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Doubt in K&B SCJP :Page 236

Jack Crifer
Greenhorn

Joined: May 18, 2008
Posts: 29
In order to save memory, two instances of the
following wrapper objects will always be == when their primitive values are the same:
* Boolean
* Byte
* Character from \u0000 to \u007f (7f is 127 in decimal)
* Short and Integer from -128 to 127


I tested Long from -128 to 127 is return TRUE when ==.
Christophe Verré
Sheriff

Joined: Nov 24, 2005
Posts: 14688
    
  16

The Java specification only states : If the value p being boxed is true, false, a byte, a char 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.

Sun's implementation also uses a cache for its Long wrapper though. It may be specific to this implementation.

The discussion in the specification goes :
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.


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