Design By Contract
One should not write a class without a specification — a contract. The contract lists the internal consistency conditions that the class will maintain (the invariant) and, for each operation, the correctness conditions that are the responsibility of the client (the precondition) and those which the operation promises to establish in return (the postcondition).
Writing a class without its contract would be similar to producing an engineering component (electrical circuit, VLSI (Very Large Scale Integration) chip, bridge, engine…) without a spec. No professional engineer would even consider the idea.~ Bertrand Meyer (1950 age:67), creator of design by contract and the Eiffel language. Description of Design By Contract