Held: ,

Back to Outline 11 - Ethics, Revisited. On to Outline 13 - Design Patterns for SaaS.

Summary

We explore issues of working with legacy code and how code smells and refactoring will help us not just with legacy code, but with all of our code.

Related Pages

Overview

• Legacy code.
• Smells and refactoring.
• Some examples.

Administrivia

• Pre-break PSA.
• 7pm, Tuesday after break: Alum on contracts (NDAs, Non-competes, etc.)
• Important news relevant to 321/322 (will be announced to the department soon).
  • Additional important news will be announced after break.
• Homework for Tuesday after break
  • Finish part 1 of the Legacy Code assignment.
  • Read Chapter 11.

Legacy Code

• What, from your perspective, is "legacy code"? (We see lots of definitions within the chapter.)
• Why do we care about it?
• What kinds of maintainance do we worry about for legacy code?
• How do we do this kind of maintainance?

Refactoring

• What do you see as the key principles of refactoring?
• What do you see as the key techniques we use to refactor?
• Sam's perspective: There are a few purposes for the list of refactoring techniques.
  • They serve to remind programmerss of the kinds of things they can do to make code better.
  • They provide us with a vocabulary to talk about the kinds of changes you might make to a program.
• You should be experienced enough as programmers to have your own list of common "code improvements". What are some of yours?

An Example

class TimeSetter
  def self.convert(d)
    y = 1980
    while (d > 365) do
      if ((y % 400 == 0) || (y % 4 == 0) && (y % 100 != 0))
        if (d > 366)
          d -= 366
          y += 1
        end
      else
        d -= 365
        y += 1
      end
    end
    return y
  end
end

Our book refactors this to the following code.

class TimeSetter
  ORIGIN_YEAR = 1980
  def self.caclculate_current_year(days_since_origin)
    @@year = ORIGIN_YEAR
    @@days_remaining = days_since_origin
    while (@@days_remaining > 365) do
      if leap_year?
        peel_off_leap_year!
      else
        peel_off_regular_year!
      end
    end
    return @@year
  end

  private
  def self.peel_off_leap_year!
    if (@@days_remaining >= 366)
      @@days_remaining -= 366
      @@year += 1
    fi
  end
  def self.peel_off_regular_year!
    @days_remaining -= 365;
    @year += 1;
  end
  def leap_year?
    @year % 400 == 0 ||
      (@year % 4 == 0 && @year % 100 != 0)
  end
end     

In contrast, we rewrote (refactored?) it to the following code.

lass TimeSetter
  def self.isLeapYear(year)
    ((year % 400) == 0) || ((year % 4 == 0) && (year % 100 != 0))
  end

  def self.daysInYear(year)
    if isLeapYear(year)
      366
    else
      365
    end
  end

  def self.convert(d)
    y = 1980;
    while (d > daysInYear(y))
      d -= daysInYear(y)
      y += 1
    end
    return y
  end
end

What do you see as athe strengths and weaknesses of each strategy?

Another Example

From Field et al., pp. 2-4.

class Movie
  REGULAR = 0
  NEW_RELEASE = 1
  CHILDRENS = 2
  attr_reader :title
  attr_accessor :price_code
  def initialize(title, price_code)
    @title, @price_code = title, price_code
  end 
end

class Rental
  attr_reader :movie, :days_rented
  def initialize(movie, days_rented)
    @movie, @days_rented = movie, days_rented
  end 
end

class Customer
  attr_reader :name

  def initialize(name)
    @name = name
    @rentals = []
  end

  def add_rental(arg)
    @rentals << arg
  end

  def statement
    total_amount, frequent_renter_points = 0, 0
    result = "Rental Record for #{@name}\n"
    @rentals.each do |element|
      this_amount = 0

      # determine amounts for each line
      case element.movie.price_code
      when Movie::REGULAR
        this_amount += 2
        this_amount += (element.days_rented - 2) * 1.5 if element.days_rented > 2
      when Movie::NEW_RELEASE
        this_amount += element.days_rented * 3
      when Movie::CHILDRENS
        this_amount += 1.5
        this_amount += (element.days_rented - 3) * 1.5 if element.days_rented > 3
      end

      # add frequent renter points
      frequent_renter_points += 1
      # add bonus for a two day new release rental
      if element.movie.price_code == Movie.NEW_RELEASE && element.days_rented > 1
          frequent_renter_points += 1
      end

      # show figures for this rental
      result += "\t" + element.movie.title + "\t" + this_amount.to_s + "\n"
      total_amount += this_amount
    end

    # add footer lines
    result += "Amount owed is #{total_amount}\n"
    result += "You earned #{frequent_renter_points} frequent renter points"
    result
  end
end