15.3. The assert Statement

An assertion is a statement containing a boolean expression. If the assertion is enabled as specified by the Java Language Specification, Third Edition, an evaluation of the assertion causes evaluation of the boolean expression and an error is reported if the expression evaluates to false. If an optional second expression exists, the error message is based on the evaluation of this expression. If the assertion is disabled, evaluation of the assertion has no effect whatsoever.

Assert Statement
[38]AssertStatement::=assert AssignmentExpressionNoRange [ ':' Expression ] ';'  

It is a compile-time error if the first expression does not have type boolean or if the optional second expression after the colon is void. The evaluation of an assert is performed as specified by the Java Language Specification, Third Edition.