CSC/ECE 517 Fall 2011/ch2 2c ds

Introduction

With the advent of object oriented languages, the designers of the language had to come up with the decision of whether or not to include the concept of multiple inheritance. Languages like C++ and Perl allow multiple inheritance whereas the designers of languages like C# and Java decided not to .

While Java like languages implemented interfaces to allow for polymorphism,languages like Ruby opted not to implement multiple inheritance but to allow the code from one module be added into another class.

Though both interfaces and Mixins simulate multiple inheritance and avoid the common pathologies associated with it , they have their own set of compromises. The below article tries to provide clarity on how this is achieved in each and also provides other distinctive features of Mixins and interfaces.

Interfaces

In an object-oriented programming languages such as Java, C#, an interface<ref>Interfaces </ref> is a reference type, similar to a class that can contain only constants, method signatures, and nested types. There are no method bodies. Interfaces cannot be instantiated and they can only be implemented by classes or extended by other interfaces. All methods are public by default and any fields declared in an interface are by default static and final.

Example:

 public interface ParkingLot {
       boolean add(Car car, int slot);
       boolean remove(Car car);
       boolean findCar(Int number);
       int freeSlotsCount();
       int occupiedSlots();
       int parkingType();
       int calculateParkingFee(int hours, int charge, int duration);
       int findParkingSlot();
 }

Interfaces play another important role in the object-oriented programming language. Interfaces are not part of the class hierarchy, although they work in combination with classes. Interfaces provide an alternative for languages like Java that does not allow multiple inheritance .

In Java, a class can inherit from only one class but it can implement more than one interface. Therefore, objects can have multiple types: the type of their own class and the types of all the interfaces that they implement. This means that if a variable is declared to be the type of an interface, its value can reference any object that is instantiated from any class that implements the interface.

Mixins

Modules provide a structure to collect classes, methods, and constants into a single, separately defined unit. They provide the simplest and an elegant way to adhere to the Don't repeat yourself principle. This is useful so as to avoid clashes with existing classes, methods, and constants.

Modules are defined in Ruby using the module keyword.

A Mixin is a class that is mixed with a module or a set of modules i.e. the implementation of the modules and the class are intertwined and combined together <ref>Mixins </ref>.

This gives us a neat and controlled way of adding new functionality to classes. Modules are similar to classes in that they hold a collection of methods, variables constants and other modules and class definitions. However, the real usage of a Mixin is exploited when the code in the Mixin starts to interact with code in the class that uses it.

Below is an example for modules and Mixins:

The module ParkingSlot consists of the methods park and unpark

 
 module ParkingSlot
       def park
          #code to park the car
       end
       def unpark
          #code to unpark the car
       end
 end

The module ParkingMeter consists of the methods calculate and printreceipt

 module ParkingMeter
       def calculate
          #code to calculate the parking fee
       end
       def printreceipt
          #code to print the receipt
       end
 end

When a class includes a module via include Module Name, all the methods on that module become instance methods on the class.

 class Parking
      include ParkingSlot
      include ParkingMeter
      def printcardetails
          #code to print the details and type of car
      end
 end

The modules ParkingSlot and ParkingMeter are “included” in the Parking class. All the variables and the instance methods of the modules ParkingSlot and ParkingMeter are now said to be “mixed in” into the class. It is now possible for class instances to use these methods defined in the modules as and when required.

 p=Parking.new   
 p.park            # -> Method park from module ParkingSlot
 p.unpark          # -> Method unpark from module ParkingSlot
 p.calculate       # -> Method calculate from module ParkingMeter
 p.printreceipt    # -> Method printreceipt from module ParkingMeter
 p.printcardetails # -> Calls printCarDetails from class

The class Parking inherits from both the modules and the module methods are now available in Parking. Hence, it is now possible to use the methods with an instance of the Parking class p.

Hence, Mixins can be thought of taking different methods and variables defined in different modules making them available as instance methods in the class as well, thereby extending the class’ functionality. Effectively mixed in modules behave as superclass.

Supported Languages

The following table shows support for Mixins and Interfaces by most commonly used languages.

Languages	Mixins	Interfaces
Ruby	Yes	No
Java	No	Yes
Python	Yes	No
Smalltalk	Yes	No
Perl	Yes	No
C#	No	Yes

Multiple Inheritance

Multiple Inheritance refers to a feature in some object oriented programming languages in which a class can inherit behaviors from more than one superclass. This contrasts with single inheritance in which a class can inherit behavior from at most one superclass. Though Multiple Inheritance provides its own advantages of improved modularity and ease of reuse of code in certain applications, it has its own set of disadvantages which sometimes outweigh the possible advantages.

Diamond Problem

This section describes the well know “Diamond Problem”<ref>Diamond Problem </ref> associated with multiple Inheritance and the succeeding sections describe how Mixins and interfaces can be used to overcome this problem. The diamond problem is the ambiguity that can occur when a class inherits from two different classes that both descend from a common superclass. For example, consider a scenario where one decides to combine the Frog and Dinosaur species which both are inherited from the Animal class. The talk() method in the Animal class is abstract and both Frog and Dinosaur needs to implement this method.

