asmhatreandClaude Sonnet 5
af615cc17c
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
48 lines
2.4 KiB
Markdown
48 lines
2.4 KiB
Markdown
# arithmetic
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Companion code for the ankurm.com post *"Add Two Numbers in Java Without Overflow."* Eighth
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module in `java-core-examples`.
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`a + b` on two Java `int`s never throws when the true mathematical result doesn't fit - it wraps
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around silently, two's-complement style. This module reproduces that wraparound with real
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numbers, then works through every real fix: the `Math` class's `*Exact()` family (which throws
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`ArithmeticException` instead of wrapping), widening to `long`, `BigInteger` for unbounded
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precision, and the actual bit trick `Math.addExact` uses internally to detect overflow without
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doing the addition twice at different widths.
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## Versions this was built and tested against
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| Component | Version | Notes |
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|---|---|---|
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| JDK | 25.0.4.1+1 (Temurin, LTS) | No preview flags needed - every API used here is finalized. |
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| JUnit Jupiter | 5.11.0 | Includes a 100,000-iteration randomized cross-check per repeat. |
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| Maven | 3.9.11 | |
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| Hardware | 2 vCPU x86-64 VM | Same sandbox as the rest of this series. |
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## Quickstart
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```bash
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export JAVA_HOME=/path/to/jdk-25
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mvn package
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java -cp target/classes com.ankurm.arithmetic.OverflowWrapDemo
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java -cp target/classes com.ankurm.arithmetic.ExactArithmeticDemo
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```
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`scripts/run-all.sh` regenerates every file in `output/` (needs `JDK25_HOME`).
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## What's in here
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| File | What it shows |
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|---|---|
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| `.../OverflowWrapDemo.java` | `int` and `long` addition and multiplication wrapping silently past their max values - no exception, just a wrong answer. |
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| `.../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. |
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| `.../WideningAndBigIntegerDemo.java` | Widening to `long` fixes the `int`-range overflow (but `long` can still overflow); `BigInteger` never overflows at all. |
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| `.../OverflowBitTrickDemo.java` | The XOR-based overflow test for addition, cross-checked against `Math.addExact()` on the same inputs. |
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| `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. |
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| `output/01`-`04` | Each demo's real run. |
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| `output/05` | JUnit correctness run. |
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## License
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MIT - see the [repo-wide LICENSE](../LICENSE).
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