We already learned what an abstract class is and what an interface is. We will now do a detailed comparison of interfaces and abstract classes, followed by a listing of points in favor of each of them, as this is also a very common interview question in Java.

Comparison of basic features of interfaces and abstract classes

1. Both abstract class and interfaces are abstractions in Java that cannot be instantiated. The main difference is that an abstract class is a class and an interface is a contract or specification without any implementation. Also, an abstract class can be invoked if it has a main method and can be used for testing its static methods.

2. Except that an abstract class cannot be instantiated, all other functionality of a regular class still exists, and its fields, methods, and constructors are all accessed in the same way. There is a common misconception among some that abstract classes can’t have constructors because they cannot be instantiated. Abstract classes have constructors and they are invoked by the subclasses using super from the subclass constructors.

3. An abstract classes can have state just like any other class. However an interface can't have behavior or state. “No behavior” is enforced by not allowing methods to have implementation, whereas abstract classes can have concrete methods and “no state” is enforced by making the fields public, static and final by default, whereas abstract classes can have non-final variables and we instantiate variables in an abstract class through constructors. All methods of an interface must be abstract, whereas abstract classes can have abstract methods and/or concrete methods. An abstract class without any abstract methods is still a valid abstract class.

4. You implement an interface using the implements keyword whereas you extend a class using the extends keyword.

Points that favor interfaces

Points that favor interfaces are also the points that are against abstract or concrete classes.

1. Java permits multiple-inheritance only through interfaces, but not through classes. A class can implement multiple interfaces, but can extend only one class. An exception to this rule is local inner classes and anonymous inner class, which can both implement or extend maximum one interface or class and not both together.

2. Existing classes can be easily modified to implement a new interface irrespective of whether it already implements another interface. But you can extend a new class only if it is not already extending another class.

3. Sometimes a class might belong to more types that does not belong to a rigid type hierarchy.  Interfaces allow the construction of these non hierarchical type frameworks. We can implement all those types or create a type which is the combination of all those interfaces. Without interfaces, we would have to use a separate class for every combination known as a bloated class hierarchy.

4. The limitation of an interface that it cannot provide any skeletal implementations can be overcome by providing implementation assistance through an abstract skeletal implementation class corresponding to an interface. Examples for this practice in Java are AbstractCollection, AbstractList, AbstractSet, AbstractMap etc. Here even if we have interfaces and abstract class, while defining a type, we should use an interface (like myMethod(Map m)) and we can either pass in an existing implementation or one we created by extending an abstract implementation. By using an interface as the type, you can change the actual implementation later without any change to the code (here myMethod()).   

Points in favor of abstract classes

Points in favor of abstract (or concrete classes) are also the points that are against interfaces.

1. It is practically not possible to add methods to an interface after it is released without changing existing classes; you have to make all its implementations implement that new method, or existing implementation classes will fail. However you can add a concrete method to an abstract class without breaking the existing classes. This is one case where defining a type in terms of abstract class has advantage.

2. It is generally a good practice to minimize the accessibility of classes and members and is an important consideration during information hiding or encapsulation. However if a class implements an interface, all of the class’s methods from interface must be declared public as all members of an interface are implicitly public. During inheritance, subclass method cannot have a lower level access than superclass method making sure that an instance of a subclass is usable anywhere an instance of super class is usable.

3. Abstract classes can be used to provide skeletal implementation, whereas interfaces cannot. A class that implements an interface must provide an implementation of all the methods of that interface. Abstract classes can be inherited without implementing the abstract methods if the derived class itself is an abstract. However this can be overcome by providing implementation assistance through an abstract skeletal implementation class.

4. Interfaces are considered to be a bit slower than abstract classes because it requires an extra indirection during virtual table lookup. A simple virtual table could be used for directly inherited methods, but finding the code that implements an interface method would involve following a pointer to a table of interfaces implemented by a class, then searching through the table to find a virtual table for the implementation of that interface. However modern JVMs are a lot smarter, and most of the lookup is done during dynamic compilation and hence the runtime differences are small or nonexistent. You can refer to the wiki link http://en.wikipedia.org/wiki/Virtual_method_table for details on virtual table.

5. An interface can extend only interfaces; an abstract class can extend another class and implement interfaces.

Summary

Both abstract classes and interfaces have their uses and that is why both of them are still there in the language. In general, you should use interfaces to define types along with a skeletal implementation if required. However if your type itself can change by adding new methods in future, you should consider using an abstract class to define types. In most code I have seen or written, interfaces are used to define types and abstract classes are used for framework code, where each layer adds some functionality to the previous layer and leaving some application specific functionality to be implemented by the application.