Java: 128 == 128 is FALSE?!PRO

Java caches Integers from -128 to 127, so == works… until it doesn't. Plus the same trap with Long.

// 127 == 127 but 128 != 128 ?!  — Java caches Integer objects from -128 to 127.
// == compares the BOXES; outside the cache you get two different boxes.
public class Main {
    public static void main(String[] args) {
        Integer a = 127, b = 127;
        System.out.println(a == b);          // true
        Integer c = 128, d = 128;
        System.out.println(c == d);          // false ?!
        System.out.println(c.equals(d));     // true ✓
        System.out.println(c.intValue() == d.intValue()); // true ✓

        // Bonus trap: Long has the same cache
        Long x = 1000L, y = 1000L;
        System.out.println(x == y);          // false ?!
    }

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