Design Principles

• Domain Models are used to visually represent important domain concepts and relationships between them.
• Domain Models help clarify and communicate important domain specific concepts and are used during the requirements gathering and designing phase.
• Domain modeling is the activity of expressing related domain concepts into a domain model.
• UML

  Unified Modelling Language
• Requirements analysis determines

  external behaviour
• Domain modelling determines

  internal behaviour
• Domain


  A sphere of knowledge particular to the problem being solved
• Domain expert


  A person expert in the domain
• Dependency

  The loosest form of relationship. A class in some way depends on another
• Association

  A class "uses" another class in some way. When undirected, it is not yet clear in what direction dependency occurs.
• Directed Association


  Refines association by indicating which class has knowledge of the other
• Aggregation


  A class contains another class. Note that the diamond it at the end with the containing class
• Composition


  Like aggregation, but the contained class is integral to the containing class. The contained class cannot exist outside of the container
• What kind of relationship does Aggregation use?

  It uses a "has-a" relationship where the part can exist without container
• What kind of relationship does Composition use?

  It uses a "is-composed-of" relationship where part cannot live without container
• What kind of relationship does Inheritance use?

  It uses a "is-a-kind-of" relationship
• should it be an attribute or a class?

  If a concept is not representable by a number or a string, most likely it is a class.
• Example of an attribute


  a lab mark can be represented by a number
• Example of a class


  a student cannot be represented by a number or a string
• Defensive programming


  Tries to address unforeseen circumstances, in order to ensure the continuing functionality of the software element.
• It makes the software behave in a predictable manner despite unexpected inputs or user actions

  Defensive programming example
• Design by Contract


  At the design time, responsibilities are clearly assigned to different software elements, clearly documented and enforced during the development using unit testing and/or language support
• Pros of Design by Contract

  clear demarcation of responsibilities helps prevent redundant checks, resulting in simpler code and easier maintenance
• Every software element should define a specification (or a contract) that governs its interaction with the rest of the software components.
• Pre-condition

  A pre-condition is a condition or predicate that must always be true just prior to the execution of some section of code
• Post-condition


  A post-condition is a condition or predicate that must always be true just after the execution of some section of code
• Invariant


  what does the contract maintain? Some values must satisfy constraints, before and after the execution (say of the method). For example: a value of mark remains between zero and 100
• Benefits of Design by Contract (DbC)

  Do not need to do error checking for conditions that not satisfy the preconditions!
• Benefits of Design by Contract (DbC)

  Prevents redundant validation tasks.
• Benefits of Design by Contract (DbC)

  Given the preconditions are satisfied, clients can expect the specified post-conditions.
• Benefits of Design by Contract

  Responsibilities are clearly assigned, this helps in locating errors and resulting in easier code maintenance
• Benefits of Design by Contract

  Helps in cleaner and faster development.
• Post-conditions are sometimes tested using guards or assertions within the code itself, and some languages have specific syntactic constructions for testing .
• In Design by Contract, the properties declared by the post-condition(s) are assured, provided the software element is called in a state in which its pre-condition(s) were true.
• An implementation or redefinition (method overriding) of an inherited method must comply with the inherited contract for the method
• v Preconditions may be weakened (relaxed) in a subclass, but it must comply with the inherited contract
• Post-conditions may be strengthened (more restricted) in a subclass, but it must comply with the inherited contract.
• An implementation or redefinition (overridden method) may increase the benefits it provides to the client, but not decrease it.
• Class Invariant


  The class invariant constrains the state (i.e. values of certain variables) stored in the object.
• Class invariants are inherited, that means, "the invariants of all the parents of a class apply to the class itself.”