Hacker News Reader

Comment by ris58h | Hacker News Reader

> A successfully constructed object should obey its documented interface contract, period.

Could you provide the contract you are talking about?

Yes, the contract of String::equals: The result is true if and only if the argument is not null and is a String object that represents the same sequence of characters as this object.

The article constructs two String objects representing the same character sequence, for which however equals() returns false, in violation of the above-quoted contract.

So they should add that mutating arguments during object construction may lead to not-so-successfully constructed (invalid) object. __Mission failed successfully__

No. The job of a constructor is to establish the class’s invariants for the new instance, or else fail with an exception. This bug in the String class fails to do so.

> The job of a constructor is to establish the class’s invariants for the new instance, or else fail with an exception.

Could you refer such statement in Java specification?

Invariants and all methods -- including constructors -- enforcing them is OOP 101. I would be surprised if they felt the need to make such a callout in the language specification.

Functions have an assumed pre-requirement not explicitly spelled out in every single javadoc, that the program does not have a data race. May as well ask for every javadoc to explicitly include a pre-requirement that no cosmic rays flip bits of memory.

But to be serious again, I'm sure if you look at Array or whatever type is involved in char[], you'll find that it is explicitly marked as not thread-safe.

> Functions have an assumed pre-requirement not explicitly spelled out in every single javadoc, that the program does not have a data race.

Maybe in your programs, not in mine. I want my classes’ instances to fulfill their contract once it has been constructed. If it can’t do so, construction should fail with an exception.

Note also that for example the OpenJDK’s String::hashCode implementation is formally not data-race free (racy single-check idiom), so your assumption as stated doesn’t hold.

If you mean that you construct your program in such a way as to not have data races, then I do the same. Even with mutable objects like char[], it's relatively straightforward to just not share such objects, and in the limited circumstances where you must share, to synchronize.

If you mean you somehow unilaterally do away with data races on shared mutable data, even if the concurrent thread does not synchronize in any way, then I am genuinely asking what you're referring to. To be specific, I'm not aware of any implementation of `String(char[])` that operates correctly if another thread is mutating the char[] without synchronization. The JMM guarantees are extremely limited even for distinct indices, hence the need for AtomicIntegerArray, etc.

If you mean you cannot eliminate the data race, but you can do a post-condition check of the String invariants, and you want that checking code to run even in non-debug builds, then fine. I think there is a place for "production asserts" like that, but I think they need careful application, and they don't belong at every level of the stack. I'd check the validity of the String in intern(), but not in every single String ctor as a post-condition. And certainly in most languages with weaker memory models than Java, but my guess is even in Java, such checks provide no real guarantee that data races can't ruin invariants. There's no magic tape to repair that crack in the foundations.

Reply on news.ycombinator.com