For any two Java objects that compare equal with equals(), hashCode() must return the same integer. Implement both methods from the same equality-defining state: this is the key to making value objects behave correctly in hash-based collections.
The contract your implementation must satisfy
Oracle’s Java SE 26 Object API states that equal objects must produce the same hash code. The API also says it is generally necessary to override hashCode() whenever you override equals(). If you define value equality but leave the inherited hash implementation in place, equal instances may yield different hashes, breaking the expectations of hash-based collections such as HashMap and HashSet.
- Required: If
a.equals(b)is true, thena.hashCode() == b.hashCode()must also be true. - Allowed: Unequal objects can have the same hash code. A hash code is not a unique identifier and does not prove that two objects are equal.
- Consistent during an execution: A given object’s hash should remain the same while information used in equality remains unchanged.
- Not guaranteed across executions: The contract does not require a hash value to stay the same between separate application runs.
These rules are set out in Oracle’s Object.hashCode() contract. Treat the result as an input to collection behavior, not as a durable identifier to store or exchange.
Choose the fields from your equality rule
Before writing hashCode(), decide what makes two instances equal. Use the same equality-relevant state in both methods. If equals() ignores a field but hashCode() includes it, two objects that compare equal could return different hashes, violating the contract.
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For example, if a person value is defined by a normalized identifier and a region, both methods should use those properties and should agree on how they treat normalization, nulls, and other special cases. The exact fields and comparisons depend on your class’s intended semantics; the important point is to keep the two methods aligned.
Oracle’s Java tutorial on hashCode() explains the role of the method in the object contract. A useful review question is: whenever equals() returns true for two instances, can every value used to calculate their hashes differ? If yes, the implementation needs correction.
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Implementing an ordinary value class
For a class with several equality-relevant fields, Objects.hash(...) is a concise option. For example:
import java.util.Objects;
final class Coordinate {
private final int x;
private final int y;
Coordinate(int x, int y) {
this.x = x;
this.y = y;
}
@Override
public boolean equals(Object other) {
if (this == other) return true;
if (!(other instanceof Coordinate)) return false;
Coordinate that = (Coordinate) other;
return x == that.x && y == that.y;
}
@Override
public int hashCode() {
return Objects.hash(x, y);
}
}Here, both methods use x and y. The API for java.util.Objects describes Objects.hash(...) as hashing the supplied values as if they were placed in an array. It is a convenience method; it does not promise a particular algorithm or exact result.
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When direct hashing may be preferable
For performance-sensitive code, or when working with a single field, you may choose a direct implementation suited to the field types rather than calling Objects.hash(...). The cited API documents the helper’s semantics but does not provide benchmarks, so make this choice based on your application’s requirements and measurements rather than assuming a performance difference.
Pay special attention to the single-argument case: Objects.hash(oneField) does not simply return oneField.hashCode(). Oracle explicitly documents this caveat in the Objects API. For primitive fields, a direct calculation must also account for the relevant primitive type; do not apply a reference field’s hashing operation to a primitive without an appropriate conversion or combination.
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Records: usually keep the generated methods
Java records provide generated equals() and hashCode() implementations based on their components. For a record whose component-based equality matches the value semantics you want, you usually do not need to override either method. Oracle’s Record API leaves the precise hash algorithm unspecified; it may change as long as the contract is satisfied.
Override a record method only when you need deliberate behavior beyond the generated component-based semantics, and ensure any custom equality and hash behavior remain consistent. Tests should verify the contract rather than assert one exact generated hash integer.
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Review and test the contract
Tests should check the relationships the API requires, not a particular hash value. For a class with value equality, cover at least these cases:
- Two separately created instances with the same equality-relevant values compare equal and have identical hash codes.
- Changing an equality-relevant value makes the instances unequal where expected; a different hash is not guaranteed, because collisions are allowed.
- Changing a field ignored by equality does not make equal instances return different hashes.
- For a record relying on generated behavior, verify equality and equal-hash behavior without pinning the result to a specific integer.
Remember that a mutable object’s hash can change if a field used in equality changes. Mutating an object while it is being used as a key in a hash-based collection can make it difficult to find again. Prefer immutable equality-relevant state for keys, or avoid changing that state while the object is stored in such a collection.
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