Lab 6: Singleton and Decorator Patterns

Objectives

During this lab session:

Students will write and test Ruby code that implements the Singleton and Decorator patterns.

This activity helps students develop the following skills, values and attitudes: ability to analyze and synthesize, capacity for identifying and solving problems, and efficient use of computer systems.

Activity Description

The lab activities can be developed individually or in pairs.

The lab report must be developed individually.

Create a folder called singleton_decorator. Inside this folder, create four files called: tigger.rb, test_tigger.rb, coffee.rb, and test_coffee.rb.

All Ruby source files must start with a comment containing the lab's title, date, and the authors' personal information. For example:
```
# Lab 6: Singleton and Decorator Patterns
# Date: 1-Mar-2011
# Authors:
# 456654  Anthony Stark 
# 1160611 Thursday Rubinstein
```
You are given the following Ruby class that models the famous Tigger character from Winnie The Pooh:
```
class Tigger
      
  def to_s
    return "I'm the only one!"
  end
    
  def roar
    'Grrr!'
  end    
        
end
```
Convert this class into a singleton using whatever technique you find fit. Place the resulting class in the tigger.rb source file.

Check your solution using the following test case (place the test in the source file test_tigger.rb):

require 'test/unit'
require './tigger'

class TiggerTest < Test::Unit::TestCase
  
  def test_tigger
    t = Tigger.instance
    assert_same(t, Tigger.instance)
    assert_raise(NoMethodError) do       # "new" method should be private!
      Tigger.new
    end
    assert_equal("I'm the only one!", t.to_s)
    assert_equal('Grrr!', t.roar)
  end
  
end

This example was taken from [FREEMAN] pp. 79-98. Write the code that allows us to model coffee beverages using the Decorator pattern. The following tables show what beverages and condiments are available, and their corresponding prices:

*Beverages*
Name	Price
Dark Roast Coffee	$0.99
Espresso	$1.99
House Blend Coffee	$0.89

*Condiments*
Name	Price
Mocha	$0.20
Whip	$0.10
Soy	$0.15
Milk	$0.10

In our design, the condiments will decorate the beverages. The following Ruby code demonstrates how your code could be used:

beverage = DarkRoast.new
beverage = Mocha.new(beverage)
beverage = Whip.new(beverage)
puts beverage.description
puts beverage.cost

For this example, the expected output should be:

Dark Roast Coffee, Mocha, Whip
1.29

The following image depicts the way the decorators work when the cost method is called from the above code.

A decorated beverage.

The following UML diagram shows the class hierarchy. Each class needs to implement one or several of these methods: initialize, description, and cost.

Beverage class hierarchy.

All these classes should be placed in the coffee.rb source file.

The following unit tests verify the correct behavior of your classes. Place the test class in the test_coffee.rb source file.

require 'test/unit'
require './coffee'

class MyTest < Test::Unit::TestCase

  def test_espresso
    beverage = Espresso.new    
    assert_equal("Espresso", beverage.description)
    assert_equal(1.99, beverage.cost)
  end
  
  def test_dark_roast
    beverage = DarkRoast.new
    beverage = Milk.new(beverage)    
    beverage = Mocha.new(beverage)
    beverage = Mocha.new(beverage)
    beverage = Whip.new(beverage)        
    assert_equal("Dark Roast Coffee, Milk, Mocha, Mocha, Whip", beverage.description)
    assert_equal(1.59, beverage.cost)
  end
  
  def test_house_blend
    beverage = HouseBlend.new
    beverage = Soy.new(beverage)
    beverage = Mocha.new(beverage)
    beverage = Whip.new(beverage)
    assert_equal("House Blend Coffee, Soy, Mocha, Whip", beverage.description)
    assert_equal(1.34, beverage.cost)
  end
  
end

Deliverables

To hand in your individual lab work, follow these instructions.

Write your lab report according to the specified guidelines. Name your LaTeX source file lab6_report_A0MMMMMMM.tex, where A0MMMMMMM is your student ID. From your LaTeX source, generate the corresponding PDF file. That file should be called lab6_report_A0MMMMMMM.pdf. Place these two files in the singleton_decorator directory.
Create a ZIP file with the full contents of the singleton_decorator directory. Call this file singleton_decorator.zip.
Using the Online Assignment Delivery System (SETA), deliver the ZIP file.

Due date is Monday, March 7.

Evaluation

This activity will be evaluated using the following criteria:

50%	Implementation of functional requirements.
50%	Lab report.
DA	The program and/or report was plagiarized.

Software Design and Architecture

Lab 6: Singleton and Decorator Patterns

Objectives

Activity Description

Deliverables

Evaluation