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asmhatreandClaude Sonnet 5 af615cc17c 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

2.4 KiB

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arithmetic

Companion code for the ankurm.com post "Add Two Numbers in Java Without Overflow." Eighth module in java-core-examples.

a + b on two Java ints never throws when the true mathematical result doesn't fit - it wraps around silently, two's-complement style. This module reproduces that wraparound with real numbers, then works through every real fix: the Math class's *Exact() family (which throws ArithmeticException instead of wrapping), widening to long, BigInteger for unbounded precision, and the actual bit trick Math.addExact uses internally to detect overflow without doing the addition twice at different widths.

Versions this was built and tested against

Component	Version	Notes
JDK	25.0.4.1+1 (Temurin, LTS)	No preview flags needed - every API used here is finalized.
JUnit Jupiter	5.11.0	Includes a 100,000-iteration randomized cross-check per repeat.
Maven	3.9.11
Hardware	2 vCPU x86-64 VM	Same sandbox as the rest of this series.

Quickstart

export JAVA_HOME=/path/to/jdk-25
mvn package
java -cp target/classes com.ankurm.arithmetic.OverflowWrapDemo
java -cp target/classes com.ankurm.arithmetic.ExactArithmeticDemo

scripts/run-all.sh regenerates every file in output/ (needs JDK25_HOME).

What's in here

File	What it shows
`.../OverflowWrapDemo.java`	`int` and `long` addition and multiplication wrapping silently past their max values - no exception, just a wrong answer.
`.../ExactArithmeticDemo.java`	The `Math` `*Exact()` family: `addExact`, `subtractExact`, `multiplyExact`, `incrementExact`, `decrementExact`, `negateExact`, `toIntExact`, `absExact` - normal case and overflow-throws case for each, plus the `Math.abs(Integer.MIN_VALUE)` trap that `absExact` exists to catch.
`.../WideningAndBigIntegerDemo.java`	Widening to `long` fixes the `int`-range overflow (but `long` can still overflow); `BigInteger` never overflows at all.
`.../OverflowBitTrickDemo.java`	The XOR-based overflow test for addition, cross-checked against `Math.addExact()` on the same inputs.
`src/test/.../ArithmeticClaimsTest.java`	Asserts every claim above, including a 100,000-sample-per-repeat randomized cross-check of the bit-trick detector against `BigInteger` ground truth.
`output/01`-`04`	Each demo's real run.
`output/05`	JUnit correctness run.

License

MIT - see the repo-wide LICENSE.

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