I've been reading about the Liskov Substitution Principle when I noticed this answer. It has a Circle and a ColoredCircle type where the constructor of ColoredCircle takes one extra argument; the color.

Doesn't this violate one of the requirements below? (taken from this answer). The only other option in the case of the ColoredCircle would be a public variable or a set_color method.

Pre-conditions cannot be strengthened: Assume your base class works with a member int. Now your sub-type requires that int to be positive. This is strengthened pre-conditions, and now any code that worked perfectly fine before with negative ints is broken.

If I'm searching in the wrong direction here, please let me know. Also, what if a subtype has many more parameters to handle, how would one normally manage those, is a new abstraction always nessacery?

CodePudding user response：

Doesn't this violate one of the requirements below?

It would depend on the language; but at least in Java, constructors are not inherited, so their signatures are not subject to the LSP, which governs inheritance.

Subtypes work best for modifying behavior (of a supertype). This is known as polymorphism, and subtypes do it well (when the LSP is followed). Subtypes work poorly for code reuse, such as sharing variables. This is the idea behind the famous principle, prefer composition over inheritance.

CodePudding user response：

When a class X has a constructor, the constructor is not a method on objects of type X. Since it is not a method on objects of type X, it doesn't have to exist as a method on objects of derived types, either -- it's irrelevant to LSP.