Java Assertion Question

Question

What exactly happens when a Java assertion fails? How does the programmer come to know that an assertion has failed?

Thanks.

Answer 1

If assertions are enabled in the JVM (via the -ea flag), an AssertionError will be thrown when the assertion fails.

This should not be caught, because if an assertion fails, it basically means one of your assumptions about how the program works is wrong. So you typically find out about an assertion failure when you get an exception stack trace logged with your thread (and possibly whole program) terminating.

Answer 2

An assertion will only fail if you enabled assertions in the JVM when it started. You can do that by specifying the parameter -ea in the command line. If you do that, then this block of code will throw an AssertionError when it is executed:

public void whatever() {
   assert false;
}

Assertions should be used to detect programming errors only. If you are validating user input or something on those lines, don't use assertions.

Answer 3

It throws an AssertionError which is a subclass of Error. As an Error generally and as a failed assertion in particular, you likely should not try to catch it since it is telling you there's a significant abnormality in your code and that if you continue you'll probably be in some undefined, unsafe state.