The Java Language Specification, Third Edition

14.10 The assert Statement BLOCKS AND STATEMENTS
376
✦ If the instance creation completes normally, the assert statement
completes abruptly by throwing the newly created AssertionError
object.
◆ If Expression2 is not present, an AssertionError instance with no "detail
message" is created.
❖ If the instance creation completes abruptly for some reason, the assert
statement completes abruptly for the same reason.
❖ If the instance creation completes normally, the assert statement completes
abruptly by throwing the newly created AssertionError object.
DISCUSSION
For example, after unmarshalling all of the arguments from a data buffer, a programmer
might assert that the number of bytes of data remaining in the buffer is zero. By verifying
that the boolean expression is indeed true, the system corroborates the programmer’s
knowledge of the program and increases one’s confidence that the program is free of bugs.
Typically, assertion-checking is enabled during program development and testing, and
disabled for deployment, to improve performance.
Because assertions may be disabled, programs must not assume that the expressions
contained in assertions will be evaluated. Thus, these boolean expressions should generally
be free of side effects:
Evaluating such a boolean expression should not affect any state that is visible after
the evaluation is complete. It is not illegal for a boolean expression contained in an assertion
to have a side effect, but it is generally inappropriate, as it could cause program behavior
to vary depending on whether assertions were enabled or disabled.
Along similar lines, assertions should not be used for argument-checking in public
methods. Argument-checking is typically part of the contract of a method, and this contract
must be upheld whether assertions are enabled or disabled.
Another problem with using assertions for argument checking is that erroneous arguments
should result in an appropriate runtime exception (such as IllegalArgumentException,
IndexOutOfBoundsException or NullPointerException). An assertion failure will not
throw an appropriate exception. Again, it is not illegal to use assertions for argument checking
on public methods, but it is generally inappropriate. It is intended that AssertionError
never be caught, but it is possible to do so, thus the rules for try statements should treat
assertions appearing in a try block similarly to the current treatment of throw statements.
DRAFT