Design patterns: Builder Pattern

1. Builder Pattern
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ttp://jyaasa.com
Sagun Shrestha
Sr. Software Engineer,
Jyaasa Technologies

2. What is Builder Pattern?
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3. ● The builder pattern is an object creation software
design pattern.
● It is a pattern designed to help you configure complex
objects
● It helps separate the construction of a complex object
from its representation.
● So that the same construction process can create
different representations.

4. Let’s suppose we are building a system that
will support computer manufacturing
business
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ttp://jyaasa.com

5. class Computer
attr_accessor :display
attr_accessor :motherboard
Attr_reader :drives
def initialize(display=:crt, motherboard=Motherboard.new,drives=[ ])
@motherboard = motherboard
@drives = drives
@display = display
end
end

6. class CPU
# Common CPU stuff...
end
class BasicCPU < CPU
# Lots of not very fast CPU-related stuff...
end
class TurboCPU < CPU
# Lots of very fast CPU stuff...
End

7. # Computer’s motherboard and drive
class Motherboard
attr_accessor :cpu
attr_accessor :memory_size
def initialize(cpu=BasicCPU.new, memory_size=1000)
@cpu = cpu
@memory_size = memory_size
end
end
class Drive
attr_reader :type # either :hard_disk, :cd or :dvd
attr_reader :size # in MB
attr_reader :writable # true if this drive is writable
def initialize(type, size, writable)
@type = type
@size = size
@writable = writable
end
end

8. # Build a fast computer with lots of memory...
motherboard = Motherboard.new(TurboCPU.new, 4000)
# ...and a hard drive, a CD writer, and a DVD
drives = [ ]
drives = << Drive.new(:hard_drive, 200000, true)
drives << Drive.new(:cd, 760, true)
drives << Drive.new(:dvd, 4700, false)
computer = Computer.new(:lcd, motherboard, drives)

9. class ComputerBuilder
attr_reader :computer
def initialize
@computer = Computer.new
end
def turbo(has_turbo_cpu=true)
@computer.motherboard.cpu = TurboCPU.new
end
def display=(display)
@computer.display=display
end
def memory_size=(size_in_mb)
@computer.motherboard.memory_size = size_in_mb
end
def add_cd(writer=false)
@computer.drives << Drive.new(:cd, 760, writer)
end
def add_dvd(writer=false)
@computer.drives << Drive.new(:dvd, 4000, writer)
end
def add_hard_disk(size_in_mb)
@computer.drives << Drive.new(:hard_disk, size_in_mb, true)
end
end

10. builder = ComputerBuilder.new
builder.turbo
builder.add_cd(true)
builder.add_dvd
builder.add_hard_disk(100000)
# new computer
computer = builder.computer

11. class DesktopComputer < Computer
# Lots of interesting desktop details omitted...
End
class LaptopComputer < Computer
def initialize( motherboard=Motherboard.new, drives=[] )
super(:lcd, motherboard, drives)
end
# Lots of interesting laptop details omitted...
end
class ComputerBuilder
attr_reader :computer
def turbo(has_turbo_cpu=true)
@computer.motherboard.cpu = TurboCPU.new
end
def memory_size=(size_in_mb)
@computer.motherboard.memory_size = size_in_mb
end
end

12. class DesktopBuilder < ComputerBuilder
def initialize
@computer = DesktopComputer.new
end
def display=(display)
@display = display
end
def add_cd(writer=false)
@computer.drives << Drive.new(:cd, 760, writer)
end
def add_dvd(writer=false)
@computer.drives << Drive.new(:dvd, 4000,
writer)
end
def add_hard_disk(size_in_mb)
@computer.drives << Drive.new(:hard_disk,
size_in_mb, true)
end
end
class LaptopBuilder < ComputerBuilder
def initialize
@computer = LaptopComputer.new
end
def display=(display)
raise "Laptop display must be lcd" unless display ==
:lcd
end
def add_cd(writer=false)
@computer.drives << LaptopDrive.new(:cd, 760,
writer)
end
def add_dvd(writer=false)
@computer.drives << LaptopDrive.new(:dvd, 4000,
writer)
end
def add_hard_disk(size_in_mb)
@computer.drives << LaptopDrive.new(:hard_disk,
size_in_mb, true)
end
end

14. Structure

15. Participants
● Builder
○ Specifies an abstract interface for creating parts of a Product object.
● ConcreteBuilder
○ Constructs and assembles parts of the product by implementing the
Builder interface.
○ Defines and keeps track of the representation it creates.
○ Provides an interface for retrieving the product
● Director
○ Constructs an object using the Builder interface.
● Product
○ Represents the complex object under construction. ConcreteBuilder
builds the product's internal representation and defines the process by
which it's assembled.
○ Includes classes that define the constituent parts, including interfaces
for assembling the parts into the final result.

16. Collaboration
● The client creates the Director object and configures it with the desired
Builder object.
● Director notifies the builder whenever a part of the product should be built.
● Builder handles requests from the director and adds parts to the product.
● The client retrieves the product from the builder.

18. def computer
raise "Not enough memory" if @computer.motherboard.memory_size < 250
raise "Too many drives" if @computer.drives.size > 4
hard_disk = @computer.drives.find {|drive| drive.type == :hard_disk}
raise "No hard disk." unless hard_disk
@computer
end
Building Sane Object

19. Thank you
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ttp://jyaasa.com