Why Does 1000 == 1000 Return false but 100 == 100 Return true in Java?

Running the code shows false for 1000 == 1000 and true for 100 == 100.

In Java, the == operator compares object references; two distinct Integer objects are unequal even if they hold the same value.

The surprising true result for 100 comes from the IntegerCache inside Integer.java, which caches Integer objects for values between -128 and 127.

When an Integer literal in that range is created, the compiler uses Integer.valueOf(int), which returns the cached instance if the value lies within the cache bounds:

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

Thus the two variables c and d both reference the same cached object, making c == d evaluate to true.

The cache reduces memory usage because small integers are used far more frequently than larger ones.

The article also shows how to access the private cache via reflection, allowing manipulation of the cached array:

Class cache = Integer.class.getDeclaredClasses()[0];
Field myCache = cache.getDeclaredField("cache");
myCache.setAccessible(true);
Integer[] newCache = (Integer[]) myCache.get(cache);
newCache[132] = newCache[133];
int a = 2;
int b = a + a;
System.out.printf("%d + %d = %d", a, a, b);