Here are a few ways to cast to a type without declaring it:1. Use a generic type parameter:<T> T cast(Object o) { return (T) o;}2. Use a raw type: Object cast(Object o) { return (Object) o; }3. Reflection:Object cast(Object o, Class<?> type) throws Exception { return type.cast(o);}4. Dynamic proxy: interface Castable { Object cast(Object o);}class CastProxy implements Castable { private Class<?> type; public CastProxy(Class

whoiam
Developer Advocate @JFrog
@jbaruch on the internetz

whoiam
Solutions Architect @Hazelcast
@gAmUssA on the internetz

1. Two entertaining guys on
stage
2. Funny Puzzling questions
3. You think and vote
4. Official twitter handles:
JAVApuzzlersng
codemash
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/sausage
B.milk/bread/sausage/eggs,don’t forget eggs!
C.milk/bread/sausage/ConcurrentModificationException
D.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.add("milk");
list.add("bread");

Going Vegan

A.milk/bread/sausage
B.milk/bread/eggs,don’t forget eggs!
C.milk/bread/ConcurrentModificationException
D.ConcurrentModificationException
list.add("milk");
list.add("bread");
list = list.subList(0, 2); //No sausage, please!

Sometimes it’s just a bug…

What’s the difference between 1 and 2?
A. 1 compiles,2 does not
B. 2 compiles,1 does not
C. Same same,both work fine
D. 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) ); // 1
ex.submit(() -> { Files.write(Paths.get("Sentence.txt"), sentence); }); // 2
}

Semicolons are the evil!

public void killAll(){
ExecutorService ex = Executors.newSingleThreadExecutor();
List<String> sentence = Arrays.asList("Punish");
ex.submit(() -> Files.write(Paths.get("Sentence.txt"), sentence) ); // 1
ex.submit(() -> { Files.write(Paths.get("Sentence.txt"), sentence); }); // 2
}
@FunctionalInterface
public interface Runnable {
public abstract void run();
}
@FunctionalInterface
public interface Callable<V> {
V call() throws Exception;
}

Mad Max

How that will work?
A. Compilation error
B. Runtime Exception
C. 3
D. Something else
System.out.println(
Stream.of(-3, -2, -1, 0, 1, 2, 3).max(Math::max).get()
);

How about now?
A.−3
B.−1
C.0
D.Something else
System.out.println(
);

• 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

Let’s upgrade the stack!

What will happen?
A. Maps will switch
B. Both will become oldSchool
C. Both will become hipster
D. 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 same
B. Two lines the same
C. All different
D. Four different

How about now?
List<String> kitties = Arrays.asList("Soft", null, "Purr");
A. All lines the same
B. Two lines the same
C. All different
D. Four different

null
Caught: java.lang.NoSuchElementException
Caught: 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 ();
}

// A. Will that work?
Dog& Catx = (Dog& Cat) obj;
x.meow ();
x.bark ();
}
// B. Will that work?
((Consumer<? extends Dog& Cat>)(x -> {
x.meow ();
x.bark ();
})).accept((Dog& Cat)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");}}
class Dogcat implements Dog, Cat{}
test(new Dogcat());
}

// A. Will that work?
Dog & Cat x = (Dog & Cat) obj;
x.meow();
x.bark();
}
// B. Will that work?
((Consumer<? extends Dog & Cat>)(x -> {
x.meow();
x.bark();
})).accept((Dog & Cat)obj); }
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");}}
static class Dogcat implements Dog, Cat{}
test(new Dogcat());
}

Bill Gates explains how that works

Optional.of((Dog & Cat) obj)
.ifPresent(x -> {
x.meow();
x.bark();
});}

Viktor Gamov and Baruch Sadogursky call customer
service:

1. HOTEL ECHO LIMA LIMA OSCAR/ HOTEL ECHO LIMA LIMA OSCAR
2. HELLO / HOTEL ECHO LIMA LIMA OSCAR
3. HOTEL ECHO LIMA LIMA OSCAR/ HELLO
4. 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());
}
}

1. HOTEL ECHO LIMA LIMA OSCAR/ HOTEL ECHO LIMA LIMA OSCAR
2. HELLO / HOTEL ECHO LIMA LIMA OSCAR
3. HOTEL ECHO LIMA LIMA OSCAR/ HELLO
4. 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";
}
}

What will happen?
1. ConcurrentModificationException
2. ArrayIndexOutOfBoundsException
3. NullPointerException
4. 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. ConcurrentModificationException
B. ArrayIndexOutOfBoundsException
C. NullPointerException
D. 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 / rtfm
B. rtfm / NullPointerException
C. NullPointerException / NullPointerException
D. NullPointerException / rtfm

System.out.println(Optional.of("rtfm").orElseGet(null));
System.out.println(Optional.empty().map(null).orElse("rtfm"));
A. rtfm /rtfm
B. rtfm / NullPointerException
C. NullPointerException / NullPointerException
D. 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

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