Regarding my previous Question, Why do == comparisons with Integer.valueOf(String) give different results for 127 and 128? , we know that Integer class has a cache which stores values between -128 and 127.
Just wondering, why between -128 and 127?
Integer.valueOf() documentation stated that it is "caching frequently requested values" . But are values between -128 and 127 frequently requested for real? I thought frequently requested values were very subjective.
Is there any possible reason behind this?
The documentation also stated: "[...] and may cache other values outside of this range."
How can this be achieved?
Just wondering, why between -128 and 127?
A larger range of integers may be cached, but at least those between -128 and 127 must be cached because it is mandated by the Java Language Specification (emphasis mine):
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 r1 and r2 be the results of any two boxing conversions of p. It is always the case that r1 == r2.
The rationale for this requirement is explained in the same paragraph:
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. [...]
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.
How can I cache other values outside of this range.?
You can use the -XX:AutoBoxCacheMax JVM option, which is not really documented in the list of available Hotspot JVM Options. However it is mentioned in the comments inside the Integer class around line 590:
The size of the cache may be controlled by the
-XX:AutoBoxCacheMax=<size>option.
Note that this is implementation specific and may or may not be available on other JVMs.