1# Best practices 2 3 4 5## <a name="interchangeable"></a>"Equals means interchangeable" 6 7Don't use AutoValue to implement value semantics unless you really want value 8semantics. In particular, you should never care about the difference between two 9equal instances. 10 11## <a name="mutable_properties"></a>Avoid mutable property types 12 13Avoid mutable types, including arrays, for your properties, especially if you 14make your accessor methods `public`. The generated accessors don't copy the 15field value on its way out, so you'd be exposing your internal state. 16 17Note that this doesn't mean your factory method can't *accept* mutable types as 18input parameters. Example: 19 20```java 21@AutoValue 22public abstract class ListExample { 23 abstract ImmutableList<String> names(); 24 25 public static ListExample create(List<String> mutableNames) { 26 return new AutoValue_ListExample(ImmutableList.copyOf(mutableNames)); 27 } 28} 29``` 30 31## <a name="simple"></a>Keep behavior simple and dependency-free 32 33Your class can (and should) contain *simple* intrinsic behavior. But it 34shouldn't require complex dependencies and shouldn't access static state. 35 36You should essentially *never* need an alternative implementation of your 37hand-written abstract class, whether hand-written or generated by a mocking 38framework. If your behavior has enough complexity (or dependencies) that it 39actually needs to be mocked or faked, split it into a separate type that is 40*not* a value type. Otherwise it permits an instance with "real" behavior and 41one with "mock/fake" behavior to be `equals`, which does not make sense. 42 43## <a name="one_reference"></a>One reference only 44 45Other code in the same package will be able to directly access the generated 46class, but *should not*. It's best if each generated class has one and only one 47reference from your source code: the call from your static factory method to the 48generated constructor. If you have multiple factory methods, have them all 49delegate to the same hand-written method, so there is still only one point of 50contact with the generated code. This way, you have only one place to insert 51precondition checks or other pre- or postprocessing. 52 53## <a name="final"></a>Mark all concrete methods `final` 54 55Consider that other developers will try to read and understand your value class 56while looking only at your hand-written class, not the actual (generated) 57implementation class. If you mark your concrete methods `final`, they won't have 58to wonder whether the generated subclass might be overriding them. This is 59especially helpful if you are *[underriding](howto.md#custom)* `equals`, 60`hashCode` or `toString`! 61 62## <a name="constructor"></a>Maybe add an explicit, inaccessible constructor 63 64There are a few small advantages to adding a package-private, parameterless constructor to your abstract class. It prevents unwanted subclasses, and prevents an undocumented public constructor showing up in your generated API documentation. Whether these benefits are worth the extra noise in the file is a matter of your judgment. 65