Is Java assert broken?

Question

While poking around the questions, I recently discovered the assert keyword in Java. At first, I was excited. Something useful I didn't already know! A more efficient way for me to check the validity of input parameters! Yay learning!

But then I took a closer look, and my enthusiasm was not so much "tempered" as "snuffed-out completely" by one simple fact: you can turn assertions off.*

This sounds like a nightmare. If I'm asserting that I don't want the code to keep going if the input listOfStuff is null, why on earth would I want that assertion ignored? It sounds like if I'm debugging a piece of production code and suspect that listOfStuff may have been erroneously passed a null but don't see any logfile evidence of that assertion being triggered, I can't trust that listOfStuff actually got sent a valid value; I also have to account for the possibility that assertions may have been turned off entirely.

And this assumes that I'm the one debugging the code. Somebody unfamiliar with assertions might see that and assume (quite reasonably) that if the assertion message doesn't appear in the log, listOfStuff couldn't be the problem. If your first encounter with assert was in the wild, would it even occur to you that it could be turned-off entirely? It's not like there's a command-line option that lets you disable try/catch blocks, after all.

All of which brings me to my question (and this is a question, not an excuse for a rant! I promise!):

What am I missing?

Is there some nuance that renders Java's implementation of assert far more useful than I'm giving it credit for? Is the ability to enable/disable it from the command line actually incredibly valuable in some contexts? Am I misconceptualizing it somehow when I envision using it in production code in lieu of statements like if (listOfStuff == null) barf();?

I just feel like there's something important here that I'm not getting.

*Okay, technically speaking, they're actually off by default; you have to go out of your way to turn them on. But still, you can knock them out entirely.

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Edit: Enlightenment requested, enlightenment received.

The notion that assert is first and foremost a debugging tool goes a long, long way towards making it make sense to me.

I still take issue with the notion that input checks for non-trivial private methods should be disabled in a production environment because the developer thinks the bad inputs are impossible. In my experience, mature production code is a mad, sprawling thing, developed over the course of years by people with varying degrees of skill targeted to rapidly changing requirements of varying degrees of sanity. And even if the bad input really is impossible, a piece of sloppy maintenance coding six months from now can change that. The link gustafc provided (thanks!) includes this as an example:

assert interval > 0 && interval <= 1000/MAX_REFRESH_RATE : interval;

Disabling such a simple check in production strikes me as foolishly optimistic. However, this is a difference in coding philosophy, not a broken feature.

In addition, I can definitely see the value of something like this:

assert reallyExpensiveSanityCheck(someObject) : someObject;

My thanks to everybody who took the time to help me understand this feature; it is very much appreciated.

Answer 1

assert is a useful piece of Design by Contract. In that context, assertions can be used in:

• Precondition checks.
• Postcondition checks.
• Intermediate result checks.
• Class invariant checks.

Assertions can be expensive to evaluate (take, for example, the class invariant, which must hold before and after calling any public method of your class). Assertions are typically wanted only in debug builds and for testing purposes; you assert things that can't happen - things which are synonymous of having a bug. Assertions verify your code against its own semantics.

Assertions are not an input validation mechanism. When input could really be correct or wrong in the production environment, i.e. for input-output layers, use other methods, such as exceptions or good old conditional checks.

Answer 2

Java's assertions aren't really made for argument validation - it's specifically stated that assertions are not to be used instead of dear old IllegalArgumentException (and neither is that how they are used in C-ish languages). They are more there for internal validation, to let you make an assumption about the code which isn't obvious from looking at it.

As for turning them off, you do that in C(++), too, just that if someone's got an assert-less build, they have no way to turn it on. In Java, you just restart the app with the appropriate VM parameters.