Object-oriented programming · interview question

Which statements about the equals/hashCode contract are true? Select all that apply.

Select all that applyMedium

If `a.equals(b)` is true, then `a.hashCode() == b.hashCode()` must be true.If two objects have the same hash code, they must be equal.Overriding `equals` without `hashCode` breaks lookups in `HashMap` and `HashSet`.`equals` must be symmetric: `a.equals(b)` implies `b.equals(a)`.`hashCode` must return a unique value for every object.

Answer and explanation

Correct answer

A. If a.equals(b) is true, then a.hashCode() == b.hashCode() must be true.
C. Overriding equals without hashCode breaks lookups in HashMap and HashSet.
D. equals must be symmetric: a.equals(b) implies b.equals(a).

Why

Hash-based collections find the bucket by hashCode, then compare with equals. Equal objects with different hash codes land in different buckets and can't be found.

References

Joshua Bloch, Effective Java, 3rd ed. (2018)
Java SE 21 API documentation

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Which statements about the equals/hashCode contract are true? Select all that apply.

Correct answer

Why

References

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