Java Integer Cache Pop Quiz

Description

A dark-background meme shows a mock exam: "The teacher: the test isn't that hard" followed by "The test:" and a Java code snippet. The code defines `Integer a = 0; Integer b = 0; while(a==b) { ++a; ++b; }` and then prints `System.out.println("The output is "+a);`, with a 100-point prompt asking for the output. The joke relies on Java autoboxing and `Integer` caching: small boxed integers compare reference-equal via `==`, but once both values become 128 they are usually boxed as separate objects, so the loop exits and prints 128.

Common Tags

Java LanguageGotchas LanguageQuirks TypeCoercion ObjectOrientedProgramming CodingChallenge SubtleBugs ComputerScienceEducation Debugging

Comments

Anonymous ★ Top Pick The answer is 128, unless someone tuned `IntegerCache` and turned the exam into a JVM configuration audit.

Anonymous ★ Top Pick

The answer is 128, unless someone tuned `IntegerCache` and turned the exam into a JVM configuration audit.
@sylfn 5y

There will be no output
@sylfn 5y

Because the program never reaches output statement
1. @illia_sahaidak 5y
  
  You don’t say?
  
  @bladefistx2 5y
  
  He's wrong the output is 0. Read the code again or run it yourself
@misesOnWheels 5y

comparing objects by reference ...
@sylfn 5y

C++ style...
@sylfn 5y

Just redefine that operator
@maxgraey 5y

Output: ⊥
Deleted Account 5y

if this was c++ you could write whatever and it will count
@KiT_BoPKiT 5y

писькодроч
@s2504s 5y

А я чёт тоже недогнал чё за ответ будет, лол
@mahdisml 5y

output is 0
1. Deleted Account 5y
  
  y tho
@s2504s 5y

Пачиму??
@s2504s 5y

Переполнение буфера может?
@yevhen_k 5y

Ноль, потому что сравниваются ссылочные типы на равенство, которое даст false
1. Deleted Account 5y
  
  java java
  
  @yevhen_k 5y
  
  Exactly, bro
  
  Deleted Account 5y
  
  wait, is that == operator is functionaly useless?
  
  @alexolexo 5y
  
  Thankfully, I write in Kotlin
2. @offpeakhours 5y
  
  є кеш інтів, тому 128
  
  @RiedleroD 5y
  
  ah bruh. That's a big brain meme if I've ever seen one.
  
  @Almamu 5y
  
  Indeed
  
  @Almamu 5y
  
  Dang, and they complain about PHP's quirks? Fml I would not be able to figure that if I had a bug in software like that
  
  Deleted Account 5y
  
  python has it too!
@s2504s 5y

Аааааа,
@RiedleroD 5y

I've seen that meme a couple of times, but I just realized: those are Integers, not ints and since Java compares Objects by reference, not by content, the loop wouldn't start and the output would be 0.
@maxgraey 5y

Otput will be 128!
1. Deleted Account 5y
  
  that's a big number ngl
  
  Deleted Account 5y
  
  lol
@maxgraey 5y

https://wiki.owasp.org/index.php/Java_gotchas#Immutable_Objects_.2F_Wrapper_Class_Caching
@CustomIcon 5y

bruh
1. Deleted Account 5y
  
  wait youre here
2. Deleted Account 5y
  
  oh you are in comments xd
  
  @CustomIcon 5y
  
  i know the python way
  
  Deleted Account 5y
  
  same
Deleted Account 5y

Hmm
@yevhen_k 5y

Java, go fuck yourself. I'm going to use python or rust instead
1. @RiedleroD 5y
  
  it's the same in python…
  
  @RiedleroD 5y
  
  well, if you compare by id, that is
2. Deleted Account 5y
  
  s/rust/C
3. Deleted Account 5y
  
  a = -5 b = -5 while (a := a + 1) is (b := b + 1): pass print(i)
  
