FN: `EqualsUsingHashCode` misses equals() that extracts hashCode() into locals

[Chordrain]

Error Prone version

Error Prone 2.49.0 (javac plugin) — javac 21.0.8, source/target 21.

Check name

EqualsUsingHashCode — https://errorprone.info/bugpattern/EqualsUsingHashCode

Description

EqualsUsingHashCode flags equals() implementations that decide equality by comparing hash codes (fragile, since hashes collide). The matcher only recognizes the direct shape hashCode() == o.hashCode(). Extracting the two hashCode() calls into locals before the == comparison is a runtime-equivalent refactor, but the rule no longer fires.

Reproducer

// BEFORE — 1 finding
public class C {
  @Override
  public boolean equals(Object o) {
    return hashCode() == o.hashCode();
  }
}

// AFTER — equivalent rewrite, 0 findings
public class C {
  @Override
  public boolean equals(Object o) {
    int a = hashCode(); int b = o.hashCode(); return a == b;
  }
}

Expected behavior

EqualsUsingHashCode should report the same finding on BEFORE and AFTER. The AFTER equals() still decides equality solely by comparing the two hash codes.

Actual behavior

• BEFORE: 1 finding (Implementing #equals by just comparing hashCodes is fragile ...).
• AFTER: 0 findings.

The matcher only recognizes the direct o.hashCode() == hashCode() comparison and misses the trivially-equivalent extracted-local form.