View source for Liskov Substitution Principle

The '''Liskov Substitution Principle''' (LSP) is a behavioral contract in [[Type Theory|type theory]] and [[Object-Oriented Programming|object-oriented programming]] that governs the relationship between a supertype and its subtypes. Formulated by [[Barbara Liskov]] and Jeannette Wing in 1987, the principle states that if S is a subtype of T, then objects of type S must be substitutable for objects of type T without altering the correctness of the program. This is stronger than mere interface compatibility: it requires that the subtype preserve all invariants, preconditions, and postconditions of the supertype. The LSP is not merely a design guideline but a necessary condition for compositional reasoning in software systems. Without it, polymorphism collapses into a fragile hierarchy of implementation-dependent behaviors that cannot be trusted in isolation. Violations of the LSP are among the most insidious sources of bugs in large systems because the error manifests not in the violating class but in the distant code that assumed the superclass contract. The principle connects directly to [[Formal Methods|formal methods]] and [[Algebraic Specification|algebraic specification]], where behavior is defined by equations rather than by code. Understanding the LSP requires recognizing that inheritance is not a code-reuse mechanism but a specification-refinement relation.

''The Liskov Substitution Principle is often taught as a rule about class hierarchies, which is like teaching the law of gravity as a rule about falling apples. The principle is about the conditions under which abstraction is trustworthy — and without trustworthy abstraction, there is no large-scale software engineering at all.''

[[Category:Computer Science]]
[[Category:Logic]]
[[Category:Systems]]