Java Puzzlers NG: The strange, the bizarre, and the wonderful
A presentation at Voxxed Days Singapore 2018 in in Singapore by Baruch Sadogursky
Java puzzlers ng The strange, the bizarre, and the wonderful @ jbaruch
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@ jbaruch @ gamussa
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Baruch “Top Hat” Sadogursky, JFrog Developer Advocate, @ jbaruch @ jbaruch
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@ tagir_valeev @ jbaruch
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@ jbaruch @ gamussa
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#devnexus2018
1.
An entertaining guy
on the
stage
2.
Funny puzzling questions
3.
Yo u t h i n k a n d
vote
4.
JFrog t
shirts are airborne 5. Official twitter hashtag
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@ jbaruch @ gamussa
javapuzzlersng #devnexus2018 jfrog.com / shownotes
Slides
Video
Links
Ratings, comments
Raffle! Shownotes !
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Which Java version are you on? @ jbaruch
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javapuzzlersng A. Java 7 B. Java 8 C. Java 9 D. Java 5 E. Java 10 F. Java 2
Watching the puzzlers like … #dafaq @ jbaruch
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Everything works (or doesn't) in the latest Java 8 and/or 9 update
Execute ’ em all
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 evil! @ jbaruch
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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 }
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; } @ jbaruch
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@ jbaruch
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What will happen? @ jbaruch
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javapuzzlersng 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); } });
@ jbaruch
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Java 8 vs Chuck Norris @ jbaruch
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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); } }); @ jbaruch
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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); @ jbaruch
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@ jbaruch
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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<Hero> expendables) { Iterator<Hero> heroes = expendables.iterator (); heroes.forEachRemaining (e
{ if ( heroes .hasNext ()) { heroes .next (); heroes .remove (); } }); System. out .println (expendables); }
@ jbaruch
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javapuzzlersng
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<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 ] @ jbaruch
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How single is a Single Abstract Method Interface? @ jbaruch
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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< T
{ 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<String> { } @ jbaruch
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@ jbaruch
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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< T
{ 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<String> { } @ jbaruch
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@ jbaruch
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@ jbaruch
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javapuzzlersng // module
info.java
module module {
requires requires;
exports exports;
opens opens;
uses uses;
}
A.
Error in every line
B.
Can’t export package with
exports
name
C.
Can’t declare uses in uses
D.
Compiles fine
@ jbaruch
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javapuzzlersng A. Error in every line B. Can’t export package with exports name C. Can’t declare uses in uses D. Compiles fine // module
info.java
module module {
requires requires;
exports exports;
opens opens;
uses uses;
}
A. Error in every line B. Can’t export package with exports name C. Can’t declare uses in uses D. Compiles fine // module
info.java
module module {
requires requires;
exports exports;
opens opens;
uses uses;
}
@
jbaruch
voxxeddayssingapore
javapuzzlersng
How do we fix that ? // module
info.java
import
opens.uses
;
module module {
requires requires;
exports exports;
opens opens;
uses uses;
}
@
jbaruch
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System.out.println ( isUltimateQuestion ? 42 : null); @ jbaruch
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@ jbaruch
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System.out.println ( isUltimateQuestion ? 42 : isUltimateQuestion ? 42 : null); @ jbaruch
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isUltimateQuestion = false A. Null B. Won’t compile C. 42 D. NullPointerException System.out.println ( isUltimateQuestion ? 42 : isUltimateQuestion ? 42 : null); @ jbaruch
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@ jbaruch
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System.out.println ( isUltimateQuestion ? 42 : isUltimateQuestion ? 42 : null); isUltimateQuestion = false A. Null B. Won’t compile C. 42 D. NullPointerException @ jbaruch
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What’s type of (b ? 42 : null) ? checking jls
15.25 : @ jbaruch
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@ jbaruch
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javapuzzlersng What’s type of (b ? 42 : null) ? isUltimateQuestion ? 42 : null isUltimateQuestion ? 42 : isUltimateQuestion ? 42 : null int null Integer int Integer int Unboxing of null à NPE
@ jbaruch
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javapuzzlersng
public class Superhero {
private Long strength;
public Superhero(Long strength) {
//null check
here
this.strength
= strength;
}
public Long
getStrength
() {
return strength;
}
}
@
jbaruch
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javapuzzlersng
What’s the correct answer? class Superhero implements Comparable<Superhero> { … @Override public int compareTo (Superhero this, Superhero that) { return this.strength.compareTo ( that.strength ); } } class Superhero implements Comparable<Superhero> { … public int compareTo (Superhero me, Superhero you) { return me.strength.compareTo ( you.strength ); } } class Superhero implements Comparator<Superhero> { … @Override public int compare(Superhero batman, Superhero superman) return batman.strength.compareTo ( superman.strength ); } } D. None ! A B C @ jbaruch
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javapuzzlersng
What’s the correct answer? class Superhero implements Comparable<Superhero> { … @Override public int compareTo (Superhero this, Superhero that) { return this.strength.compareTo ( that.strength ); } } class Superhero implements Comparable<Superhero> { … public int compareTo (Superhero me, Superhero you) { return me.strength.compareTo ( you.strength ); } } class Superhero implements Comparator<Superhero> { … @Override public int compare(Superhero batman, Superhero superman) return batman.strength.compareTo ( superman.strength ); } } D. None ! A B C @ jbaruch
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@ jbaruch
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import static java.util.stream.IntStream.range ; range(10, 0). forEach ( System.out :: println ); A. 0..10 B. IllegalArgumentException C. Nothing D. 10 .. 0 (10..0). each { println it } @ jbaruch
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import static java.util.stream.IntStream.range ; range(10, 0). forEach ( System.out :: println ); A. 0..10 B. IllegalArgumentException C. Nothing D. 10 .. 0 @ jbaruch
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vs. IntStream.rangeClosed (0, 10) .map(x
10 – x) . forEach ( System.out :: println ) (10..0). each { println it } Srsly , how do you do it?! @ jbaruch
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@ jbaruch
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Conclusions @ jbaruch
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@ jbaruch @ gamussa
javapuzzlersng #devnexus2018
Wr i te readab le co de!
Comment all the tricks
Sometimes it’s just a bug
Static code analysis FTW
I ntelliJ IDEA!
RT F M !
Don’t abuse lambdas and streams !
@ jbaruch @ gamussa
javapuzzlersng #devnexus2018
Tr u s t u s, w e h av e m u c h m o r e
where those came from
.
Puzzlers? Gotchas ? Fetal position inducing behavior ?
puzzlers @ jfrog. co m
We pay in t
shirts and glory!
jfrog.com / shownotes
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Praise us on twitter!
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Didn’t like it?
/d e v/n u l l
Not sure about you, but working with the latest versions Java made me lose all of my hair and most of my sleep (or was it the jet lag?). If you still haven’t reached the level of Brian Goetz in mastering lambdas and strings, this session is for you. And if you think you have, here’s some bad news for you: you should attend as well. You’ll see some surprising, bizarre, and inconceivable parts of the latest versions of Java, so you won’t be (too) surprised when they hit you in production. Like any other puzzlers session, this one consists lots of puzzling questions, answers from the audience, t-shirts flying around, and a lot of fun—all for one purpose: to make you understand the latest versions of Java better.
