Design Pattern: Builder

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Design Pattern Builder

• Timothy Holper

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The Builder design pattern What is it?

• The Builder pattern is a way to
• Separate the construction of a complex object
  from its representation so that the same
  construction process can create different
  representations Grady Booch

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The Builder design pattern When to use it?

• You know you should be using the Builder design
  pattern when you
• Want to create different complex objects, but use
  the same set of construction steps.
• Allows you to vary the products internal
  representation.
• Want to separate the steps used to create a
  complex object from the following
• The parts involved in the creation of the
  product.
• How the parts are actually assembled in the
  creation of the product.
• Want to hide the implementation of each
  construction step from other objects.
• There is only one object that DOES need to know
  how the product is assembled.

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Why is the Builder design pattern significant?

• Gives greater control over the object
  construction process
• Product may only be created by going through one
  object.
• That one object can keep track of how many
  products are created.
• Product may only be created by following specific
  steps.
• Allows you to easily vary the products internal
  representation without affecting other parts.
• If you want to change how the product is actually
  built, while keeping the same set of steps, only
  one object needs to know about it.
• This is good encapsulation.
• The Code for construction and the code for
  representation are isolated.
• Only interested parties need to know their part.
• Once again, good encapsulation.
• Overall, simplifies the project specification by
  only indicating the type and parts that the final
  product REQUIRES.
• We can say, definitively, that given these parts
  and this set of steps, we will be able to produce
  a finished product.

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Example An auto manufacturing plant

• The plant builds only 2 types of cars, the AMC
  Gremlin and the AMC Pacer.
• Both cars are built off of the same vehicle
  chassis (i.e. same frame and axles)
• There are 2 teams which are in charge of building
  these cars, a Gremlin team and a Pacer team.
• Both teams are using the same set of steps, in
  order, to construct these cars
• Add transmission
• Add engine
• Add seats
• Add body panels

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Participants in the Builder design pattern

• Director
• Builds the complex object using the Builder
  interface.
• Responsible for the overall construction process.
• Delegates the actual creation/assembly of the
  object to the Concrete Builder/s.
• Returns complex object to client.
• Builder
• Specifies the interface for creating the parts of
  the complex object.
• Concrete Builder
• Implements the Builder interface it chooses the
  parts that go into the complex object and
  assembles these parts that make up the object.
• Provides interface for returning the object that
  is created.
• Product
• This is the complex object, the deliverable.
• It is created by the Concrete Builder,
  implementing the Builder interface.

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Your Concrete Builders Building different
products using the same set of construction steps

• Add 3spd. Automatic Transmission
• Add 6 Cylinder Engine
• Add vinyl seats
• Add Pacer body panels

• Add 4spd. Manual Transmission
• Add 4 Cylinder Engine
• Add denim seats
• Add Gremlin body panels

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Code Example Driver
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Code Example Director and Builder
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Code Example Concrete Builders
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Code Example Vehicle (Product)
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Code Example Output
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Code Example Director and Builder
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Code Example Concrete Builders