[Design Pattern] Composite Pattern

• Buildings are composed of rooms
  • Although buildings and rooms have slightly different functions, they are essentially treated as generic housing structures
    • An individual can be entered and exited from a building or a room
    • A building or a room can both be used to store furnitures
  • Despite being unique types of objects, they can be dealt with uniformly

Composite Design Pattern

• Achieves two goals
  • 1. To compose nested structures of objects 
  • 2. To deal with the classes for these objects uniformly

How it Works

• 1. A component interface serves as the supertype for a set of classes so they all can be dealt with uniformly
  • This is done by enforcing polymorphism
  • All implementing classes conform to the same interface
  • An abstract superclass can also be used in place of an interface as both are allowed in polymorphism

• 2.Composite class is used to aggregate any class that implements the component interface
  • Composite class allows you to traverse through and potentially manipulate the component objects that the composite object contains
  • Composite class is a subtype of component interface

 

• 3. leaf class represents a non-composite type. It is not composed of other components
  • Leaf class is a subtype of component interface

 

Recursive Composition

• is where a composite object contains other composite object
• Addresses two issues:
  • 1. How do we use individual types of objects to build a tree-like structure?
  • 2. How can we treat the individual types of objects uniformly without checking their types
• The easiest way to think about this is by thinking a tree
  • At root level, there is the main composite object
    • This is made up of other component objects or leaf objects
  • At each level, more components can be added below each composite object

 

 

Example

• 1. The following is called the component interface
  • Is used to describe a building, floor, or a room
    • Each of these areunique type of objects

• 2. The following is called the composite class
  • The housing class is the composite class (recursive)
  • A housing is a type of structure which could also contain other structures
    • Example
      • A building could be represnted by a housing object
      • A building could contain floors
      • Each floor is represented by another housing object
      • This allows to traverse through the building from floor to floor because the building is composed of floors all of which are housing objects