Java Reference
In-Depth Information
type wrapper class instance in the collection—see the following example:
Collection<Integer> col = ...; // This code does not com-
pile because of the “...”.
int x = 27;
col.add(new Integer(x)); // Indirectly store int value 27
via an Integer object.
Thereversesituationisalsotedious.Whenyouwanttoretrievethe
int
from
col
,
you must invoke
Integer
's
intValue()
method (which, if you recall, is inher-
itedfrom
Integer
's
java.lang.Number
superclass).Continuingonfromthisex-
ample,youwouldspecify
int y = col.iterator().next().intValue();
to assign the stored 32-bit integer to
y
.
Toalleviatethistedium,Java5introducedautoboxingandunboxing,whichareapair
of complementary syntactic sugar-based language features that make primitive values
appear more like objects. (This “sleight of hand” is not complete because you cannot
specify expressions such as
27.doubleValue()
.)
Autoboxing
automatically
boxes
(wraps)aprimitivevalueinanobjectoftheappro-
priateprimitivetypewrapperclasswheneveraprimitivetypeisspecifiedbutareference
isrequired.Forexample,youcouldchangetheexample'sthirdlineto
col.add(x);
and have the compiler box
x
into an
Integer
object.
Unboxing
automatically
unboxes
(unwraps)aprimitivevaluefromitswrapperobject
whenever a reference is specified but a primitive type is required. For example, you
couldspecify
int y = col.iterator().next();
andhavethecompilerunbox
the returned
Integer
object to
int
value 27 prior to the assignment.
Althoughautoboxingandunboxingwereintroducedtosimplifyworkingwithprimit-
ivevaluesinacollectionscontext,theselanguagefeaturescanbeusedinothercontexts,
andthisarbitraryusecanleadtoaproblemthatisdifficulttounderstandwithoutknow-
ledge of what is happening behind the scenes. Consider the following example:
Integer i1 = 127;
Integer i2 = 127;
System.out.println(i1 == i2); // Output: true
System.out.println(i1 < i2); // Output: false
System.out.println(i1 > i2); // Output: false
System.out.println(i1+i2); // Output: 254
i1 = 30000;
i2 = 30000;
