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

Developer Advocate @JFrog@jbaruch on the internetz

Developer Advocate @Hazelcast@gAmUssA on the internetz

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

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

1. Two entertaining guys on the stage

2. Funny puzzling questions3. You think and vote4.T-shirts are airborne5. Official twitter hashtags:

#javapuzzlersng#DevoxxUS

Which Java version are you on?

A. Java 7B. Java 8C. Java 9D. Java 6E. Java 5F. Java 2


Watching the puzzlers like… #dafaq


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


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 negativeB. UnsupportedOperationException – can’t drain when negativeC. 0D. -42

A. IllegalArgumentException – can’t create semaphore with negativeB. UnsupportedOperationException – can’t drain when negativeC. 0D. -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());}

}

Available -42?!


A. true/trueB. true/falseC. false/trueD. false/false

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



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

Singleton Strikes Back!



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


Even empty Stream has state!


”Identical”

1. Has the same state

2. Not related to “equals and hashcode” contract

3. Not related to references to objects in memory


List[] twins = new List[2];Arrays.setAll(twins, ArrayList::new);

A. Absolutely identical empty listsB. Absolutely identical non-empty listsC. Non-identical empty listsD. Non-identical non-empty lists


How single is a Single Abstract Method Interface?


A. WTF?! ’Single’ means one, not three!B. Problem is with partyHard(T), remove it and it will workC. Problem is the drinkIn methods, removing one of them and it will

workD. It will work fine! Both partyHard() and drinkIn() are merged in

SingleAndHappy, leaving one abstract method

public interface Single<T> {default void partyHard(String songName) { System.out.println(songName); }void partyHard(T songName);void drinkIn(T drinkName);void drinkIn(String dringName);

}@FunctionalInterfacepublic interface SingleAndHappy extends Single<String> { }


A. WTF?! ’Single’ means one, not three!B. Problem is with partyHard(T), remove it and it will workC. Problem are the drinkIn methods, removing it will leave one abstract

methodD. Yes! Both partyHard() and drinkIn() are merged in SingleAndHappy,

leaving one abstract method

public interface Single<T> {default void partyHard(String songName) { System.out.println(songName); }void partyHard(T songName);void drinkIn(T drinkName);void drinkIn(String dringName);

}@FunctionalInterfacepublic interface SingleAndHappy extends Single<String> { }


Hacking the bank

☑Bank software written in Java☑Hack into it☑Analyze the accounts


Given the code above, which statement is wrong:A. The Set is ordered by hashcodeB. The order is predictable across multiple runs of the JVM on the same machineC. The order of elements in Set is not predictableD. Statements A & B are correct

Set<String>accounts=newHashSet<>(Arrays.asList("Gates","Buffett","Bezos","Zuckerberg"));System.out.println(”accounts="+accounts);


public boolean add(E e) {return map.put(e, PRESENT)==null;

}


Your turn, FBI



Set<String>accounts=Set.of("Gates","Buffett","Bezos","Zuckerberg");System.out.println(”accounts="+accounts);


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;}

}}


Juggling Accident

What’s correct?A. If you convert your application to module, classpath

dependencies will still be resolved correctlyB. If one of the dependencies was converted to a module, you

have to declare it in module-info in order to useC. Once you added the module-info to your project you have to

declare the dependencies twice, in classpath and in module-info

D. None of the above@jbaruch@gamussa #DevoxxUS http://jfrog.com/shownotes

A. You killed them allB. You killed only even onesC. They all survivedD. You killed only odd onesE. All answers are correct

static void killThemAll(Collection<Hero> expendables) {Iterator<Hero> heroes = expendables.iterator();heroes.forEachRemaining(e -> {

if (heroes.hasNext()) {heroes.next();heroes.remove();

}});System.out.println(expendables);

}

Don’t do that. Really, don’t.

killThemAll(new ArrayList<String>(Arrays.asList("N","S","W","S","L","S","L","V")));[]

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

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

killThemAll(new TreeSet<String>(Arrays.asList("N","S","W","S","L","S","L","V")));[N,W,L,L]


Subtle Difference


A. Both work just fineB. Lambda works, method ref failsC. Method ref works, lambda failsD. Won’t compile

@FunctionalInterfacepublic interface OriginalPredicate<T> {

boolean test(T t);}

OriginalPredicate<Object> lambda = (Object obj) -> ”adidas".equals(obj);OriginalPredicate<Object> methodRef = ”adidas"::equals;

A. Both work just fineB. Lambda works, method ref failsC. Method ref works, lambda failsD. Not a functional interface, will fail on annotation processing

@FunctionalInterfacePublic interface CopyCatPredicate {

<T> boolean test(T t);}

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

Agenericfunctiontypeforafunctionalinterfacemaybeimplementedbyamethodreferenceexpression(§15.13),butnotbyalambdaexpression(§15.27)asthereisnosyntaxforgenericlambdaexpressions.“


A.[Data, Kirk, Spock]B.[Spock, Kirk, Data, Data, Kirk, Spock]C.[Spock, Kirk, Data]D.[Data, Data, Kirk, Kirk, Spock, Spock]E.Are you nuts? Won’t compile! Data with

Kirk?!

List<String> list = Stream.of("Spock", "Kirk", "Data", "Data", "Kirk", "Spock").sequential()

.filter(new TreeSet<>()::add).collect(Collectors.toList());System.out.println(list);


filter(new TreeSet<>()::add) filter(i -> new TreeSet<>().add(i))!=

Newinstanceiscreatedeverytime!

Instancemethodiscreatedonce!


A. obvious / obviousB. obvious / NullPointerExceptionC. NullPointerException / obviousD. NullPointerException / NullPointerException

Optional.of("obvious").orElseGet(null);Optional.empty().map(null).orElse("obvious");




WillneverhappenWillneverhappen


Identical?


A. AllB. 3 and 4C. Only 3D. Other

1. Consumer<String> agentA = s -> System.out.println(s);Consumer<String> agentB = s -> System.out.println(s);

2. Consumer<String> agentA = System.out::println;Consumer<String> agentB = System.out::println;

3. Supplier<Consumer<String>> supplier = () -> s -> System.out.println(s);Consumer<String> agentA = supplier.get();Consumer<String> agentB = supplier.get();

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

When agentA == agentB?

Reuse is only possible for pure functions

Consumers accept parameters == have state

Supplier in 4 has state – the resolved method reference


Conclusions


-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!

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

-Puzzlers? Gotchas? Fetal position inducing behavior?

[email protected]

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

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

-#javapuzzlersng #DevoxxUS-@gamussa-@jbaruch

-Didn’t like it?-/dev/null

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

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