What does the Java assert keyword do, and when should it be used?

Question

What are some real life examples to understand the key role of assertions?

Answer 1

Here's an assertion I wrote in a server for a Hibernate/SQL project. An entity bean had two effectively-boolean properties, called isActive and isDefault. Each could have a value of "Y" or "N" or null, which was treated as "N". We want to make sure the browser client is limited to these three values. So, in my setters for these two properties, I added this assertion:

assert new HashSet(Arrays.asList("Y", "N", null)).contains(value) : value;

Notice the following.

1. This assertion is for the development phase only. If the client sends a bad value, we will catch that early and fix it, long before we reach production. Assertions are for defects that you can catch early.

2. This assertion is slow and inefficient. That's okay. Assertions are free to be slow. We don't care because they're development-only tools. This won't slow down the production code because assertions will be disabled. (There's some disagreement on this point, which I'll get to later.) This leads to my next point.

3. This assertion has no side effects. I could have tested my value against an unmodifiable static final Set, but that set would have stayed around in production, where it would never get used.

4. This assertion exists to verify the proper operation of the client. So by the time we reach production, we will be sure that the client is operating properly, so we can safely turn the assertion off.

5. Some people ask this: If the assertion isn't needed in production, why not just take them out when you're done? Because you'll still need them when you start working on the next version.

Some people have argued that you should never use assertions, because you can never be sure that all the bugs are gone, so you need to keep them around even in production. And so there's no point in using the assert statement, since the only advantage to asserts is that you can turn them off. Hence, according to this thinking, you should (almost) never use asserts. I disagree. It's certainly true that if a test belongs in production, you should not use an assert. But this test does not belong in production. This one is for catching a bug that's not likely to ever reach production, so it may safely be turned off when you're done.

BTW, I could have written it like this:

assert value == null || value.equals("Y") || value.equals("N") : value;

This is fine for only three values, but if the number of possible values gets bigger, the HashSet version becomes more convenient. I chose the HashSet version to make my point about efficiency.