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Design by contract

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  • 1. Design by Contract Jiří Kiml, 18/08/2011, Bernwww.querity.cz
  • 2. Agenda ►Motivation ►DBC ►Support in JAVA ►Eclipse examples ►Summary ►Linkswww.querity.cz
  • 3. Motivation Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live. ~Martin Golding ►Wouldnt it be nice if all Java classes that you use, including your own, lived up to their promises?www.querity.cz
  • 4. Design by Contract (1) ►The Design by Contract (DBC) software development technique ensures high- quality software by guaranteeing that every component of a system lives up to its expectations. As a developer using DBC, you specify component contracts as part of the components interface. The contract specifies what that component expects of clients and what clients can expect of it. ►Bertrand Meyer developed DBC as part of his Eiffel programming language. Regardless of its origin, DBC is a valuable design technique for all programming languages, including Java.www.querity.cz
  • 5. Design by Contract (2) ►Central to DBC is the notion of an assertion ►At runtime we evaluate the assertions at specific checkpoints during the systems execution.www.querity.cz
  • 6. Kinds of Assert expressions ►Preconditions ►Postconditions ►Invariantswww.querity.cz
  • 7. Preconditions ►Preconditions specify conditions that must hold before a method can execute. As such, they are evaluated just before a method executes. Preconditions involve the system state and the arguments passed into the method. ►Preconditions specify obligations that a client of a software component must meet before it may invoke a particular method of the component. ►If a precondition fails, a bug is in a software components client.www.querity.cz
  • 8. Postconditions ►In contrast, postconditions specify conditions that must hold after a method completes. Consequently, postconditions are executed after a method completes. Postconditions involve the old system state, the new system state, the method arguments, and the methods return value. ►Postconditions specify guarantees that a software component makes to its clients. If a postcondition is violated, the software component has a bug.www.querity.cz
  • 9. Invariants ►An invariant specifies a condition that must hold any time a client could invoke an objects method. Invariants are defined as part of a class definition. In practice, invariants are evaluated any time before and after a method on any class instance executes. A violation of an invariant may indicate a bug in either the client or the software component.www.querity.cz
  • 10. Support in JAVA ►Assert keyword ►Many third party solutions (annotations, AspectJ) ►DBC classwww.querity.cz
  • 11. Eclipse examples ► XXXXwww.querity.cz
  • 12. Summary ►Contracts in code are good ►Easy to use ►Correctness ►Robustnesswww.querity.cz
  • 13. Links ►Two presentations: – http://www.eiffel.com/developers/presentations/ ►http://en.wikipedia.org/wiki/Design_by_con tractwww.querity.cz
  • 14. Questions … … and maybe answerswww.querity.cz
  • 15. Thank you Jiří Kiml , 18/08/2011, Bernwww.querity.cz