETE

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Watching the puzzlers like… #dafaq @gamussa #PhillyETE

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Everything works (or doesn't) 
 in the latest Java 8 and/or 9 update @gamussa #PhillyETE

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RUSSIAN HACKERS HACKING THE BANK

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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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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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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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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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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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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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@gamussa #PhillyETE

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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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@gamussa #PhillyETE

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Available -42?! @gamussa #PhillyETE

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Available -42?! @gamussa #PhillyETE

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@gamussa #PhillyETE

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Collections.emptyList() == Collections.emptyList();
 Collections.emptyIterator() == Collections.emptyIterator();
 @gamussa #PhillyETE

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A. true/true Collections.emptyList() == Collections.emptyList();
 Collections.emptyIterator() == Collections.emptyIterator();
 @gamussa #PhillyETE

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A. true/true B. true/false Collections.emptyList() == Collections.emptyList();
 Collections.emptyIterator() == Collections.emptyIterator();
 @gamussa #PhillyETE

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A. true/true B. true/false C. false/true Collections.emptyList() == Collections.emptyList();
 Collections.emptyIterator() == Collections.emptyIterator();
 @gamussa #PhillyETE

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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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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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Spliterators.emptySpliterator() == Spliterators.emptySpliterator();
 Stream.empty() == Stream.empty();


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A. true/true Spliterators.emptySpliterator() == Spliterators.emptySpliterator();
 Stream.empty() == Stream.empty();


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


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


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


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


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Singleton Strikes Back! @gamussa #PhillyETE

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Singleton Strikes Back! @gamussa #PhillyETE

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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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Even empty Stream has state! @gamussa #PhillyETE

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Even empty Stream has state! @gamussa #PhillyETE

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Even empty Stream has state! @gamussa #PhillyETE

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”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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List[] twins = new List[2];
 Arrays.setAll(twins, ArrayList::new);
 @gamussa #PhillyETE

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List[] twins = new List[2];
 Arrays.setAll(twins, ArrayList::new);
 A. Absolutely identical empty lists @gamussa #PhillyETE

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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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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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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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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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@gamussa #PhillyETE

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

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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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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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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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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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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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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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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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@gamussa #PhillyETE

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RUSSIAN HACKERS STILL TRYING TO HACK THE BANK

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

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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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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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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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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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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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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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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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public boolean add(E e) {
 return map.put(e, PRESENT)==null;
 } @gamussa #PhillyETE

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Your turn, FBI @gamussa #PhillyETE

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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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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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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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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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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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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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@gamussa #PhillyETE

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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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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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Juggling Accident

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Juggling Accident

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What’s correct? @gamussa #PhillyETE

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

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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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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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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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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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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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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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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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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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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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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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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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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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

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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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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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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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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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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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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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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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Subtle Difference @gamussa #PhillyETE

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@FunctionalInterface
 public interface OriginalPredicate {
 boolean test(T t);
 } OriginalPredicate lambda = (Object obj) -> "adidas".equals(obj);
 OriginalPredicate methodRef = "adidas"::equals;

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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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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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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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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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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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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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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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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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@gamussa #PhillyETE

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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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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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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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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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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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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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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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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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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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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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@gamussa #PhillyETE

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

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

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

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

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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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@gamussa #PhillyETE

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

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A. obvious / obvious Optional.of("obvious").orElseGet(null);
 Optional.empty().map(null).orElse("obvious"); @gamussa #PhillyETE

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A. obvious / obvious B. obvious / NullPointerException Optional.of("obvious").orElseGet(null);
 Optional.empty().map(null).orElse("obvious"); @gamussa #PhillyETE

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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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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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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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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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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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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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@gamussa #PhillyETE

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@gamussa #PhillyETE

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Identical? @gamussa #PhillyETE

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When agentA == agentB?

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

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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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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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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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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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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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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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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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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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@gamussa #PhillyETE

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@gamussa #PhillyETE

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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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Reuse is only possible for pure functions @gamussa #PhillyETE

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

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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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Conclusions @gamussa #PhillyETE

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-Write readable code!

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-Write readable code! -Comment all the tricks

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

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

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

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

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

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

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-Did you like it?

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

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

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

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

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