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The Liskov substitution principle is a particular definition of a subtyping relation, called strong behavioral subtyping, that was initially introduced by Barbara Liskov in a 1988 conference keynote address titled Data abstraction and hierarchy. It is based on the concept of "substitutability" – a principle in object-oriented programming stating that an object and a sub-object must be interchangeable without breaking the program. It is a semantic rather than merely syntactic relation, because it intends to guarantee semantic interoperability of types in a hierarchy, object types in particular. Barbara Liskov and Jeannette Wing described the principle succinctly in a 1994 paper as follows:
Subtype Requirement: Let ϕ {\displaystyle \phi } be a property provable about objects x {\displaystyle x} of type T. Then ϕ {\displaystyle \phi } should be true for objects y {\displaystyle y} of type S where S is a subtype of T.
In the same paper, Liskov and Wing detailed their notion of behavioral subtyping in an extension of Hoare logic, which bears a certain resemblance to Bertrand Meyer's design by contract in that it considers the interaction of subtyping with preconditions, postconditions and invariants.