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