Java Reference
In-Depth Information
trueatthecommentlocationsissufficientfordeterminingcorrectness.However,com-
ments are useless for preventing bugs because the compiler ignores them.
Many languages address this problem by providing a language feature called asser-
tionsthatletsthedevelopercodifyassumptionsaboutapplicationcorrectness.Whenthe
applicationruns,andifanassertionfails,theapplicationterminateswithamessagethat
helps the developer diagnose the failure's cause.
ThissectionintroducesyoutoJava'sassertionslanguagefeature.Afterdefiningthis
term,showingyouhowtodeclareassertions,andprovidingexamples,thesectionlooks
atusingandavoidingassertions.Finally,youlearnhowtoselectivelyenableanddisable
assertions via the
javac
compiler tool's command-line arguments.
Declaring Assertions
An
assertion
isastatementthatletsyouexpressanassumptionofprogramcorrectness
viaaBooleanexpression.Ifthisexpressionevaluatestotrue,executioncontinueswith
the ext statement. Otherwise, an error that identifies the cause of failure is thrown.
There are two forms of the assertion statement, each of which begins with reserved
word
assert
:
assert
expression1
;
assert
expression1
:
expression2
;
In both forms of this statement,
expression1
is the Boolean expression. In the
secondform,
expression2
isanyexpressionthatreturnsavalue.Itcannotbeacall
to a method whose return type is
void
.
When
expression1
evaluates to false, this statement instantiates the
Asser-
tionError
class. The first statement form calls this class's noargument constructor,
which does not associate a message identifying failure details with the
Asser-
tionError
instance.
The second form calls an
AssertionError
constructor whose type matches the
type of
expression2
's value. This value is passed to the constructor and its string
representation is used as the error's detail message.
Whentheerroristhrown,thenameofthesourcefileandtheumberofthelinefrom
wheretheerrorwasthrownareoutputtotheconsoleaspartofthethrownerror'sstack
trace. In many situations, this information is sufficient for identifying what led to the
failure, and the first form of the assertion statement should be used.
Listing 3-31
demonstrates the first form of the assertion statement.
