Akshat-coder2106 opened a new pull request, #1554:
URL: https://github.com/apache/commons-lang/pull/1554
Summary
Replace the commutative XOR-based hashCode() implementation in Pair and
Triple with Objects.hash() to improve hash distribution and eliminate
collisions for permuted elements.
The Problem
The current implementation uses a simple XOR of the element hash codes:
return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
This leads to two significant issues:
Commutativity: Pair.of("A", "B") and Pair.of("B", "A") result in identical
hash codes, causing 100% collisions for permutations.
Self-Cancellation: For identical elements like Pair.of("A", "A"), the hash
code becomes 0 because X⊕X=0.
The Fix
Updated both Pair.java and Triple.java to use Objects.hash(...), which
employs a prime multiplier (31) to ensure that the order of elements
significantly impacts the resulting hash.
Modified Files:
src/main/java/org/apache/commons/lang3/tuple/Pair.java
src/main/java/org/apache/commons/lang3/tuple/Triple.java
Technical Verification
The fix was verified by adding assertions to existing test methods in
ImmutablePairTest and ImmutableTripleTest to maintain a clean test suite.
Local Results:
Permutation Check: Pair.of("A", "B").hashCode()
= Pair.of("B", "A").hashCode().
Identity Check: Pair.of("A", "A").hashCode()
=0.
Build Status: mvn clean test -Dtest=ImmutablePairTest,ImmutableTripleTest
passed with 41 successful tests.
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