What is the underscore actually doing in this Java code?

Question

I just began to learn Java.

My friend who is helping me study just sent me this and said 'figure this out'.

Unfortunately I am unable to read this. It looks like Perl to me.

class _{_ _;_(){_=this;}}

What does it mean?

This question is being discussed [on meta](http://meta.stackexchange.com/questions/173714/can-we-nuke-this-too-localized-question). Let's take arguments for or against closing/deleting it there. — Adam Lear, Mar 26 '13 at 22:32
@anotherordinary You should probably ditch your friend as a programming teacher if he/she thought telling you to figure this out would be a good way to learn this language. Remembering irreverent trivia isn't useful at all. Much less when you are starting to learn a language. — Enno Shioji, Mar 27 '13 at 00:44
@EnnoShioji How is this "irrelevant trivia" that needs remembering? Anyone that calls himself a Java programmer should be able to understand this easily. — phant0m, Jun 11 '13 at 13:21
@phant0m: The "trivia" here I think is the fact that Java allows a single underscore as an identifier, which I don't think is useful knowledge. — Enno Shioji, Jun 11 '13 at 13:56

tckmn · Accepted Answer · 2013-10-20T19:37:49.597

155

_ is the class name. It's a very confusing one, but it works!

With the class renamed:

class Something {Something something;Something(){something=this;}}

And cleaned up:

class Something {
    Something something;
    Something() {
        something=this;
    }
}

And you can go crazy with this odd naming :)

class _{_ __;_ ____;_(){__=this;____=__;}_(_ ___){__=___;}}

In fact, Unicode is even supported, so this is valid:

class 合法類別名稱{合法類別名稱(){}}

edited Oct 20 '13 at 19:37

answered Mar 26 '13 at 22:08

tckmn

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I personally love the @Alvin's edit :) – Bartek Banachewicz Jun 25 '13 at 09:40
4

There are few mistakes in your `get crazy` example. (1) you forgot to add semicolon after `____=__` (two times), (2) if you are trying to overload `_` method then you need to also pass type so `_(___)` should be `_(_ ___)`. Here is how it can look `class _{_ __;_ ____;_(){__=this;____=__;}_(_ ___){__ = ___;}}`. Wait a minute. Did I just debugged underscores? It is time for a brake. – Pshemo Oct 18 '13 at 22:25
@Pshemo Lol, I never really meant to be serious about that code :P fixed – tckmn Oct 20 '13 at 19:37
Get a +1 for the comment on my answer. Could not think of another way of rewarding you. – Ed Heal Nov 11 '13 at 02:50

score 71 · Answer 2 · edited Jun 11 '13 at 13:06

71

_ is the class name, underscore is a valid Java variable name, you just need to indent your code to deobfuscate it:

class _{
    _ _;
    _(){
     _=this;
   }
}

Like:

class A{
    A A;
    A(){
     A=this;
   }
}

Edit: thanks to @Daniel Fischer

Type names and variable names have different namespaces. and for example code class FOO { FOO FOO; } is valid in Java.

Summary

_ is a class name e.g at class _{
_ is a class member name e.g at _ _; and _=this
_ is a constructor name e.g. at _()

Remember: Java uses six different namespaces:

Package names,

type names,

field (variable) names,

method names,

local variable names (including parameters), and

labels.

In addition, each declared enum has its own namespace. Identical names of different types do not conflict; for example, a method may be named the same as a local variable.

edited Jun 11 '13 at 13:06

phant0m

16,595
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answered Mar 26 '13 at 22:09

Grijesh Chauhan

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20
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3

There is no mistake. It compiles just fine as I typed. I just didn't understand it till now. – another ordinary Mar 26 '13 at 22:21
2

@GrijeshChauhan type names and variable names have different namespaces. `class FOO { FOO FOO; }` works. – Daniel Fischer Mar 26 '13 at 22:39
@DanielFischer Thanks..This is in Java only **?** ..any other language do this? – Grijesh Chauhan Mar 26 '13 at 22:40
1

In Haskell, for example, types and values also have different namespaces. A difference there is that case has meaning, a type name must begin with an upper case letter, so you can't name any old value like a type, but only constructors, but `data Foo = Foo | Bar Int` works. It can only work, of course, in languages where you can figure out the category of an identifier from where it appears. – Daniel Fischer Mar 26 '13 at 22:45
@DanielFischer thanks a lot Deniel Fischer... I found it very interesting. – Grijesh Chauhan Mar 26 '13 at 22:51
2

@anotherordinary read my updated answer I added a link also – Grijesh Chauhan Mar 26 '13 at 23:01
@DanielFischer I have a cross question: In c++ also contractors and class name, function name is overloading and overriding are same. Its dues to namespace or c++ compiler internally represent every function with different name **?** Sorry for bothering you again. – Grijesh Chauhan Mar 26 '13 at 23:14
1

Uh, C++ isn't really my domain. But unless things changed since I last looked, overloading is resolved via name-mangling, with parameter types included in the "real" name in some way. I think type names and function/constructor names are in the same namespace in C++, but I'm not sure, and too lazy to look it up now. – Daniel Fischer Mar 26 '13 at 23:27
1

Aren't field names and local variables in the same namespace? Otherwise you wouldn't be able to shadow the variables, as is commonly done in setter methods: `void setValue(String value) { this.value = value;}` – Jörn Horstmann Mar 27 '13 at 14:38
@JörnHorstmann No I mean yes both are in same *namespace*. – Grijesh Chauhan Mar 27 '13 at 14:42
@anotherordinary Hi Did you read this question and its answer [Obfuscated C Code Contest 2006.](http://stackoverflow.com/questions/15393441/obfuscated-c-code-contest-2006-please-explain-sykes2-c/15395030#15395030) – Grijesh Chauhan Mar 28 '13 at 14:08
@GrijeshChauhan: In the comment you wrote that they're in the same namespace, but this contradicts your answer listing 6 different namespaces. I'd say "field vs local variable vs enum member" is all just scoping. – maaartinus Nov 14 '13 at 03:49
@maaartinus Yes I notice. But I think because we use `this` so both are in same name space. I didn't understand what do you mean by `'this is just scoping'` ..please add some info/link I am myself confused. – Grijesh Chauhan Dec 04 '13 at 12:00

score 12 · Answer 3 · answered Mar 27 '13 at 12:19

12

well that’s good example . Java allows unicode to be identifiers so you can write something like:

class ⲥlass {
ⲥlass claѕѕ;
}

here class name's c is 'ⲥ' (U+2CA5 COPTIC SMALL LETTER SIMA) and

object name's 'ѕ' (U+0455 CYRILLIC SMALL LETTER DZE).

answered Mar 27 '13 at 12:19

ashgkwd

195
11

You can't have a variable named `class`, it's a keyword. – nickb Mar 27 '13 at 16:36
14

@nickb : it is not Java keyword `'class'`. as I have mentioned that object name `'claѕѕ'` has `'ss'` which are non ascii characters. so actually object's name is `claѕѕ` is `cla\u0455\u0455'` I think this clears misunderstanding. :) – ashgkwd Mar 27 '13 at 17:31

What is the underscore actually doing in this Java code?

3 Answers3

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