Today is my first time to try Java language. When I try this code, I feel very strange:
int a =500;
System.out.println(System.identityHashCode(500));
System.out.println(System.identityHashCode(500));
System.out.println(System.identityHashCode(a));
System.out.println(System.identityHashCode(a));
All of these results is different. But when I changed 500 to 50, It become the same result.
Why is it?
But when I changed 500 to 50, It become the same result.
Autoboxing caches the conversion of primitives to Object. Small values get the same object, larger values do not.
Note: while values between -128 and 127 are always cached, Higher values can be cached depending on command line settings. See the source for Integer for more details.
This is also called a Flyweight Pattern
You can set the maximum size of the Integer cache with
-Djava.lang.Integer.IntegerCache.high=NNNN
-XX:AutoBoxCacheMax=NNNN
-XX:+AggressiveOpts // sets it higher depending on the version e.g. 10000
http://martykopka.blogspot.co.uk/2010/07/all-about-java-integer-cache.html
http://www.javaspecialists.eu/archive/Issue191.html
I feel very strange
I know what you mean reading this question. ;)
private static class IntegerCache used by Integer.valueOf(int i), which is what reminded me of memoization.
IntegerCache is the data structure and valueOf() is the static factory that recycles cached instances. +1 from earlier, BTW.It caches the int value -128 and 127 (inclusive), So it will refer to same instance in memory within that range
When you pass primitive value to (here 10)
System.identityHashCode(10);
It autoboxes it to Integer object and it inturns uses valueOf() method of Integer class for conversion
For Example
Integer a = 10;
will get converted in to internally it uses valueOf()
1: invokestatic #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
Integer.valueOf() which has got the cache implementation
public static Integer valueOf(int i) {
if(i >= -128 && i <= IntegerCache.high)
return IntegerCache.cache[i + 128];
else
return new Integer(i);
}
so if you pass the value from -128 to 127(inclusive) it will use the cached version as you can see from
if(i >= -128 && i <= IntegerCache.high)
return IntegerCache.cache[i + 128];
See Also
500.