Blocks and Statements - The Java Language Specification - page 377

• If the value is true , then the first contained Statement (the one before the else
keyword) is executed; the if-then-else statement completes normally if and only if
execution of that statement completes normally.
• If the value is false , then the second contained Statement (the one after the else
keyword) is executed; the if-then-else statement completes normally if and only if
execution of that statement completes normally.
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 if the host system has determined that the top level class that lexically contains
the assert statement enables assertions.
Whether or not a top level class enables assertions is determined no later than the earliest
of the initialization of the top level class and the initialization of any class nested in the top
level class, and cannot be changed after it has been determined.
An assert statement that is executed before its class has completed initialization is enabled.
This rule is motivated by a case that demands special treatment. Recall that the asser-
tion status of a class is set no later than the time it is initialized. It is possible, though
generally not desirable, to execute methods or constructors prior to initialization. This
can happen when a class hierarchy contains a circularity in its static initialization, as
in the following example:
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public class Foo {
public static void main(String[] args) {