Java 8 Puzzlers as it was presented at Codemash 2017

whoiam

Developer Advocate @JFrog@jbaruch on the internetz

whoiam

Solutions Architect @Hazelcast@gAmUssA on the internetz

@tagir_valeev

1. Two entertaining guys on stage

2. Funny Puzzling questions3. You think and vote4. Official twitter handles:

JAVApuzzlersngcodemash

5. Jfrog.com/shownotes

Watching the puzzlers like… #dafaq

@jbaruch@gamussa #javapuzzlersng #codemash http://jfrog.com/shownotes

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


Broken Eggs Tale


What will be the output?

A. milk/bread/sausageB. milk/bread/sausage/eggs, don’t forget eggs!C. milk/bread/sausage/ConcurrentModificationExceptionD. ConcurrentModificationException

List<String> list = new ArrayList<>();list.add("milk");list.add("bread");list.add("sausage");Stream<String> stream = list.stream();list.add("eggs, don’t forget eggs!");stream.forEach(System.out::println);

Late binding, duh…

List<String> list = new ArrayList<>();list.add("milk");list.add("bread");list.add("sausage");Stream<String> stream = list.stream();list.add("eggs, don’t forget eggs!");stream.forEach(System.out::println);


Going Vegan



A. milk/bread/sausageB. milk/bread/eggs, don’t forget eggs!C. milk/bread/ConcurrentModificationExceptionD. ConcurrentModificationException

List<String> list = new ArrayList<>();list.add("milk");list.add("bread");list.add("sausage");list = list.subList(0, 2); //No sausage, please!Stream<String> stream = list.stream();list.add("eggs, don’t forget eggs!");stream.forEach(System.out::println);

Sometimes it’s just a bug…


Execute ’em all

What’s the difference between 1 and 2?

A. 1 compiles, 2 does notB. 2 compiles, 1 does notC. Same same, both work fineD. Same same, both won’t compile

public void killAll(){ExecutorService ex = Executors.newSingleThreadExecutor();List<String> sentence = Arrays.asList("Punish");ex.submit(() -> Files.write(Paths.get("Sentence.txt"), sentence) ); // 1ex.submit(() -> { Files.write(Paths.get("Sentence.txt"), sentence); }); // 2

}

Semicolonsaretheevil!


What’s the difference between 1 and 2?

A. 1 compiles, 2 does notB. 2 compiles, 1 does notC. Same same, both work fineD. Same same, both won’t compile


}


}

@FunctionalInterfacepublic interface Runnable {

public abstract void run();}

@FunctionalInterfacepublic interface Callable<V> {

V call() throws Exception;}


Mad Max


How that will work?

A. Compilation errorB. Runtime ExceptionC. 3D. Something else

System.out.println(Stream.of(-3, -2, -1, 0, 1, 2, 3).max(Math::max).get()

);


How about now?

A. −3B. −1C. 0D. Something else


);


• Math.max(−3, −2) = −2 < 0 à −3 < −2, selecting−2• Math.max(−2, −1) = −1 < 0 à −2 < −1, selecting−1• Math.max(−1, 0) = 0 à −1 == 0, keeping−1• Math.max(−1, 1) = 1 > 0 à −1 > 1, keeping−1• Math.max(−1, 2) = 2 > 0 à −1 > 2, keeping−1• Math.max(−1, 3) = 3 > 0 à −1 > 3, keeping−1

Stream.of(-3, -2, -1, 0, 1, 2, 3).max(Math::max).get()


Let’s upgrade the stack!


What will happen?

A. Maps will switchB. Both will become oldSchoolC. Both will become hipsterD. Really?! That won’t even compile!

Map<String, String> oldSchool = initOldSchoolStack();// oldSchool = {buildTool=maven, lang=java, db=db2}

Map<String, String> proper = initHipsterStack();// proper = {buildTool=npm, lang=javascript, db=elastic}

oldSchool.replaceAll(proper::put);

void replaceAll(BiFunction<? super K, ? super V, ? extends V> function)

V put(K key, V value);

Map interface

oldSchool.replaceAll(proper::put);


void replaceAll(BiFunction<? super K, ? super V, ? extends V> function)

V put(K key, V value);

Map interface

final BiFunction<String, String, String> function = (key, value) -> proper.put(key, value);

for (Map.Entry<String, String> entry : oldSchool.entrySet())entry.setValue(function.apply(entry.getKey(), entry.getValue()));


How many lines will be the same?

List<String> kitties = Arrays.asList("Soft", "Warm", "Purr");Comparator<String> kittiesComparator= Comparator.nullsLast(Comparator.naturalOrder());

System.out.println(Collections.max(kitties, kittiesComparator));System.out.println(kitties.stream().collect(Collectors.maxBy(kittiesComparator)).get());System.out.println(kitties.stream().max(kittiesComparator).get());

A. All lines the sameB. Two lines the sameC. All differentD. Four different


How about now?

List<String> kitties = Arrays.asList("Soft", null, "Purr");Comparator<String> kittiesComparator= Comparator.nullsLast(Comparator.naturalOrder());




How about now?






System.out.println(Collections.max(kitties, kittiesComparator));



System.out.println(kitties.stream().collect(Collectors.maxBy(kittiesComparator)).get());



System.out.println(kitties.stream().max(kittiesComparator).get());


nullCaught: java.lang.NoSuchElementExceptionCaught: java.lang.NullPointerException

Consistency, yeah.


