Section 10.3 Interfaces and Polymorphism
Like a class, an interface can be used as a type. Variables may be defined as type interface. Furthermore, any instance of a class that implements an interface may be assigned to a variable of type interface. When a derived class object is assigned to a variable of type base class, only methods implemented in the base class may be accessed using the base class typed variable. Similarly, when an object that implements an interface is assigned to a variable of type interface, only methods included in the interface definition may be accessed through the variable.
You will recognize this ability to assign any object that implements an interface to a variable of type interface as a form of subtype polymorphism, also known as polymorphic reference variables. In addition to inheritance, interfaces represent the second form of subtype polymorphism available in Java.
Subtype polymorphism joins method polymorphism as the two broader forms of polymorphism implemented by Java. Recall that method polymorphism itself has two forms, that is, overloading of a method by defining two method having the same name but different signatures, and overriding a method in a base class by redefining the exact same signature in a dervied class.
With the option to implement multiple interfaces, a single Java class has the ability to "morph" into multiple other types. This ability provides a kind of enhancement over what is possible with inheritance alone. An object can take on as many forms as the number of interfaces it implements. In this way, it can become the subject of a wide variety of algorithms, each knowing that all methods in a given interface are implemented fully.
To summarize, refer to Figure 10.3.1. The two broad types of polymorphism in Java are subtype polymorphism and method polymorhism. Subtype polymorphism is divided into inheritance (also known as implementation inheritance), and interfaces (also known as interface inheritance). Method polymorphism is divided into overloading (two methods in the same class having the same name but different signatures), and overriding (a method in a dervied class with the same signature as a method in a base class).
One minor shortcoming of multiple interface inheritance (implementing multiple interfaces) is that an object cannot be considered all implemented interface types simultaneously. This means that an object must be cast from one interface type to another before it is considered to be of that type and usable as such. This minor limitation is vastly outweighed by the flexibility we get from implementing multiple interfaces, coupled with the help of the compiler to verify that all interface methods are imlemented as required to fulfill the interface contract.
