Programming by Contract

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Put the responsibility on the client to fufill the preconditions and postconditions

• Must Trust Clients to do it right

Classes: Levels of Complexity

1. POD (Plain Old Data)
   • Dont have to worry about designing
2. Computed Fields
   • Only rely on data inside class
     • Internal data
3. Uses contained objects
   • Objects that contain other complicated objects
4. Orchestrates interactions between objects
   • Two different objects, need to do something with both of them in a specific order or way

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Room Class
Person Class

List<Person> occupants
  int maxOccupancy;     // If at max occ want to reject new person entering room
  
Room currentRoom;

How do you know if room can accept person? Can person move? Updating of fields?

Create mediator that helps keep track of everything. Knows about other objects, updates rooms and persons.

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Programming by Contract

Design by Contract, other name

• Original idea created with eiffle
• Preconditions: needs to be true beforehand
  • In order to remove something from the stack or queue, there needs to be something in the stack or queue
• Postconditions: will be true after method is executed
  • For end user to help them understand the class
  • File.close()
    • file will no longer be locked
    • Write operations with this object will fail
  • remove() breaks any open iterators
• Class invariants
  • True for the lifetime of instances
  • Health is always non-negative for player in video game
• Implementation invariants
  • Implementation notes
• Interface invariant

Method

• Name
• Parameters
• Preconditions
• Postcoditions
• budy

BCPL (Basic Common Programming Language) -> B -> C -> C++ -> Java -> C#

• Comments
• Assert
• Language Feautre
  • Usually compiled out in producted (Assert too)

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Defensive Programming

• Have a back up plan to every single case
• Leads to exceptions
• Not trusting user to not do this
• Many if statements, null checks

• Will never go super wrong, cause all cases are covered

• All exceptions add overhead, slower