Java Puzzle: Square Root • Hacker News Board

Re: Java Puzzle: Square Root

#31

Post by craigching » Wed, Mar 20, 2013, 2:45 AM UTC

wheaties wrote: post #30

I hate to break it to you but "n" is not final. Hence, you can reassign any BigInteger to it. Why not just set n = 9 and the root = 3... Immutability is your friend. Use it or lose it.

But n is also private, so you can't reassign it without using setAccessible or changing the security manager, both forbidden by the rules.

Re: Java Puzzle: Square Root

#32

Post by tantalor » Wed, Mar 20, 2013, 2:45 AM UTC

wheaties wrote: post #30

I hate to break it to you but "n" is not final. Hence, you can reassign any BigInteger to it. Why not just set n = 9 and the root = 3... Immutability is your friend. Use it or lose it.

You can't assign anything to a private variable.

Re: Java Puzzle: Square Root

#33

Post by coolbrow » Wed, Mar 20, 2013, 2:46 AM UTC

wheaties wrote: post #30

I hate to break it to you but "n" is not final. Hence, you can reassign any BigInteger to it. Why not just set n = 9 and the root = 3... Immutability is your friend. Use it or lose it.

But "n" is private and the rules state the solution must be in a different class. Thus you can't simply reassign it.

Re: Java Puzzle: Square Root

#34

Post by craigching » Wed, Mar 20, 2013, 2:50 AM UTC

powera wrote: post #5

I'm not sure this is interesting. Unless I'm missing something, the question is "find what way of breaking the compartmentalization of the JVM we haven't forbidden in these English-language rules".

Yes, I believe you and cromwellian have hit upon the main point. Or, to be more succinct, "do you know about Java spi."

Re: Java Puzzle: Square Root

#35

Post by wisesascha » Wed, Mar 20, 2013, 2:51 AM UTC

is using the ASM library not a legitimate way or would that violate the rules in some way?

Re: Java Puzzle: Square Root

#36

Post by tantalor » Wed, Mar 20, 2013, 2:53 AM UTC

I'd start by trying to create an evil subclass of BigInteger, as it is not a final class.

Re: Java Puzzle: Square Root

#37

Post by mattvanhorn » Wed, Mar 20, 2013, 3:01 AM UTC

cromwellian wrote: post #6

Consider that SecureRandom is really a facade around multiple providers that can plug in varying implementations. :)

...and that you can remove all the existing ones and insert your own.

Re: Java Puzzle: Square Root

#38

Post by mattvanhorn » Wed, Mar 20, 2013, 3:06 AM UTC

IvyMike wrote: post #27

I'm surprised nobody's given the obvious algorithm yet: while(i Before anyone complains, this algorithm is correct and does not break any of the rules as far as I can tell. :) I think the timing attack is probably what he's really looking for. Edit: as mattvanhorn pointed out, answer() is void, but that's ok... changed to a "constant time" algorithm. :)

answer() has a void return, although I suppose you might be able to watch System.out to see if it worked.

Re: Java Puzzle: Square Root

#39

Post by [deleted] » Wed, Mar 20, 2013, 3:14 AM UTC

GhotiFish wrote: post #9

cromwellian wrote: post #6

Consider that SecureRandom is really a facade around multiple providers that can plug in varying implementations. :)

ah ha! That might of being too big of a hint, good sir.

[deleted]

Re: Java Puzzle: Square Root

#40

Post by raylu » Wed, Mar 20, 2013, 3:14 AM UTC

cromwellian wrote: post #6

Consider that SecureRandom is really a facade around multiple providers that can plug in varying implementations. :)

SquareRoot.n is static, so it runs before any code in your main().