The Java® Language Specification, Java SE 7 Edition, Fourth Edition

14.10. The assert Statement

An assertion is an assert statement containing a boolean expression. An assertion is either enabled or disabled. If the assertion is enabled, execution of the assertion causes evaluation of the boolean expression and an error is reported if the expression evaluates to false. If the assertion is disabled, execution of the assertion has no effect whatsoever.

AssertStatement:    assert Expression1 ;    assert Expression1 : Expression2 ;

It is a compile-time error if Expression1 does not have type boolean or Boolean.

In the second form of the assert statement, it is a compile-time error if Expression2 is void (§15.1).

An assert statement that is executed after its class has completed initialization is enabled if and only ...