Interface vs. abstract class

Choosing interfaces and abstract classes is not an either/or proposition. If you need to change your design, make it an interface. However, you may have abstract classes that provide some default behavior. Abstract classes are excellent candidates inside of application frameworks.

Abstract classes let you define some behaviors; they force your subclasses to provide others. For example, if you have an application framework, an abstract class may provide default services such as event and message handling. Those services allow your application to plug in to your application framework. However, there is some application-specific functionality that only your application can perform. Such functionality might include startup and shutdown tasks, which are often application-dependent. So instead of trying to define that behavior itself, the abstract base class can declare abstract shutdown and startup methods. The base class knows that it needs those methods, but an abstract class lets your class admit that it doesn't know how to perform those actions; it only knows that it must initiate the actions. When it is time to start up, the abstract class can call the startup method. When the base class calls this method, Java calls the method defined by the child class.

Many developers forget that a class that defines an abstract method can call that method as well. Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.

Q: Why use interfaces to develop Java applications?
A: It is advisable to design relatively large applications using interfaces because it makes the whole system easier to modify, extend and integrate new features. To start with, you may only have one implementation of a given interface, but if you find you need slightly different behaviour in special circumstances, you only need write a class that conforms to one of the existing interfaces and it will drop in place without major modifications.

Interfaces also allow you to adapt a class from a different hierarchy to work in an existing application. The class only needs to declare it implements the interface, provide the necessary methods and it can be integrated directly as if it were created for the job.

Q: What is the difference between abstract classes and interfaces?
A: A Java interface is a definition of a class type without any concrete implementation. Typically, an interface consists of one or more method signatures that a subclass must fulfil to conform to the type. In effect, all their methods are abstract, and interfaces cannot be instantiated

Q: Do interfaces have member variables?
A: Interfaces may have member variables, but these are implicitly final and static because they are not inherited by extension. In effect interface variables are constants that are available to all implementations and may be used as key references for method arguments for example.

Q: Can we create an object for an interface?
A: Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be instantiated in their own right, so you must write a class that implements the interface and fulfil all the methods defined in it.

public class Concrete implements ExampleInterface {

...
}
Q: What is a marker interface?
A: Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The java.io.Serializable interface is a typical marker interface. It does not contain any methods, but classes must implement this interface in order to be serialized and de-serialized.

Q: Should I use a public access modifier for interface methods?
A: Java interfaces are used to define a public Application Programming Interface (API) for classes to implement, so a public modifier is redundant in this context. Non-public modifiers are not valid for interfaces, so the compiler should fail and warn you in this case.

Interface usage
Q: What are the rules for passing subclasses for method arguments?
A: It may help to discuss an example using birds: an interface called Avian, a superclass FlyingBird that implements Avian and two concrete classes: Parrot and Penguin. Parrot extends FlyingBird, so is implicitly an Avian type, but Penguin does not, it only implements Avian. The examples below work through all possibilities, passing references to methods soundBirdCall(FlyingBird) and displayPlumage(Avian)

Q: When should I use abstract classes rather than interfaces?
A: Abstract classes are are often used to provide methods that will be common to a range of similar subclasses, to avoid duplicating the same code in each case. Each subclass adds its own features on top of the common abstract methods.

Q: Can an interface be declared final?
A: It is not permitted to declare an interface as final, it will cause a compilation error. It does not make sense to declare an interface final because it contains no implementation code and cannot instantiated in its own right. The final modifier is used to prevent the extension of concrete classes.

Q: This code seems to be instantiating an interface!
A: The code you are looking at declares an inline anonymous class that implements an interface, which has a similar effect. This declaration does not instantiate the interface, but defines the type of the anonymous class, which has no name of its own. This approach is often used in AWT or Swing applications where a class is required to fulfil a minimal interface and it is not necessary to retain a reference to it by assignment.