The builder pattern is a design pattern used to separate the construction of a complex object from its representation. It allows the same construction process to create different representations. The document describes the builder pattern using the example of building computers. It defines classes for components like CPU, motherboard, drives, and their assembly into computer objects using either a general ComputerBuilder or specialized builders for desktop and laptop computers. The builder pattern participants are the builder, concrete builders, director and product, and their collaboration is described.

1. Builder Pattern
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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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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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