  @yevhen_k 5y
  
  Yeah, but in python you explicitly use reference comparison
  
  @Almamu 5y
  
  Yeah, it's doesn't shoot you in the feet without warning
  
  Deleted Account 5y
  
  yes i agree java bad
  
  @CustomIcon 5y
  
  hey guess what. i learnt nim instead of kotlin. and i dont regret it
  
  Deleted Account 5y
  
  fuck you
  
  Deleted Account 5y
  
  kotlin is best
  
  Deleted Account 5y
  
  jvm tho
  
  Deleted Account 5y
  
  Yep
  
  Deleted Account 5y
  
  jvm is literally faster than C
  
  Deleted Account 5y
  
  HAHAHAHAHAN'T
  
  Deleted Account 5y
  
  refer to the conversation above
  
  Deleted Account 5y
  
  what, the test code with a and b?
  
  Deleted Account 5y
  
  no, with Slava S.
  
  Deleted Account 5y
  
  what
  
  Deleted Account 5y
  
  if you are not in chat then the post "C++ devs when their thousand line code..."
  
  Deleted Account 5y
  
  oh i'm here
  
  Deleted Account 5y
  
  what's slava s
  
  Deleted Account 5y
  
  https://benchmarksgame-team.pages.debian.net/benchmarksgame/performance/mandelbrot.html
  
  @RiedleroD 5y
  
  lol even node.js is faster
  
  Deleted Account 5y
  
  Lol
  
  Deleted Account 5y
  
  this page literally says you have to read the code to see which are threaded
  
  Deleted Account 5y
  
  And indeed both Java and C programs are multithreaded
  
  Deleted Account 5y
  
  Yes
  
  @Almamu 5y
  
  PHP POWA
  
  Deleted Account 5y
  
  Shut the fuck up...
  
  @Almamu 5y
  
  Just kidding, but C# or C++ all the way
  
  Deleted Account 5y
  
  Oh, still I don't like C# nor C++ but I mean still better than that shitwell
  
  Deleted Account 5y
  
  They both suck
@bit69tream 5y

btw this looks completely unlogical
1. @RiedleroD 5y
  
  most tests do. ever did a math test? yeah it's the same with programming.
@CustomIcon 5y

a = 0 b = 0 while a==b: a += 1 b += 1 print ("The output is: {output}".format(output=a))
1. @RiedleroD 5y
  
  that'd be infinity
2. Deleted Account 5y
  
  no
3. Deleted Account 5y
  
  a = -5 b = -5 while (a := a + 1) is (b := b + 1): pass print(a, b)
  
  @CustomIcon 5y
  
  walrus i must hate
  
  @RiedleroD 5y
  
  a bit simpler would be: a=0 b=0 while a is b: a+=1 b+=1 print(a,b) also, keep in mind that the walrus operator is new in 3.8, and therefore incompatible with most python installations. 3.6 is usually the target version.
  
  Deleted Account 5y
  
  fuck 3.6 it doesn't even support f strings
  
  @RiedleroD 5y
  
  yes it does…
  
  Deleted Account 5y
  
  does it? i don't remember that
  
  @RiedleroD 5y
  
  I think 3.6 is the version where they were first introduced.
  
  Deleted Account 5y
  
  i thought it was 3.7
  
  @RiedleroD 5y
  
  3.7 was the asyncio update
Deleted Account 5y

Language: ruby Source: a = b = 0 while a == b a += 1 b += 1 end puts(a) Errors: Kill signal (SIGKILL)
@yevhen_k 5y

Numpy is the fastest language 😂
@panzer_maus 5y

128 :)
1. @mahdisml 5y
  
  nice ! my answer was right
2. Red Я. 5y
  
  Old school rules)
Deleted Account 5y

Primer comentario en español
@panzer_maus 5y

This is because under 127 all Integers have same reference
@Withouthatewithoutfear 5y

To will get a output when it adventually crashes
1. @bladefistx2 5y
  
  It will always output 0
  
  Deleted Account 5y
  
  it will output 128
  
  @bladefistx2 5y
  
  Lol i read while a=b
  
  Deleted Account 5y
  
  i thought i was going insane, but you just deleted that message
@Cuwano 5y

But why 128?

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Java Integer Cache Pop Quiz

Description

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Comments

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