API Design: Interfaces versus Abstract Classes
By darcy on May 12, 2008
Jake Gittes: Why are you doing it?
How much better can you eat?
What can you buy that you can't already afford?
Noah Cross: The future, Mr. Gitts, the future.
Quoting, Effective Java, first edition, Item 16: Prefer Interfaces to abstract classes
To summarize, an interface is generally the best way to define a type that permits multiple implementations. An exception to this rule is the case where ease of evolution is deemed more important than flexibility and power.
As discussed in that item, the ease of evolution of abstract classes comes from the ability to add new methods having "reasonable default implementations" without almost surely causing source of all existing subtypes to no longer compile. The flexibility and power of interfaces involve ease of retrofitting to existing classes, allowing nonhierarchical type relations, and so on. An additional benefit of interfaces is the ability to use dynamic proxies; one notable use of dynamic proxies is creating the annotation objects returned at runtime by getAnnotation. One potential difference not worth considering with modern virtual machines is the speed difference between invoking a method on an interface versus invoking a method on a class.
While there is a sound rationale backing the conventional wisdom, in my estimation the compatible evolution advantages of abstract classes are smaller than they appear at first, further tipping the balance in favor of using interfaces in more situations.
The two alternatives to be considered to define the initial desired type abstraction are:
Declare an interface.
Declare an abstract class, all of whose initial methods are public and abstract.
In neither case are fields being defined. In both cases a skeletal abstract implementation class, like java.util.AbstractList, could be used to share implementation code. If the type abstraction is defined by an abstract class, the skeletal class and abstract class might be able to be combined, saving a type compared to the pair of an interface plus a skeletal class. However, forcing all implementations to be based on the same skeletal class may be awkward. Interfaces can easily have multiple independent skeletal helper classes. Subclasses can blunt inheritance issues by using an intermediate subclass to abstract-ify any problematic implementations from the parent.
Table 1 outlines the different kinds of compatibility impacts, source, binary, and behavioral, from adding a method to an interface and an abstract class. The effects of adding a method to an abstract class depend on whether or not the added method is abstract or has an implementation. For the purposes of discussion, we will assume the method does have an implementation (otherwise, there would be no advantage to using an abstract class).
|Binary compatibility||Adding a method to an interface is binary compatible. Note that existing clients will continue to link, but attempted calls to the missing new method will result in an AbstractMethodError.||Adding a method to an abstract class is binary compatible.|
|Source compatibility||Adding a method to an interface has the full range from possible impacts, from being binary-preserving source compatible to breaking compilation.||Adding a method to an abstract class has the full range from possible impacts, from being binary-preserving source compatible to breaking compilation.|
|Behavioral compatibility||No direct behavioral impact to existing code calling existing methods.||No direct behavioral impact for the cases under consideration.|
Technically, adding a method to an interface and adding a method to an abstract class are both binary compatible since programs using those types will continue to link. However, in the case of an interface type, if a program calls the new method on an existing implementation of the interface (unless the implementation presciently had a method with a matching signature declared), an AbstractMethodError will be thrown, which is an awkward situation to recover from. Also, for the call to the new interface method to work on an existing implementor of the old interface, the method in the implementor must be an exact match, signature and return type, for the added method; if the return type in the implementor is a subtype of the added method, a covariant return, a recompile of the implementor is needed to create the bridge method joining the method from the interface with the method declared in the class.
Adding a method to an interface has a wide range of possible source compatibility effects on existing code. It is possible that an implementation anticipated future developments and already has a method matching the newly added method. In that case, adding the method is binary-preserving source compatible with that particular class. Of course in general it is much more likely that existing implementations do not already have the new method, in which case they won't compile against the modified interface declaration. Therefore, the worst possible outcome is that existing implementations will stop compiling after the method is added to the interface; this worst case outcome is also the most likely outcome in the absence of other information.
Adding a concrete method to an abstract class also has a range of source compatibility outcomes. If no existing extending class has a method with the new name, there is no conflict and the addition is binary-preserving source compatible given the set of actual programs. If not the expected outcome, this is certainly the hoped for outcome of adding a method to an abstract class! However, it is possible existing subclass already declare a method with the new name. If the parameter types match but the return types conflict, existing subclasses will stop compiling after the method is added. If the parameter types are not the same, an overloading situation is introduced or expanded. This can change method resolution of call sites using the existing subclass, which may or may not lead to behaviorally equivalent class files since different methods might be called. One technique to avoid changing resolution at existing call sites is for the new method to include in its parameter list a new type added at the same time as the method. If the new type is not related to existing types, then no method in an existing subclass will interact with the new method during method resolution. Therefore, the worst possible outcome is that some existing subclasses will stop compiling after the method is added to the abstract class; this can be avoided depending on the parameter list of the new method, at the potential cost of introducing new overloadings that change existing method resolution.
Not counting introspective operations like core reflection, adding methods to an interface or abstract class does not have much direct appreciable behavioral compatibility impact because adding methods doesn't directly affect the code run by existing clients of the class. If an abstract class were not at the conceptual root of a type hierarchy, adding a concrete method could intercept calls to a method with the same signature in the superclass. However, if the children of an abstract superclass already have a concrete implementation for the newly added method, existing calls to the children's method would not be intercepted by the method added in the superclass.
Since adding a method to an interface or an abstract class is binary compatibly and in both cases the worst case source compatibility outcome is breaking compilation of existing subtypes, any evolution advantage of abstract classes hinges on the ability to have a reasonable default implementation for new methods. But what can such a new method implementation really do? Some viable options are:
Throw new UnsupportedOperationException or some other exception.
Call existing methods on the abstract class.
A no-op method.
(Other sorts of behavior could potentially be added to skeletal classes, but those classes aren't an alternative to interfaces.) Adding a default implementation that throws an exception isn't necessarily very useful; throwing AbstractMethodError would mimic adding a method to an interface! If the functionality of the new method can be expressed in terms of existing methods on the abstract class, the new method could also be written as convenience static method in a helper class. In that case, the convenience method could just as easily be written in terms of methods on an interface instead. Proposals for extension methods would add syntactic support for this helper class pattern. A no-op method could be added to optionally advise subclasses to some condition or event, but it would have no useful effect on existing subclasses. While it is straightforward to add simple concrete methods to an abstract class, with sufficient advance planning, such methods could also be automatically added to implementations of an interface at compile time.
Starting in JDK 6, Java compilers must support standardized annotation processing. Annotation processing is a general meta-programming framework not directly tied to annotations. Before annotation processing, the types being compiled can be incomplete, including references to types to be generated during annotation processing. The to-be-generated types can include the superclass of a class being compiled. Supporting the generation of superclasses is a very powerful technique for modifying the semantics of the child class. In this case, a class implementing an interface expected to change in the future could refer to a private superclass. With the original definition of the interface, the superclass would be empty. However, when methods were added to the interface, the annotation processor could generate implementations of those methods in the superclass. This would have the effect of adding the new methods to the class at compile time. Annotations could drive what the synthesized implementation actually did, such as throw an exception or a no-op.
Compared to adding methods to an interface, adding concrete methods to an abstract class seems to be much more compatible. However, both operations are binary compatible, and while adding a method to an abstract class usually has a better "average" impact on existing subtypes, the worst possible impact is the same, breaking the compilation of existing code. As for the functionality that can be added in a concrete method, convenience methods can be put in separate class and the other sorts of limited functionality methods that can readily be added could also be generated via annotation processing for implementors of an interface. Therefore, the practical evolutionary benefits of using an abstract class rather than an interface should be considered carefully since interfaces may still be a better choice when limited evolution is anticipated.