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Subtyping is a notion in programming language theory where a subtype, which is a data type, is related to a supertype based on the notion of substitutability, where program elements such as functions and subroutines that are written for the supertype will still operate if given the subtype instead. Subtypes are an essential concept in object-oriented programming and are substitutable to supertypes and sometimes even have more stringent specifications than the supertype.
Subtyping is a method for substitution and code reuse used in object-oriented programming languages to prevent unnecessary copying of largely similar code and promote code readability and prevent bugs. A subtype is basically a substitute for a supertype that can fulfill all of the latter's specifications, and then some. So if B is a subtype of A, then B can always be used to substitute for an A and any property that is guaranteed by A must also be guaranteed by B.
The subtype is allowed to strengthen and even add properties that are not on the supertype, which means that it allows the supertype to be extended. So, instead of creating a new supertype every time something new is required and then copying properties and conditions from another supertype, a subtype could simply be created that extends the supertype with additional properties or features without changing the supertype. Then, in this case, everything that is provable to the supertype will also be provable to the subtype and more. If a subtype is treated purely the same way as the supertype by using and querying only supertype methods and fields, then the results would be consistent with objects of the supertype.