Lets say both Frog and Dinosaur implement the talk method like this:

   public class Frog extends Animal {
         void talk() {
              System.out.println(“I am Frog, I can croak “);
         } 
  }

and,

   public class Dinosaur extends Animal {
         void talk() {
              System.out.println(“I am Dinosaur, I can roar “);
         } 
  }

When the Frogsaur is created, it inherits from both Frog and Dinosaur.

   public class Frogsaur extends Frog, Dinosaur {
      // Note below syntax is used just for demonstration
   }

The real problem occurs when someone tries to invoke talk() method on a Frogsaur object from the Animal reference.

   Animal animal = new Frogsaur();
   animal.talk(); // which method to invoke ???

Due to the ambiguity caused by the diamond problem, it isn’t clear whether the runtime system should invoke Frog’s talk() method or Dinosaur talk() method.

Approaches

Different programming languages have addressed the diamond problem in different ways

In Java, interfaces solve all these ambiguities caused by the diamond problem. Through interfaces, Java allows multiple inheritance of interface but not of implementation. Implementation, which includes instance variables and method implementations, is always singly inherited. As a result, confusion will never arise in Java over which inherited instance variable or method implementation to use.

Ruby does not support multiple inheritance. Mixin provide a way which eliminates the need for multiple inheritance. Ruby has modules which are just like abstract classes in Java. Modules can have different methods implemented in them. They cannot be instantiated like abstract classes. They cannot inherit from other modules or classes. Ruby classes can inherit from only one superclass but can have unlimited number of modules in them to exploit the usage of predefined implementations in these modules. This provides functionality similar to multiple inheritance avoiding any ambiguities.

The below code in Ruby demonstrates two modules Frog and Dinosaur, each of which contain the talk method defined in them. The Frogsaur class includes these two methods and defines its own talk method. This example was chosen to exhibit on how Ruby resolves method calls at run time.

 module Frog
     def talk
         puts "I can croak"
     end
 end

 module Dinosaur
     def talk
         puts "I can roar"
     end
 end

 class Frogsaur
     include Frog 
     include Dinosaur
        def talk
            puts "I can roar like a dinosaur and croak like a frog"
        end
 end

 fs = Frogsaur.new
 fs.talk         # -> I can roar like a dinosaur and croak like a frog

When the method talk is called on the Frogsaur object, Ruby makes sure that the latest method defined in the hierarchy is chosen. Here in this example, the talk method in the Frogsaur class is defined the latest and hence “I can roar like a dinosaur and croak like a frog” is printed. If there was no method defined in the class, method from the Dinosaur is printed since it is next in the hierarchy specified by the include statements.

Comparison between Mixins and Interfaces

This is a general comparision:

Mixins	Interfaces
In Ruby, modules have the implementation of the methods. So all the classes which include the modules gets the same Implementation because classes create a reference to these modules.	In Java, all classes which implement the interfaces should provide implementation for each of the methods declared in the interface. This might duplicate the code if the 2 or more classes have similar functionality.
Relatively smaller because the classes automatically get access to methods defined in the modules. Modules server as a central repository	Relatively larger because a class implementing more than one interfaces has to provide implementation.
Having more modules in the classes can make code less readable. Ideally mixins are suitable for small teams having few modules.	The code is more readable here as the implementation is provided to each method and can be found in place
In Ruby, a method inside a module can have Module name as qualifier. Hence it’s possible for a class to have inherit two or more modules which have same names.	In Java, the class itself has to provide the implementation of the methods. Inheriting methods of same names from different interfaces will signal an error message.
Modules cannot be inherited and cannot form is-a hierarchy	Interfaces can extend another interface and hence can form is-a hierarchy

Drawbacks of Mixins and Interfaces

The drawbacks of the technique of Mixins are very much debated. Though Mixins provide us with an easy way to write flexible and decoupled code and also help us solve the diamond problem, they pose their own problems. In large programs there could be large number of modules and each module could have loads of methods each performing a certain task, to trace the origin of the methods and to keep in mind the hierarchy is practically impossible. Another issue associated with Mixins pose is silent method overriding.

Although Java provides polymorphic behavior with the use of interfaces, they sometimes tend to be very slow. The implementation of interfaces are also limited to public methods and constants with no implementation.

Conclusion

Although mixins have certain advantages over interfaces, they possess their own share of disadvantages. Mixins are generally used in small frameworks and interfaces , more than solving the problem of multiple inheritance provide polymorphism feature which makes object-oriented code more flexible and easy to modify and maintain.

References

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See Also

1. http://ruby.about.com/od/beginningruby/a/mixin.html
2. http://juixe.com/techknow/index.php/2006/06/15/mixins-in-ruby/
3. https://www.re-motion.org/blogs/team/2008/02/20/introducing-mixins-finally/
4. https://www.cs.washington.edu/education/courses/cse413/11sp/lectures/ruby_interfaces-etc.pdf
5. http://pg-server.csc.ncsu.edu/mediawiki/index.php?title=CSC/ECE_517_Fall_2010/ch3_3b_sv&printable=yes
6. http://csis.pace.edu/~bergin/patterns/multipleinheritance.html