Java Reference
In-Depth Information
If the value
p
being boxed is
true
,
false
, a
byte
, or a
char
in the range
\u0000
to
\u007f
, or an
int
or
short
number between
-128
and
127
(inclusive), then let
r
1
and
r
2
be the results of any two
boxing conversions of
p
. It is always the case that
r
1
==
r
2
.
Ideally, boxing a given primitive value
p
, would always yield an identical reference.
In practice, this may not be feasible using existing implementation techniques. The
rules above are a pragmatic compromise. The final clause above requires that certain
common values always be boxed into indistinguishable objects. The implementation
may cache these, lazily or eagerly. For other values, this formulation disallows any
assumptions about the identity of the boxed values on the programmer's part. This
would allow (but not require) sharing of some or all of these references.
This ensures that in most common cases, the behavior will be the desired one, without
imposing an undue performance penalty, especially on small devices. Less memory-
limited implementations might, for example, cache all
char
and
short
values, as well as
int
and
long
values in the range of -32K to +32K.
A boxing conversion may result in an
OutOfMemoryError
if a new instance of one of the
wrapper classes (
Boolean
,
Byte
,
Character
,
Short
,
Integer
,
Long
,
Float
, or
Double
) needs to be al-
located and insufficient storage is available.
5.1.8. Unboxing Conversion
Unboxing conversion converts expressions of reference type to corresponding expressions
of primitive type. Specifically, the following eight conversions are called the
unboxing con-
versions
:
• From type
Boolean
to type
boolean
• From type
Byte
to type
byte
• From type
Short
to type
short
• From type
Character
to type
char
• From type
Integer
to type
int
• From type
Long
to type
long
• From type
Float
to type
float
• From type
Double
to type
double
At run time, unboxing conversion proceeds as follows:
• If
r
is a reference of type
Boolean
, then unboxing conversion converts
r
into
r
.booleanValue()
