Why does Java evaluate 1000==1000 as false while 100==100 evaluates to true?

In Java, the == operator compares object references, not their numeric values. When two Integer objects refer to the same cached instance, == returns true; otherwise it returns false.

Running the following code illustrates the difference:

Integer a = 1000, b = 1000;
System.out.println(a == b); //1
Integer c = 100, d = 100;
System.out.println(c == d); //2

produces the output:

false
true

The reason is that Java caches Integer objects for the range -128 to 127 in an internal class IntegerCache. Assignments such as Integer c = 100; are actually compiled to Integer i = Integer.valueOf(100); and valueOf returns a cached instance when the value lies within the cache range:

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

Consequently, both c and d reference the same cached object, making c == d evaluate to true. For values outside the cache (e.g., 1000), new objects are created, so a == b is false.

The article also shows how to manipulate the cache via reflection:

public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
    Class cache = Integer.class.getDeclaredClasses()[0]; //1
    Field myCache = cache.getDeclaredField("cache"); //2
    myCache.setAccessible(true); //3

    Integer[] newCache = (Integer[]) myCache.get(cache); //4
    newCache[132] = newCache[133]; //5

    int a = 2;
    int b = a + a;
    System.out.printf("%d + %d = %d", a, a, b);
}

This demonstrates that the cache can be altered, which may affect the result of == comparisons for cached values.