Excerpted from DomainDrivenDesignBook

Definition: A class that can be understood and tested without reference to any others, except system primitives and basic libraries.

Problem: Even within a Module, the difficulty of interpreting a design increases wildly as dependencies are added. This adds to mental overload, limiting the design complexity a developer can handle. Implicit concepts contribute to this load even more than explicit references.

Solution: Low coupling is fundamental to object design. When you can, go all the way. Eliminate all other concepts from the picture. Then the class will be completely self-contained and can be studied and understood alone. Every such self-contained class significantly eases the burden of understanding a Module.

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StandaloneClasses is mentioned on: MessagesByTopic