Mutants


How to cast to a type without declaring it?

interface Cat{ default void meow() {System.out.println(”meow ");}}interface Dog{ default void bark() {System.out.println(”woof ");}}

public static void main(String[] args) {class Dogcatimplements Dog, Cat{}test(new Dogcat());

}

static void test(Object obj) {def x = (?)obj;x.meow ();x.bark ();

}



static void test(Object obj) {// A. Will that work?

Dog& Catx = (Dog& Cat) obj;x.meow ();x.bark ();}

static void test(Object obj) {// B. Will that work?((Consumer<? extends Dog& Cat>)(x -> {x.meow ();x.bark ();

})).accept((Dog& Cat)obj); }

static void test(Object obj) {// C. Will that work?Optional.of((Dog& Cat) obj)

.ifPresent(x -> {x.meow ();x.bark ();

});}

// D. You’re two sick bastards.

interface Cat{ default void meow() {System.out.println(”meow");}}interface Dog{ default void bark() {System.out.println(”woof");}}

public static void main(String[] args) {class Dogcat implements Dog, Cat{}test(new Dogcat());

}



static void test(Object obj) {// A. Will that work?

Dog & Cat x = (Dog & Cat) obj;x.meow();x.bark();}

static void test(Object obj) {// B. Will that work?((Consumer<? extends Dog & Cat>)(x -> {x.meow();x.bark();

})).accept((Dog & Cat)obj); }

static void test(Object obj) {// C. Will that work?Optional.of((Dog & Cat) obj)

.ifPresent(x -> {x.meow();x.bark();

});}

// D. You’re two sick bastards.

interface Cat{ default void meow() {System.out.println(”meow");}}interface Dog{ default void bark() {System.out.println(”woof");}}

public static void main(String[] args) {static class Dogcat implements Dog, Cat{}test(new Dogcat());

}


Bill Gates explains how that works


static void test(Object obj) {// C. Will that work?Optional.of((Dog & Cat) obj)

.ifPresent(x -> {x.meow();x.bark();

});} @jbaruch@gamussa #javapuzzlersng #codemash http://jfrog.com/shownotes

Viktor Gamov and Baruch Sadogursky call customer service:



1. HOTEL ECHO LIMA LIMA OSCAR/ HOTEL ECHO LIMA LIMA OSCAR2. HELLO / HOTEL ECHO LIMA LIMA OSCAR3. HOTEL ECHO LIMA LIMA OSCAR/ HELLO4. HELLO/HELLO

public class Test {String str;

void run() {str = "hello ";Supplier<String> s1 = str::toUpperCase;Supplier<String> s2 = () -> str.toUpperCase();str = "Hotel Echo Lima Lima Oscar ";System.out.println(s1.get());System.out.println(s2.get());

}}


1. HOTEL ECHO LIMA LIMA OSCAR/ HOTEL ECHO LIMA LIMA OSCAR 2. HELLO / HOTEL ECHO LIMA LIMA OSCAR3. HOTEL ECHO LIMA LIMA OSCAR/ HELLO4. HELLO/HELLO

public class Test {String str;

void run() {str = ”hello";Supplier<String> s1 = str::toUpperCase;Supplier<String> s2 = () -> str.toUpperCase();str = ”Hotel Echo Lima Lima Oscar";System.out.println(s1.get());System.out.println(s2.get());

}}

What will happen?

1. ConcurrentModificationException2. ArrayIndexOutOfBoundsException3. NullPointerException4. No exceptions, all good

List<String> list = new ArrayList<>(Arrays.asList("Arnie", "Chuck", "Slay"));list.stream().forEach(x -> {

if(x.equals("Chuck")) {list.remove(x);

}});


Java 8 vs Chuck Norris


What will happen?

A. ConcurrentModificationExceptionB. ArrayIndexOutOfBoundsExceptionC. NullPointerExceptionD. No exceptions, all good

List<String> list = new ArrayList<>(Arrays.asList("Arnie", "Chuck", "Slay"));list.stream().forEach(x -> {

if(x.equals("Chuck")) {list.remove(x);

}});


Here’s why:

stream().forEach() à spliterator().forEachRemaining()

forEachRemaining checks for mod count once, in the end

Removing element adds null to the end of the array:["Arne", "Chuck", "Slay"] à ["Arne", "Slay", null]

On the last iteration if(null.equals("Chuck")) fails with NPE (didn’t get to CME)

Use list.removeIf("Chuck"::equals);


System.out.println(Optional.of("rtfm").orElseGet(null));System.out.println(Optional.empty().map(null).orElse("rtfm"));


A. rtfm / rtfmB. rtfm / NullPointerExceptionC. NullPointerException / NullPointerExceptionD. NullPointerException / rtfm


System.out.println(Optional.of("rtfm").orElseGet(null));System.out.println(Optional.empty().map(null).orElse("rtfm"));


A. rtfm /rtfmB. rtfm / NullPointerExceptionC. NullPointerException / NullPointerExceptionD. NullPointerException / rtfm


Conclusions


- Write readable code!- Comment all the tricks- Sometimes it’s 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?

- puzzlers jfrog.com

Did you like it?Praise us on twitter and in the feedback form!

- javapuzzlersng- gamussa- Jbaruch-http://jfrog.com/shownotes

Didn’t like it?/dev/null

Java 8 Puzzlers as it was presented at Codemash 2017

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