Java Reference
In-Depth Information
Atypeparameter'sscopeisitsgenerictypeexceptwheremasked.Thisscopeincludes
the formal type parameter list of which the type parameter is a member.
Topreservetypesafety,youarenotallowedtoviolatethefundamentalruleofgeneric
types:
for a given subtype
x
of type
y
, and given
G
as a raw type declaration,
G<x>
is not a subtype of
G<y>
.Inotherwords,multipleparameterizedtypesthatdifferonly
inregardtoonetypeparameterbeingasubtypeofanothertypeparameterarenotpoly-
morphic.Forexample,
List<String>
isnotaspecializedkindof
List<Object>
.
Thisrestrictioncanbenamelioratedwithoutviolatingtypesafetybyusingwildcards.
For example, where a
void output(List<Object> list)
method can only
outputa
List
thatcontains
Object
s(toadheretotheaforementionedrule),a
void
output(List<?> list)
method can output a
List
of arbitrary objects.
Wildcardsalonecannotsolvetheproblemwhereyouwanttocopyone
List
toan-
other.Thesolutionistouseagenericmethod,a
static
oron-
static
methodwitha
type-generalizedimplementation.Forexample,a
<T> void copyList(List<T>
src, List<T> dest)
methodcancopyasource
List
ofarbitraryobjects(whose
typeisspecifiedby
T
)toanother
List
ofarbitraryobjects(havingthesametype).The
compiler infers the actual type arguments from the context in which the method is in-
voked.
Reification is representing the abstract as if it was concrete -- for example, making
amemoryaddressavailablefordirectmanipulationbyotherlanguageconstructs.Java
arraysarereifiedinthatthey'reawareoftheirelementtypes(anelementtypeisstored
internally) and can enforce these types at runtime. Attempting to store an invalid ele-
ment in an array causes the JVM to throw an instance of the
ArrayStoreExcep-
tion
class.
Unlikewitharrays,agenerictype'stypeparametersarenotreified.They'renotavail-
able at runtime because they're thrown away after the source code is compiled. This
“throwingawayoftypeparameters”isaresultoferasure,whichalsoinvolvesinserting
caststoappropriatetypeswhenthecodeisn'ttypecorrect,andreplacingtypeparamet-
ers by their upper bounds (such as
Object
).
When you invoke a varargs method whose parameter is declared to be a parameter-
izedtype(asin
List<String>
),thecompileremitsawarningmessageatthepointof
call.Thismessagecanbeconfusingandtendstodiscouragetheuseofvarargsinthird-
party APIs.
Thewarningmessageisrelatedtoheappollution,whichoccurswhenavariableofa
parameterized type refers to an object that is not of that parameterized type.
