Java Reference
In-Depth Information
Note A class must implement the java.lang.Cloneable interface or its in-
stances cannot be shallowly cloned via Object 's clone() method—this method
performs a runtime check to see if the class implements Cloneable .(Iwill discuss
interfaceslaterinthischapter.)Ifaclassdoesnotimplement Cloneable , clone()
throws java.lang.CloneNotSupportedException . (Because
CloneNotSupportedException isacheckedexception,itisnecessaryfor List-
ing2-25 tosatisfythecompilerbyappending throws CloneNotSupportedEx-
ception to the main() method's header. I will discuss exceptions in the next
chapter.) String is an example of a class that does not implement Cloneable ;
hence, String objects cannot be shallowly cloned.
Afterassigningthe Employee object'sreferencetolocalvariable e1 , main() calls
the clone() methodonthisvariabletoduplicatetheobject,andthenassignstheres-
ulting reference to variable e2 . The (Employee) cast is needed because clone()
returns Object .
Toprovethattheobjectswhosereferenceswereassignedto e1 and e2 aredifferent,
main() nextcomparesthesereferencesvia == andoutputstheBooleanresult,which
happens to be false. To prove that the Employee object was shallowly cloned,
main() nextcomparesthereferencesinboth Employee objects' name fieldsvia ==
and outputs the Boolean result, which happens to be true.
Note Object 's clone() methodwasoriginallyspecifiedasa public method,
accesswaslaterchangedto protected ,whichmeansthatonlycodewithinthesame
packageastheclasswhose clone() methodistobecalled,orcodewithinasubclass
of this class (regardless of package) can call clone() .
to the same object via their equivalent reference fields. For example, each of Listing
2-25 ' s two Employee objects refers to the same String object via its name field.
Although not a problem for String , whose instances are immutable, changing a
see the same change via its reference field. For example, suppose you add a reference
fieldnamed hireDate to Employee .Thisfieldisoftype Date with year , month ,
and day instancefields.Because Date isintendedtobemutable,youcanchangethe
contents of these fields in the Date instance assigned to hireDate .
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