Question

You can have a super class Shape, Square and Rectangle are two children class, but can you have Square subclass Rectangle as Square is a special Rectangle with four sides equal?

I tried to examine using the principle that whenever a super class is used, it should be able to be replaced by its child class, but if a child class is used, it may not be able to be replaced by its super class. Looks like it is fine, but I always feel Square cannot be a child of Rectangle for some reason? Can anyone shed me some light?

Answer 1

You are referring to the Liskov Substition Principle

Substitutability is a principle in object-oriented programming. It states that, in a computer program, if S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., objects of type S may be substituted for objects of type T) without altering any of the desirable properties of that program (correctness, task performed, etc.).

Specific to your question:

A typical example that violates LSP is a Square class that derives from a Rectangle class, assuming getter and setter methods exist for both width and height. The Square class always assumes that the width is equal with the height. If a Square object is used in a context where a Rectangle is expected, unexpected behavior may occur because the dimensions of a Square cannot (or rather should not) be modified independently. This problem cannot be easily fixed: if we can modify the setter methods in the Square class so that they preserve the Square invariant (i.e., keep the dimensions equal), then these methods will weaken (violate) the postconditions for the Rectangle setters, which state that dimensions can be modified independently. Violations of LSP, like this one, may or may not be a problem in practice, depending on the postconditions or invariants that are actually expected by the code that uses classes violating LSP. Mutability is a key issue here. If Square and Rectangle had only getter methods (i.e., they were immutable objects), then no violation of LSP could occur.

In plain English, you can’t use a Square where a Rectangle is expected because a Square has behavior that a Rectangle doesn’t. If someone is trying to use what they think should be an instance of Rectangle, but it’s really an instance of Square, they might set the Width and be surprised that the Height automatically changes (unexpected side effect).