CSC/ECE 517 Fall 2009/wiki2 13 ncs

Introduction

Ruby offers more concise implementations of certain design patterns than Java does. We will look at some design patterns and compare the implementation in Ruby vs Java with the goal to answer the following:

• Is this typical of dynamic object-oriented languages?

• Are there design patterns that any dynamically typed language can realize better than a statically typed language?

• Are there instances when a different pattern should be used in a dynamic language than a static language?

Design Patterns in Java vs Ruby

Singleton Pattern

Singleton pattern is a design pattern used to restrict the instantiation of the class to one object.

The implementation of the pattern involves:

• making the constructor private
• declare a instance variable
• implement a getInstance method and make the it thread safe.

Below is the comparison between singleton class written in Java and Ruby.

It is clear that Ruby provides a more concise way of implementing the singleton pattern. Ruby uses the singleton module which automatically provides all the functionality that we need to write manually in Java. By writing include singleton in Ruby, we prevent ourselves from having to repeat writing the same code for each class that we wish to make singleton. In contrast, we need to write the Java class below for every singleton objects we create.

Java

 public class SingletonExample { 
    // declare instance variable
    private static SingletonExample singletonEx; 
    
    // Make the constructor is private
    private SingletonExample() {
    }
    // make this thread safe use synchronize
    synchronize public static SingletonExample getSingletonObject() {
      if (singletonEx == null) {
         singletonEx = new SingletonExample();
      }
      return singletonEx;
    }
 }

Ruby (using singleton module)

 require singleton
 class SingletonExample
   include singleton
 end

Factory Pattern

Factory pattern is implemented by isolating the code to create an object from the concrete implementation of the class.

In Java, we typically write a similar code as below:

 interface IObjectFactory {
    IGear createObject();
 }
 class ConcreteObjectFactory implements IObjectFactory{
 IObject createObject() {
    if ( some condition )
       return new XObject();
    else
       return new YObject();
 }
 class ObjectUser {
    public void doSomething(IObjectFactory factory ) {
       IObject my_Object = factory.createObject();
    }
 }

We can implement the same code skeleton in Ruby. However, there is no special classes or class methods in Ruby since it is a dynamic language. Ruby allows us to rename our factory method to be called new.

 class ObjectFactory
   def new() 
     if ( ... some condition )
        return XObject.new()
     else
        return YObject().new()
   end
 end

Our client class now becomes:

 class ObjectUser 
   def doSomething(factory)
     ...
     my_object = factory.new()
     ...
   end
 end

Ruby lets us redefine the new method and allows us to implement the Factory pattern with a class that looks very much like an ordinary class. The client does not need to know about the special createObject method.

Iterator

Iterator pattern is design pattern used to access the elements of an aggregate object sequentially without exposing its underlying representation.

In order to use Iterator patter, we need:

• An iterator declared and initialized to point to the collection
• Write a loop and use the iterator to get the value of the next sequential object in the collection.

In Java, we will need to know the type of objects in the collection and cast them to the right type in order to use them. Whereas in Ruby, iterators are built right in.

Here are some examples that show the ease with which we can use iterators in Ruby:

 a = [ 10, 20, 30, 40 ]
 a.each { |element| print "The element is #{element}\n" }

 Thread.list.each { |t| print "Thread: #{t}\n" }

 ObjectSpace.each_object { |o| print "Object: #{o}\n" }

 open("data.txt").each_line { |line| print "The line is #{line}\n" }

Conclusion

Design pattern is a concept that applicable to solve problems regardless of the languages we use to implement the solution. So I cannot think of a case where a specific pattern will be more useful or pertinent to be used in a dynamic language than a static language. However given the features of dynamic languages like Ruby that support meta-programming and reflection via Modules and Mixins and support extending/reopening classes and redefining methods at run time definitely make it easier to implement patterns with fewer lines of code. Ruby lets us hide the details of implementations of design patterns much more effectively. You can make a class a singleton with a simple include Singleton statement. You can make factories that look exactly like ordinary classes. These features in Ruby do make implementing design patterns very easy and let a programmer focus on solving the problem at hand rather than spending time implementing the design patterns.

References

• Gamma, Erich; Richard Helm, Ralph Johnson, and John Vlissides (1995). Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley. ISBN 0-201-63361-2.

• "Design Patterns in Ruby" by Russ Olsen, February 27, 2006.