CSC/ECE 517 Fall 2007/wiki1b 6 aa

Take a case of the Singleton pattern and implement it as succinctly as possible in Ruby and Java.Compare the two implementations in terms of clarity and succinctness.The example should be a "real-world" example. While it may be grossly oversimplified for the purpose of illustration, it should not be totally contrived (i.e., should not raise the question, Why would anyone ever want to do that?).

Comparison

 Ruby can implement singleton pattern by simply referencing the singleton module and all its methods will be included. The singleton mixin makes the constructor inaccessible, override clone and dup method, and take care of all the threading complications. 

Java has no mixins, so singleton implementation in Java has to do the following:

1. block off access to constructor by making it private
2. provide a static method for getting an instance of the singleton,
3. prevent cloning by overriding the clone() method explicitly
4. ensure thread safety

Example

Let's take the example of the Earth. Because we only have one earth,it is appropriate to implement the Earth class in singleton.In side the class, currentCondition describes the state (current condition) of the earth.

Ruby Implementation

require 'Singleton'

class Earth
  include Singleton  # include singleton mixin
  attr_accessor :currentCondition
end

Java Implementation

public class Earth {
	  private Earth()
	  {
	   // override the contructor to make it private
	  }
	  private static Earth ref;

	  // make the method thread safe
	  public synchronized static Earth getEarth()
	  {
	    if (ref == null)
            synchronized (Earth.class) {   // for thread-safety
                if (ref == null) ref = new Earth ( );
          } 	
	    return ref;
	  }

	  public Object clone()
		throws CloneNotSupportedException
	  {
	    throw new CloneNotSupportedException(); 
	  }

	  private String currentCondition="I was beautiful and clean";
	  
	  public String getCurrentCondition(){
		  return currentCondition;
	  }
	  
	  public void setCurrentCondition(String str){
		  currentCondition=str;
	  }
	  
	}

Test

Ruby Test code

class People
 def pollute(a)  # pollute an earth object
 a.currentCondition="i am old and polluted"
end

def test
 p=People.new
 earth1=Earth.instance
 earth2=Earth.instance

 earth1.currentCondition="I am young and clean"
 puts "Earth1 says: " + earth1.currentCondition
 puts "Earth2 says: " + earth2.currentCondition

 # pollute earth1
 p.pollute(earth1)
 puts "After people pollute earth1"

 puts "Earth1 says: " + earth1.currentCondition
 puts "Earth2 says: " + earth2.currentCondition
end
end

 p=People.new
 p.test

Java test code

public class People {
	public void pollute(Earth earth){
		earth.getEarth().setCurrentCondition("I am dirty and polluted");
	}
	public static void main(String args[]){
		People p=new People();
		Earth earth1= Earth.getEarth();
		Earth earth2= Earth.getEarth();	
		System.out.println("earth1 says: " +earth1.getCurrentCondition());
		System.out.println("earth2 says: " +earth2.getCurrentCondition());	
		p.pollute(earth1);
		System.out.println("After people pollute earth1");
		System.out.println("earth1 says: " +earth1.getCurrentCondition());
		System.out.println("earth2 says: " +earth2.getCurrentCondition());
		
	}
}

Test Result

Both the java code and the ruby code will produce the following output

 earth1 says: I was beautiful and clean
 earth2 says: I was beautiful and clean
 After people pollute earth1
 earth1 says: I am dirty and polluted
 earth2 says: I am dirty and polluted

References

Books

1. Programming Ruby: The programmatic programmer’s guide
2. Head First Design Patterns

Links

1. - Ruby standard library documentation
2. Design Patterns - Wikipedia
3. - Patterns in Ruby: Singleton Pattern

See Also

1. []