Java Reference
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CAP#1 extends Object from capture of ?
1 error
Thiserrormessageassumesthat copyList() ispartofaclassnamed CopyList .
Although it appears to be incomprehensible, the message basically means that the
dest.add(src.get(i)) method call violates type safety. Because ? implies that
element type is incompatible with the source list's element type.
Forexample,supposeyoucreatea List of String asthesourcelistanda List of
Employee asthedestinationlist.Attemptingtoaddthesourcelist's String elements
tothedestinationlist,whichexpects Employee s,violatestypesafety.Ifthiscopyop-
erationwereallowed,a ClassCastException instancewouldbethrownwhentry-
ing to obtain the destination list's elements.
You could avoid this problem by specifying void copyList(List<String>
src, List<String> dest) , but this method header limits you to copying only
listsof String objects.Alternatively,youmightrestrictthewildcardargument,which
is demonstrated here:
static void copyList(List<? extends String> src,
List<? super String> dest)
for (int i = 0; i < src.size(); i++)
per bound or (unlike with a type parameter) a lower bound to limit the types that can
bepassedasactualtypeargumentstothegenerictype.Specifyanupperboundvia ex-
tends followed by the upper bound type after the ? , and a lower bound via super
followed by the lower bound type after the ? .
You interpret ? extends String to mean that any actual type argument that
is String or a subclass can be passed, and you interpret ? super String to
implythatanyactualtypeargumentthatis String orasuperclasscanbepassed.Be-
cause String cannotbesubclassed,thismeansthatyoucanonlypasssourcelistsof
String and destination lists of String or Object .
The problem of copying lists of arbitrary element types to other lists can be solved
throughtheuseofa generic method (aclassorinstancemethodwithatype-generalized
implementation). Generic methods are syntactically expressed as follows:
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