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Quirk: BigDecimal equals vs compareTo - #105
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# Conflicts: # FirstSteps/README.md
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What you'll learn
BigDecimal.equals()compares both unscaled value and scale, so"2.0"and"2.00"are not equalBigDecimal.compareTo()only compares numeric value, so it treats them as equalHashSet(usesequals/hashCode) keeps both, whileTreeSet(usescompareTo) keeps only onenew BigDecimal(double)exposes the exact binary floating-point value, whileBigDecimal.valueOf(double)goes throughDouble.toStringfor the expected decimal;stripTrailingZeros()normalizes scale soequalsagrees withcompareToWhy it matters
Mixing
equals/hashCodewithcompareTo-based collections is a classic source of subtle bugs — deduplication that silently "half-works" depending on whichSetimplementation you pick, orBigDecimalconstants that look equal but aren't.Read more
BigDecimal#equals Javadoc
Quiz
Given
Set<BigDecimal> set = new TreeSet<>(); set.add(new BigDecimal("2.0")); set.add(new BigDecimal("2.00"));, what isset.size()?Answer
1 — `TreeSet` orders and deduplicates using `compareTo`, which ignores scale, so the second `add` is treated as a duplicate and discarded. A `HashSet` in the same scenario would keep both (size 2), since it uses `equals`/`hashCode`, which do consider scale.Note on local verification: this environment only had JDK 21 installed, and the Gradle build requires JDK 25 via toolchain; JDK 25 auto-provisioning was blocked by network policy here, so I couldn't run
./gradlewlocally. I compiled and ran the test manually against JDK 21 beforehand to check the logic, and CI (JDK 25) has now confirmed all tests pass.Closes #92