Java Reference
In-Depth Information
The fourth line assigns
empListArray
to
objArray
. This assignment is legal
becausearraysarecovariantanderasureconverts
List<Employee>[]
tothe
List
runtime type, and
List
subtypes
Object
.
Because oferasure,theJVMdoesn'tthrow
ArrayStoreException
wheniten-
counters
objArray[0] = strList;
.Afterall,we'reassigninga
List
reference
to a
List[]
array at runtime. However, this exception would be thrown if generic
types were reified because we'd then be assigning a
List<String>
reference to a
List<Employee>[]
array.
However,thereisaproblem.A
List<String>
instancehasbeenstoredinanarray
thatcanonlyhold
List<Employee>
instances.Whenthecompiler-insertedcastop-
erator attempts to cast
empListArray[0].get(0)
's return value (
"string"
) to
Employee
, the cast operator throws a
ClassCastException
object.
PerhapsafutureversionofJavawillreifytypeparameters,makingitpossibletospe-
cify array-creation expressions that involve type parameters or actual type arguments.
Varargs and Generics
When you invoke a varargs (variable umber of arguments) method whose parameter
is declared to be a parameterized type (as in
List<String>
), the compiler emits a
warning message at the point of call. This message can be confusing and tends to dis-
courage the use of varargs in third-party APIs.
Thewarningmessageisrelatedto
heap pollution
,whichoccurswhenavariableofa
parameterizedtypereferstoanobjectthatisnotofthatparameterizedtype.Heappollu-
tioncanonlyoccurwhenanapplicationperformsanoperationthatwouldgiverisetoan
unchecked warning at compile time. (
The Java Language Specification, Third Edition
discussestheconceptofheappollution[
http://java.sun.com/docs/books/
Unchecked warnings occur in calls to varargs methods whose parameter types are
notreifiable.Inotherwords,theparameter'stypeinformationcannotbecompletelyex-
pressed at runtime because of erasure.
Varargsareimplementedviaarraysandarraysarereified.Inotherwords,anarray's
element type is stored internally and used when required for various runtime type
checks. However, this stored type information cannot include information required to
represent a parameterized type that is onreifiable.
