CSC/ECE 517 Fall 2009/wiki3 20 i7

Liskov substitution principle

The Liskov substitution principle was introduced by Barbara Liskov in 1987 and is concerned with subtype contractual correctness. Based on a paper from 1994 the principle states, "Let q(x) be a property provable about objects x of type T. Then q(y) should be true for objects y of type S where S is a subtype of T.". This means that for every parent type T, subtype S should be able to be substituted in for it and the behavior of the program should remain exactly the same.

Preconditions

Preconditions cannot be strengthened. <Explain in english> <Example>

Postconditions

Postconditions cannot be weakened. <Explain in english>

Invariants

Invariants must be preserved. <Explain in english>

• http://en.wikipedia.org/wiki/Liskov_substitution_principle
• http://javaboutique.internet.com/tutorials/JavaOO/
• http://www.codingefficiency.com/2009/07/21/solid-l-liskov-substitution-principle/