Weird Integer boxing in Java

Question

I just saw code similar to this:

public class Scratch
{
    public static void main(String[] args)
    {
        Integer a = 1000, b = 1000;
        System.o         


        

Answer 1

If we check the source code of the Integer class, we can find the source of the valueOf method just like this:

public static Integer valueOf(int i) {
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
}

This explains why Integer objects, which are in the range from -128 (Integer.low) to 127 (Integer.high), are the same referenced objects during the autoboxing. And we can see there is a class IntegerCache that takes care of the Integer cache array, which is a private static inner class of the Integer class.

There is another interesting example that may help to understand this weird situation:

public static void main(String[] args) throws ReflectiveOperationException {
    Class cache = Integer.class.getDeclaredClasses()[0];
    Field myCache = cache.getDeclaredField("cache");
    myCache.setAccessible(true);

    Integer[] newCache = (Integer[]) myCache.get(cache);
    newCache[132] = newCache[133];

    Integer a = 2;
    Integer b = a + a;
    System.out.printf("%d + %d = %d", a, a, b); // The output is: 2 + 2 = 5
}