Structural Patterns

Posted on May 20, 2022


  • Wrapper for a constructor that builds an object instance in a single call
  • Has methods that build parts of the object, and combines them together (eg. might have a wheel constructor, brakes constructor, frame constructor, saddle constructor, combines them together to make a bike)
  • Can simplify code significantly: by extending the factory class, we can create another class which can call super methods and create variants on the factory’s output (eg. extend bike with a gear system constructor for a mountain bike)
  • Good at reducting repeated code.


  • Subtle difference: whereas a factory builds an object in one call, a builder builds a different aspect of the new object with each call, and returns the new object by calling get().
  • More flexible - good for making customisable, complex objects with optional parameters.
  • Focuses on cutting down long constructors, rather than cuttong down repeated code.


  • Start by making one instance of an object you need many copies of.
  • Anytime you need a new one, clone() the original prototype.


Placeholders for objects, redirects to the correct resource. Comes with added bonuses.

  • Virtual proxy: Lazy loading - delays loading the resource until nessecary
  • Protection proxy: Provides access control, eg. Chip & PIN as a proxy for money
  • Remote proxy: Also handles communications to access a remote resource
  • Logging proxy: Keeps track of accesses and requests in a service object
  • Caching proxy: Saves results from resource and keeps them to avoid repeated resource queries
  • Smart referencing: Garbage collection - remove heavy service objects if unused for some time


  • Adds new behaviour to an object at runtime
  • Wrapper for existing objects - eg. to change colour, wrap in redColourWrapper(obj)


  • Takes common resource out as it’s own object
  • Other objects now access just the flyweight object
  • Useful for eg. game textures, where many objects need the same texture, saves on memory

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