There are many traps and pitfalls in Java. Things which looks obvious at a glance may be tricky. Take a look how is it possible that 4+1=15 or how to read from non-existent file.

1. It’s a trap
Karol Wrótniak

2. Ramazan wanted to write "You change the
topic every time you run out of
arguments" (sounds familiar enough) but
what Emine read was, "You change the
topic every time they are fucking
you" (sounds familiar too.)
Source: Jesus Diaz, A Cellphone's Missing Dot Kills Two People, Puts Three More in Jail (2008)
Retrieved from: https://gizmodo.com/382026/a-cellphones-missing-dot-kills-two-people-puts-three-more-in-jail

3. When using
LOWER_CASE_WITH_DASHES or
LOWER_CASE_WITH_UNDERSCORES as
FieldNamingPolicy it could happen that
certain fields won't get serialized/deserialized.
I've seen that behavior on devices with a
turkish (tr-TR) locale.
Source: Sebastian Clan, Fix issues if runing in an environment with a Turkish locale (2015)
Retrieved from: https://github.com/google/gson/pull/652/files

4. Dotted and dotless I
Lowercase Uppercase
I
İ
I
i
i
ı
English
Turkish
Turkish
dotted
dotless

5. Casing done right
•Default (or specific) locale for UI
•ROOT locale for machine to machine communication like JSON keys or HTTP headers
value.toUpperCase(Locale.ROOT);

6. Casing depends on context
•Dotted and dotless I in Turkish: İi, Iı
•Final and non-final small Greek sigmas with single capital letter: ς, σ, Σ
•Retained dot in a lowercase i and j when followed by accents in Lithuanian: i̇̀
Unicode casing: https://unicode.org/Public/UNIDATA/SpecialCasing.txt

7. Where is my uppercase?
•No uppercase for some ligatures: ß SS, ffl FFL
•No uppercase for some precomposed characters: ΐ (1 character) Ϊ́ (3 characters)
"ß".toUpperCase() == "SS";
"dzwon".length() == 4;

8. Collator
String[] words = {"ćma", "kres", "czas", "chyba"};
Arrays.sort(words);
System.out.println(Arrays.toString(words));
//[chyba, czas, kres, ćma]
Arrays.sort(words, Collator.getInstance());
•Collator PL: [chyba, czas, ćma, kres]
•Collator CS: [ćma, czas, chyba, kres]
•Hungarian SZ+SZ=SSZ
•[kaszinó, kassza]

9. Source: DailyHaha

10. Number pitfalls
x == -x && x != 0
int x = Integer.MIN_VALUE;
x != x
float x = Float.NaN;
x = x + y;
x += y;
short x = 1;
int y = 2;

11. The missing days
Calendar calendar = Calendar.getInstance();
System.out.println(calendar.getTime()); //Sat Oct 06 12:25:24 CEST 2018
calendar.add(YEAR, -436);
System.out.println(calendar.getTime()); //Sat Oct 16 12:25:24 CET 1582

12. 1-1 = ?
Calendar calendar = Calendar.getInstance();
calendar.set(YEAR, 1);
System.out.println(calendar.get(YEAR)); //1
calendar.add(YEAR, -1);
System.out.println(calendar.get(YEAR)); //1

13. Source: java-gaming.org

14. java.io.File internals
file.exists() == true;
file.isDirectory() == false;
file.isFile() == false;
File file = new File("/dev/null");
InputStream stream = new FileInputStream(file);
file.delete();
stream.read();

15. Initialization order
abstract class Parent {
Parent() {
log();
}
abstract void log();
}
class Child extends Parent {
final String foo;
Child() {
foo = "bar";
}
@Override
protected void log() {
System.out.println(foo);
}
}

16. Abnormal finally block completion
public static void main(String[] args) {
System.out.println(foo());
}
private static boolean foo() {
try {
return false;
} finally {
return true;
}
}

17. Exceptional exceptions
public static <T extends Throwable> void sneakyThrow(Throwable t) throws T {
throw (T) t;
}
public static void main(String[] args) {
sneakyThrow(new IOException());
}

18. Surprising escapes
/*
String specialChars = "!@#$%*/()";
*/
String x = "au0022;
String x = "a";
//TODO add u000A support

19. Volatile
private static boolean isReady;
public static void main(String[] args) throws Exception {
new Thread(() -> {
while (!isReady);
System.out.println("Ready!");
}).start();
Thread.sleep(1000);
isReady = true;
}

20. REFERENCES
•Java Puzzlers
•Edge Cases to Keep in Mind. Part 1 – Text
•Edge Cases to Keep in Mind. Part 2 – Files
•Unicode casing
•Java Magic. Part 4: sun.misc.Unsafe

21. THANKS!
ANY QUESTIONS?
karol.wrotniak@droidsonroids.pl
koral--
karol-wrotniak
@karol.wrotniak
Karol Wrótniak
Android Dev @Droids On Roids
Co-organizer @GDG Wrocław