Liskov Substitution Principle

Liskov Substitution Principle: let $\varphi (x)$ be a provable property about objects $x$ of type $T$. Then $\varphi (y)$ should be true for objects $y$ of type $S$, where $S$ is a subtype of $T$.

For example, if $S$ extends $T$, some method myMethod should act the same on $S$ as it does on $T$ and we should be able to swap the two. You also cannot change pre or post conditions.

Example Violation: lets say we want to define a Square, if we extend a Rectangle, then we introduce methods such as setLength and setHeight which we do not want. These would have to be prohibited or to introduce side-effects which violates the principle.