Java assertions underused

Question

I\'m wondering why the assert keyword is so underused in Java? I\'ve almost never seen them used, but I think they\'re a great idea. I certainly much prefer the

Answer 1

It's an abuse of assertions to use them to test user input. Throwing an IllegalArgumentException on invalid input is more correct, as it allows the calling method to catch the exception, display the error, and do whatever it needs to (ask for input again, quit, whatever).

If that method is a private method inside one of your classes, the assertion is fine, because you are just trying to make sure you aren't accidentally passing it a null argument. You test with assertions on, and when you have tested all the paths through and not triggered the assertion, you can turn them off so that you aren't wasting resources on them. They are also useful just as comments. An assert at the start of a method is good documentation to maintainers that they should be following certain preconditions, and an assert at the end with a postcondition documents what the method should be doing. They can be just as useful as comments; moreso, because with assertions on, they actually TEST what they document.

Assertions are for testing/debugging, not error-checking, which is why they are off by default: to discourage people from using assertions to validate user input.

Answer 2

From Programming with Assertions

By default, assertions are disabled at runtime. Two command-line switches allow you to selectively enable or disable assertions.

This means that if you don't have complete control over the run-time environment, you can't guarantee that the assertion code will even be called. Assertions are meant to be used in a test-environment, not for production code. You can't replace exception handling with assertions because if the user runs your application with assertions disabled (the default), all of your error handling code disappears.

Answer 3

@Don, you are frustrated that assertion are turned off by default. I was also, and thus wrote this little javac plugin that inlines them (ie emits the bytecode for if (!expr) throw Ex rather than this silly assert bytecode.

If you include fa.jar in your classpath while compiling Java code, it will do its magic and then tell

Note: %n assertions inlined.

@see http://smallwiki.unibe.ch/adriankuhn/javacompiler/forceassertions and alternatively on github https://github.com/akuhn/javac

Answer 4

Assertions are very limited: You can only test boolean conditions and you need to write the code for a useful error message every time. Compare this to JUnit's assertEquals() which allows to generate a useful error message from the inputs and even show the two inputs side by side in the IDE in a JUnit runner.

Also, you can't search for assertions in any IDE I've seen so far but every IDE can search for method invocations.

Answer 5

Assertions are useful because they:

• catch PROGRAMMING errors early
• document code using code

Think of them as code self-validation. If they fail it should mean that your program is broken and must stop. Always turn them on while unit testing !

In The Pragmatic Programmer they even recommend to let them run in production.

Leave Assertions Turned On

Use Assertions to Prevent the Impossible.

Note that assertions throw AssertionError if they fail, so not caught by catch Exception.