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OCPJP(83%)
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OCPJP(83%)
ankur trapasiya wrote:Is it maintained for each wrapper class ??? e.g Short,Byte etc ... ???
Adolfo Eloy
Software Developer
OCPJP 6
G
... 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.
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.
Adolfo Eloy
Software Developer
OCPJP 6
G
Gary Marshall wrote: I understand that caching of certain wrapper objects will occur. Further, I understand that the Character, Short and Integer wrapper objects will be cached IF their given values are within a certain range. For example, for Short and Integer that range is -128 to +127. If the given value of a Short or Integer object is outside of this stated range of values then caching of that Short or Integer object will NOT occur, and therefore, if their given values are the same, "==" on those objects will be "false" and ".equals" (dot equals) will be "true".
Am I understanding this correctly?
The JLS specifies the data types and range values that is to be cached. It does *not* state what happens to the data types and values outside of the range. It is within specification for an implementation to cache data types and ranges larger than required.
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Gary Marshall wrote:
OK. By "larger than required" are you referring to, for example, an Integer whose given value is less than -128 or greater than +127? Are saying that the JVM will cache a data element when its given type and range is outside of a specified range, meaning that an Integer type that has a value of 129 will be cached? And then another Integer type with the identical value of 129 will always be "=="?
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