22.1. Polymorphism
When related alternatives or behaviors vary by type (class), assign responsibility for the behavior—using polymorphic operations—to the types for which the behavior varies.[1]
[1] Polymorphism has several related meanings. In this context, it means “giving the same name to services in different objects” [Coad95] when the services are similar or related. The different object types usually implement a common interface or are related in an implementation hierarchy with a common superclass, but this is language-dependent; for example, dynamic binding languages such as Smalltalk do not require this.
Solution
Corollary: Do not test for the type of an object and use conditional logic to perform varying alternatives based on type. ...
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