Programming by Contract

• An important aspect of object oriented design is reuse
• Main goal of object oriented design and programming is to improve the quality of software
• Design by contract presents a set of principles to produce dependable and robust object oriented software
• Client


  requests a service
• Supplier


  supplies the service
• Contract


  the agreement between the client and the supplier
• Each party expects some benefits from the contract and is prepared to incur some obligations to obtain them. These benefits and obligations are documented in a contract document
• Benefit of the client is the obligation of the supplier, and vice versa.
• A contract document between a client and a supplier protects both sides
• If a party fulfills its obligations it is entitled to its benefits
• No Hidden Clauses Rule


  no requirement other than the obligations written in the contract can be imposed on a party to obtain the benefits
• Design by contract promotes using pre and post-conditions (written as assertions) as a part of module design
• The pre and postconditions are assertions, i.e., they are expressions which evaluate to true or false
• If there is no precondition or postcondition, then the precondition or postcondition is assumed to be true
• Defensive programming


  an approach that promotes putting checks in every module to detect unexpected situations
• In Design by Contract the responsibility assignment is clear and it is part of the module interface
• Frequently a programmer must communicate precisely what a function accomplishes, without any indication of how the function does its work.
• precondition


  statement indicates what must be true before the function is called.
• postcondition


  statement indicates what will be true when the function finishes its work.