Subcontracting

Contracts and Assertions - slide 24 : 32

Subcontracting

Due to polymorphism an instance of a subclass can act as stand in for an instance of the superclass

Consequently, the contracts of the subclass must comply with the contract of the superclass

The contract of a subclass must therefore be a subcontract of the superclass' contract

Assertions in a subclass
- The precondition must not be stronger than the precondition in the superclass
- The postcondition must not be weaker than the postcondition in the superclass
- The class invariant must not be weaker than the invariant in the superclass

Operations in subclasses cannot arbitrarily redefine/override operations in superclasses