Liskov substitution principle - no overriding/virtual methods?

Question

My understanding of the Liskov substitution principle is that some property of the base class that is true or some implemented behaviour of the base class, should be true for the derived class as well.

I guess this would mean when a method is defined in a base class, it should never be overrided in the derived class - since then substituting the base class instead of the derived class would give different results. I guess this would also mean, having (non-pure) virtual methods is a bad thing?

I think I might have a wrong understanding of the principle. If I don't, I do not understand why is this principle good practice. Can someone explain this to me? Thanks

Answer 1

Subclasses overriding methods in the base class are totally allowed by the Liskov Substituion Principle.

This might be simplifying it too much, but I remember it as "a subclass should require nothing more and promise nothing less"

If a client is using a superclass ABC with a method something(int i), then the client should be able to substitute any subclass of ABC without problems. Instead of thinking about this in terms of variable types, perhaps think about it in terms of preconditions and postconditions.

If our something() method in the ABC base class above has a relaxed precondition that permits any integer, then all subclasses of ABC must also permit any integer. A subclass GreenABC is not allowed to add an additional precondition to the something() method that requires the parameter to be a positive integer. This would violate the Liskov Substitution Principle (i.e., requiring more). Thus if a client is using subclass BlueABC and passing negative integers to something() the client won't break if we need to switch to GreenABC.

In reverse, if the base ABC class something() method has a postcondition - such as guaranteeing it will never return a value of zero - then all subclasses must also obey that same postcondition or they violate the Liskov Substitution Principle (i.e., promising less).

I hope this helps.

Answer 2

There is one popular example which says if it swims like a duck, quack likes a duck but requires batteries, then it breaks Liskov Substitution Principle.

Put it simply, you have a base Duck class which is being used by someone. Then you add hierarchy by introduction PlasticDuck with same overridden behaviors (like swimming, quacking etc.) as of a Duck but requires batteries to simulate those behaviors. This essentially means that you are introducing an extra pre-condition to the behavior of Sub Class to require batteries to do the same behavior that was earlier done by the Base Duck class without batteries. This might catch the consumer of your Duck class by surprise and might break the functionality built around the expected behavior of Base Duck class.

Here is a good link - http://lassala.net/2010/11/04/a-good-example-of-liskov-substitution-principle/

Answer 3

No, it tells that you should be able to use derived class in the same way as its base. There're many ways you can override a method without breaking this. A simple example, GetHashCode() in C# is in base for ALL classes, and still ALL of them can be used as "object" to calculate the hash code. A classic example of breaking the rule, as far as I remember, is derivin Square from Rectangle, since Square can't have both Width and Height - because setting one would change another and thus it's no more conforms to Rectangle rules. You can, however, still have base Shape with .GetSize() since ALL shapes can do this - and thus any derived shape can be substituted and used as Shape.

Answer 4

Overriding breaks Liskov Substitution Principle if you change any behavior defined by a base method. Which means that:

1. The weakest precondition for a child method should be not stronger than for the base method.
2. A postcondition for the child method implies a postcondition for the parent method. Where a postcondition is formed by: a) all side effects caused by a method execution and b) type and value of a returned expression.

From these two requirements you can imply that any new functionality in a child method that does not affect what is expected from a super method does not violate the principle. These conditions allow you to use a subclass instance where a superclass instance is required.

If these rules are not obeyed a class violates LSP. A classical example is the following hierarchy: class Point(x,y), class ColoredPoint(x,y,color) that extends Point(x,y) and overridden method equals(obj) in ColoredPoint that reflects equality by color. Now if one have an instance of Set<Point> he can assume that two points with the same coordinates are equal in this set. Which is not the case with the overridden method equals and, in general, there is just no way to extend an instantiable class and add an aspect used in equals method without breaking LSP.

Thus every time you break this principle you implicitly introduce a potential bug that reveals when invariant for a parent class that is expected by the code is not satisfied. However, in real world often there is no obvious design solution that does not violate LSP, so one can use, for example, @ViolatesLSP class annotation to warn a client that it is not safe to use class instances in a polymorphic set or in any other kind of cases that rely on the Liskov substitution principle.

Answer 5

I think that you're literally correct in the way you describe the principle and only overriding pure virtual, or abstract methods will ensure that you don't violate it.

However, if you look at the principle from a client's point of view, that is, a method that takes a reference to the base class. If this method cannot tell (and certainly does not attempt to and does not need to find out) the class of any instance that is passed in, then you are also not violating the principle. So it may not matter that you override a base class method (some sorts of decorators might do this, calling the base class method in the process).

If a client seems to need to find out the class of an instance passed in, then you're in for a maintenance nightmare, as you should really just be adding new classes as part of your maintenance effort, not modifying an existing routine. (see also OCP)