Java Reference
In-Depth Information
The
emptyList()
method is implemented as follows:
return (List<T>)
EMPTY_LIST;
. This statement returns the single
List
instance assigned to the
EMPTY_LIST
class field in the
Collections
class.
Youmightwanttoworkwith
EMPTY_LIST
directly,butyou'llrunintoanunchecked
warning message if you do, because
EMPTY_LIST
is declared to be of the raw type
List
,andmixingrawtypeswithgenerictypesleadstosuchmessages.Althoughyou
could suppress the warning, you're better off using the
emptyList()
method.
Suppose you add a
void setBirds(List<String> birds)
method to
Birds
, and pass an empty list to this method, as in
birds.setBirds(Collections.emptyList());
.Thecompilerwillrespond
with an error message stating that it requires the argument to be of type
List<String>
,butinsteadtheargumentisoftype
List<Object>
.Itdoessobe-
causethecompilercannotfigureoutthepropertypefromthiscontext,andsoitchooses
List<Object>
.
Thereisawaytosolvethisproblem,whichwillprobablylookverystrange.Specify
birds.setBirds(Collections.<String>emptyList());
, where the
formaltypeparameterlistanditsactualtypeargumentappearafterthememberaccess
operatorandbeforethemethodname.Thecompilerwillnowknowthatthepropertype
argument is
String
, and that
emptyList()
is to return
List<String>
.
Legacy Collections APIs
Java 1.2 introduced the Collections Framework. Prior to the framework's inclusion in
Java, developers had two choices where collections were concerned: create their own
frameworks, or use the
Vector
,
Enumeration
,
Stack
,
Dictionary
,
Hasht-
able
,
Properties
, and
BitSet
types, which were introduced by Java 1.0.
Vector
isaconcreteclassthatdescribesagrowablearray,muchlike
ArrayList
.
Unlike an
ArrayList
instance, a
Vector
instance is synchronized.
Vector
has
beengenerifiedandalsoretrofittedtosupporttheCollectionsFramework,whichmakes
statements such as
List<String> list = new Vector<String>();
legal.
TheCollectionsFrameworkprovides
Iterator
foriteratingoveracollection'sele-
ments.Incontrast,
Vector
's
elements()
methodreturnsaninstanceofaclassthat
implements the
Enumeration
interface for
enumerating
(iterating over and return-
ing)a
Vector
instance'selementsvia
Enumeration
's
hasMoreElements()
and
nextElement()
methods.
