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

Question

Regarding my previous Question, Why do == comparisons with Integer.valueOf(String) give different results for 127 and 128? , we know that Integer class

Answer 1

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() {}
}