Why can comparing two Integer objects with == sometimes return true and sometimes false?

Integer caches boxed values from -128 to 127 (the IntegerCache), so == on two autoboxed values in that range compares the same cached reference and returns true.

Outside that range each autoboxing creates a new object, so == returns false even when the numeric values are equal. The lesson is to always use .equals() or unbox to int before comparing, and to never rely on identity for boxed numbers.