Interfaces and Abstract Classes

1. Interfaces and Abstract Classes March 4, 2005

2. Outline • More abstract trivia • Java interfaces

3. Stuff from last time • Abstract methods do not have implementations • Static member functions can not be overridden • Classes that are declared final can not have derived classes • The super keyword can be used within a derived class in order to call a method currently overridden by the derived class.

4. Constructor Trivia • Constructors are not inherited • Use this for calling other constructors of the same class • Calling a different constructor using this or super must be the first line in the constructor’s implementation

5. Guidelines About Abstract Classes • When you want to declare a method to indicate something that should be done, but you want to defer the implementation to a subclass, you use an abstract class. • You can mix implementing (concrete) methods and abstract methods in an abstract class.

6. Multiple Inheritance • Allowed in C++ • Not allowed in Java

7. Interfaces • Java’s version of a completely abstract class • Generally, a interface defines what a class does, instead of what a class is. • A class can implement zero or more interfaces. • If a class does implement an interface, it must implement ALL methods defined in the interface. • Unless it is abstract • Interfaces can extend other interfaces.

8. Example public interface MyInterface { void aMethod(int i); } public class MyClass implements MyInterface { public void aMethod(int i) { //implementation } }

9. Why use Interfaces? • They encourage smart application design • They promote readability • They promote maintainability • Interface as a Supertype • They allow flexibility in your class definitions • Multiple “Inheritance”

10. Rules for Writing an Interface • Interfaces can not extend classes. • Interface methods are implicitly abstract; they can not be final. • Interface methods are implicitly public. • Interfaces can define variables, but each variable must be declared public, static, and final.

11. Is and Does • Whereas an abstract class defines what something is, an interface defines what it can do • Abstract class or Interface? • Person • Employee • Student • Programmer • Skier • Wild Animal • Data Accessor

12. Subtypes and Interfaces • If interface I extends interface J, then I is a subtype of J. • If class C implements interface I, then C is a subtype of I. •  interfaces I, I is a subtype of Object.

13. Name Collisions among Interfaces • A class implementing two different interfaces containing functions with the same name • Different signatures -> overloading • Same signature/return type -> collapsed into 1 function • Same signature/different return type -> compilation error