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Fundamentals · Hard · Free

Why must equals() and hashCode() stay consistent in Java?

FresherFundamentalsOOP & Language Fundamentals
Model answer

equals and hashCode contract

If two objects are equal per equals(), they must have the same hashCode. Violating this breaks HashMap/HashSet—objects become "lost" in buckets.

java
@Override public boolean equals(Object o) {
  if (!(o instanceof User u)) return false;
  return id.equals(u.id);
}
@Override public int hashCode() { return Objects.hash(id); }
Say you use IDE generate or Objects.hash—never override equals without hashCode.