The Java Language Specification, Third Edition

CONVERSIONS AND PROMOTIONS Boxing Conversion 5.1.7
◆ If p is not NaN, boxing conversion converts p into a reference r of class and
type Double, such that r.doubleValue() evaluates to p
◆ Otherwise, boxing conversion converts p into a reference r of class and type
Double such that r.isNaN() evaluates to true.
• If p is a value of any other type, boxing conversion is equivalent to an identity
conversion (5.1.1).
If the value p being boxed is true, false, abyte, achar in the range \u0000 to
\u007f, or an int or short number between -128 and 127, then let r1 and r2 be
the results of any two boxing conversions of p. It is always the case that r1 ==
r2.
DISCUSSION
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 characters and shorts, as well as integers
and longs in the range of -32K - +32K.
DRAFT
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 allocated and insufficient storage is available.
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