arithmetic: companion code for the Add Two Numbers Without Overflow post

Silent int/long overflow reproduced with real numbers, the Math *Exact() family (addExact/subtractExact/multiplyExact/incrementExact/decrementExact/ negateExact/toIntExact/absExact) with the abs(MIN_VALUE) trap absExact exists to catch, widening to long vs BigInteger, and the XOR-based overflow bit trick Math.addExact uses internally, cross-checked against BigInteger ground truth over 100k random int pairs per JUnit repeat. Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01FhzLY5p6okFva3qsnsRyvM
2026-09-30 09:16:49 +00:00
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+$ java ExactArithmeticDemo
+
+=== addExact / subtractExact / multiplyExact: normal case ===
+Math.addExact(20, 22)      = 42
+Math.subtractExact(50, 8)  = 42
+Math.multiplyExact(6, 7)   = 42
+
+=== addExact / multiplyExact: overflow case - throws instead of wrapping ===
+Math.addExact(Integer.MAX_VALUE, 1) threw: integer overflow
+Math.multiplyExact(100000, 100000) threw: integer overflow
+
+=== incrementExact / decrementExact / negateExact ===
+Math.incrementExact(41)              = 42
+Math.decrementExact(43)              = 42
+Math.negateExact(-42)                = 42
+Math.incrementExact(Integer.MAX_VALUE) threw: integer overflow
+
+=== toIntExact: the safe narrowing cast from long back to int ===
+Math.toIntExact(2000000000L)         = 2000000000
+Math.toIntExact(3000000000L) threw: integer overflow
+
+=== The classic trap: Math.abs(Integer.MIN_VALUE) does NOT throw, and is wrong ===
+Math.abs(Integer.MIN_VALUE)          = -2147483648  (still negative! +2147483648 doesn't fit in an int)
+Math.absExact(Integer.MIN_VALUE) threw: Overflow to represent absolute value of Integer.MIN_VALUE