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

Answer 1

Assertions aren't for the end user to see. They're for the programmer, so you can make sure the code is doing the right thing while it's being developed. Once the testing's done, assertions are usually turned off for performance reasons.

If you're anticipating that something bad is going to happen in production, like listOfStuff being null, then either your code isn't tested enough, or you're not sanitizing your input before you let your code have at it. Either way, an "if (bad stuff) { throw an exception }" would be better. Assertions are for test/development time, not for production.

Answer 2

Assertions are really a great and concise documentation tool for a code maintainer.

For example I can write:

foo should be non-null and greater than 0

or put this into the body of the program:

assert foo != null;
assert foo.value > 0;

They are extremely valuable for documenting private/package private methods to express original programmer invariants.

For the added bonus, when the subsystem starts to behave flaky, you can turn asserts on and add extra validation instantly.

Answer 3

Assertions are meant to ensure things you are sure that your code fulfills really are fulfilled. It's an aid in debugging, in the development phase of the product, and is usually omitted when the code is released.

What am I missing?

You're not using assertions the way they were meant to be used. You said "check the validity of input parameters" - that's precisely the sort of things you do not want to verify with assertions.

The idea is that if an assertion fails, you 100% have a bug in your code. Assertions are often used for identifying the bug earlier than it would have surfaced otherwise.

Answer 4

Every language I've ever seen with assertions comes with the capability of shutting them off. When you write an assertion you should be thinking "this is silly, there's no way in the universe this could ever be false" -- if you think it could be false, it should be an error check. The assertion is just to help you during development if something goes horribly wrong; when you build the code for production you disable them to save time and avoid (hopefully) superfluous checks

Answer 5

This doesn't directly answer your question about assert, but I'd recommend checking out the Preconditions class in guava/google-collections. It allows you to write nice stuff like this (using static imports):

// throw NPE if listOfStuff is null
this.listOfStuff = checkNotNull(listOfStuff);

// same, but the NPE will have "listOfStuff" as its message
this.listOfStuff = checkNotNull(listOfStuff, "listOfStuff"); 

It seems like something like this might be what you want (and it can't be turned off).

Answer 6

If asserts could not be turned off, then why should they even exist.

If you want to performa validity check on an input, you can easily write

if (foobar<=0) throw new BadFoobarException();

or pop up a message box or whatever is useful in context.

The whole point of asserts is that they are something that can be turned on for debugging and turned off for production.