Design by contract :Design by contract, DBC or Programming 
by contract is an approach to designing computer software. 
It prescribes that software designers should define precise 
verifiable interface specifications for software components 
based upon the theory of abstract data types and the 
conceptual metaphor of a business contract.Because "Design 
by Contract" is a trademark of Eiffel Software, the 
maintainers of Eiffel, many developers will refer to it 
only as programming by contract or contract-first 
development. The principal idea of Design by Contract (DBC) 
is that a class and its clients have a contract with each 
other: The client must guarantee certain conditions before 
calling a method specialized on the class (the 
preconditions), the class guarantees certain properties 
after the call (the postconditions). If the pre- and 
postconditions are included in a form that the compiler can 
check, then any violation of the contract between caller 
and class can be detected immediately.