Q10: What is the equals() and hashCode() contract, and why must both be overridden together?

The contract dictates:

If a.equals(b) is true, then a.hashCode() == b.hashCode() MUST be true.
If a.hashCode() == b.hashCode(), a.equals(b) may or may not be true (hash collision). If you override .equals() without .hashCode(), key lookups in hash-based collections (HashMap, HashSet) will fail because duplicate objects resolve to different bucket indices.

Overriding equals and hashCode

public class Person {
    private final int id;
    private final String name;

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Person person)) return false;
        return id == person.id && Objects.equals(name, person.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, name);
    }
}

💡 Both methods evaluate the exact same identity fields to satisfy the Java collection contract.

Key Takeaways

Equal objects MUST have equal hash codes.
Failing to override hashCode() breaks HashMap and HashSet lookups.

Knowledge Check

What happens if two distinct instances return the exact same hashCode() value?

Q10: What is the equals() and hashCode() contract, and why must both be overridden together?

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