library
beginnerequals() & hashCode() Contract
Understand the contract between equals and hashCode, and what breaks when you violate it.
The equals/hashCode contract is the most important invariant in Java collections:
hashCode = a filing cabinet drawer number (tells you which drawer to look in). equals = comparing the actual document to confirm it's the right one. Wrong drawer number = you'll never find the document.
Key Concepts
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1. If a.equals(b) is true, then a.hashCode() == b.hashCode() must be true.
2. If hashCode() differs, equals() must return false.
3. Objects that are not equal MAY have the same hash code (collisions are allowed).
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If you override equals() without overriding hashCode(), HashMap/HashSet break. The map puts the key in a bucket based on hashCode(). When you look up with an equal key that has a different hashCode, it searches the wrong bucket and returns null.
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equals() rules (contract):
- Reflexive: x.equals(x) is true
- Symmetric: x.equals(y) ↔ y.equals(x)
- Transitive: if x.equals(y) and y.equals(z), then x.equals(z)
- Consistent: multiple calls return the same result if objects don't change
- x.equals(null) is always false
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hashCode() rules:
- Consistent: must return the same value during a single execution (unless equals-relevant fields change)
- Equal objects must have equal hash codes
- Use Objects.hash() for convenience, or hand-roll with prime multiplication for performance
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A good hashCode distributes values evenly across buckets. A bad hashCode (e.g., always returning 1) turns a HashMap into a linked list — O(n) instead of O(1).