Interfaces

• Interfaces are like templates for classes.
• They define the methods and constants, but they do not implement them.
• Interfaces are not classes themselves, they are simply definitions of what an inheriting class must contain.
• You can only extend one class, but you can implement one or more interfaces.
• What's the difference?
• Extending a class means you are inheriting the behavior of that parent class.
• Implementing an interface simply means you agree to implement the methods of the interface.
• Multiple inheritance is not permitted in Java.  Multiple inheritance is where you can extend more than one class, but this presents a problem.  What if you extend two classes that happen to have the same  method name?  Which one does your subclass actually use?
• Well, that is why Java does not permit multiple inheritance.  But that's also where interfaces come in - since they don't actually implement any methods, two interfaces can have the same method name and not conflict.  Granted, if this does happen, it is generally a sign of poor design or programming.
• All methods of an interface are public.  You do not need to specify public when you write the method definitions in an interface, it is just assumed.  But, in your implementation, you must specify the public modifier or the compiler will complain.
• Interfaces cannot...
• Contain instance fields.  This would bring us to the multiple inheritance problem.
• Implement methods.  See the discussion above.
• So what's the use?
• Polymorphism applies to interfaces.
• For a class to implement an interface, you simply need to add 'implements interfaceName' after the class definition.
• Example: public class Stock { is now public class Stock implements Securities {
• Example: Implementing the Comparable interface.


 Interface Properties
 
 

Created by:  Brandan Jones January 4, 2002