Exceptions and Assertions - The Java Programming Language - page 438

i++;
assert i < max;
12.8.1. The assert Statement
The syntax for an assertion is:
assert eval-expr [: detail-expr ];
where eval-expr is either a boolean or Boolean expression and detail-expr
is an optional expression that will be passed to the AssertionError con-
structor to help describe the problem. The detail expression is optional;
if present, it provides information to a person who looks at the state-
ment or thrown exception. If the detail is a Throwable it will become
the cause of the AssertionError ; that is, it will be the value returned by
the error's getCause method. Otherwise, the detail will be converted to a
string and become the detail message for the error.
When an assert is encountered in the code, the first expression is evalu-
ated. If the resulting value is true , the assertion passes. If it is false , the
assertion fails, and an AssertionError will be constructed and thrown.