Java Reference
In-Depth Information
ownshallowcopyingofnonreferencefields),itisn'tnecessaryfortheclasscontaining
theoverriding
clone()
methodtoimplement
Cloneable
,butitshouldimplement
this interface for consistency.
String
does not override
clone()
, so
String
ob-
jects cannot be deeply cloned.
Equality
The
==
and
!=
operatorscomparetwoprimitivevalues(suchasintegers)forequality
(
==
) or inequality (
!=
). These operators also compare two references to see whether
theyrefertothesameobjectornot.Thislattercomparisonisknownasan
identity check
.
You cannot use
==
and
!=
to determine whether two objects are logically the same
(ornot).Forexample,two
Truck
objectswiththesamefieldvaluesarelogicallyequi-
valent. However,
==
reports them as unequal because of their different references.
Note
Because
==
and
!=
perform the fastest possible comparisons, and because
stringcomparisonsneedtobeperformedquickly(especiallywhensortingahugenum-
berofstrings),the
String
classcontainsspecialsupportthatallowsliteralstringsand
string-valuedconstantexpressionstobecomparedvia
==
and
!=
.(Iwilldiscussthis
these comparisons:
System.out.println("abc" == "abc"); // Output: true
System.out.println("abc" == "a"+"bc"); // Output: true
System.out.println("abc" == "Abc"); // Output: false
System.out.println("abc" != "def"); // Output: true
System.out.println("abc" == new String("abc")); // Output:
false
Recognizing the need to support logical equality in addition to reference equality,
Java provides an
equals()
method in the
Object
class. Because this method de-
faultstocomparingreferences,youneedtooverride
equals()
tocompareobjectcon-
tents.
Beforeoverriding
equals()
,makesurethatthisisnecessary.Forexample,Java's
equals()
.Perhapsthisclass'sdesignersdidnotthinkitnecessarytodetermineiftwo
StringBuffer
objects are logically equivalent.
