Why Integer class caching values in the range -128 to 127?

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= option.

Note that this is implementation specific and may or may not be available on other JVMs.

-128 to 127 is the default size. But javadoc also says that the size of the Integer cache may be controlled by the -XX:AutoBoxCacheMax=<size> option. Note that it sets only high value, low value is always -128. This feature was introduced in 1.6.

As for why -128 to 127 - this is byte value range and it is natural to use it for a very small cache.

The reason for caching small integers, if that's what you're asking, is that many algorithms use small integers in their calculations, so avoiding the object-creation overhead for these values tends to be worthwhile.

The question then becomes which Integers to cache. Again, speaking in general, the frequency with which constant values are used tends to decrease as the absolute value of the constant increases -- everyone spends a lot of time using the values 1 or 2 or 10, relatively few few use the value 109 very intensively; fewer will have performance depend on how quickly one can obtain an Integer for 722.. Java chose to allocate 256 slots spanning the range of a signed byte value. This decision may have been informed by analyzing programs in existence at the time, but is just as likely to have been a purely arbitrary one. It's a reasonable amount of space to invest, it can be accessed rapidly (mask to find out if the value's in the cache's range, then a quick table lookup to access the cache), and it will definitely cover the most common cases.

In other words, I think the answer to your question is "it isn't as subjective as you thought, but the exact bounds are largely a rule-of-thumb decision ... and experiemental evidence has been that it was good enough."

Max high integer value that can be cached can be configured through system property i.e java.lang.Integer.IntegerCache.high(-XX:AutoBoxCacheMax) . The cache is implemented using an array.

    private static class IntegerCache {
    static final int high;
    static final Integer cache[];

    static {
        final int low = -128;

        // high value may be configured by property
        int h = 127;
        if (integerCacheHighPropValue != null) {
            // Use Long.decode here to avoid invoking methods that
            // require Integer's autoboxing cache to be initialized
            int i = Long.decode(integerCacheHighPropValue).intValue();
            i = Math.max(i, 127);
            // Maximum array size is Integer.MAX_VALUE
            h = Math.min(i, Integer.MAX_VALUE - -low);
        }
        high = h;

        cache = new Integer[(high - low) + 1];
        int j = low;
        for(int k = 0; k < cache.length; k++)
            cache[k] = new Integer(j++);
    }

    private IntegerCache() {}
}

When you encounter with Integer class and always boxed within the range -128 to 127 it's always better to convert the Integer object into int value as below.

<Your Integer Object>.intValue()