Layer (object-oriented design)

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In object-oriented design, a layer is a group of classes that have the same set of link-time module dependencies to other modules. In other words, a layer is a group of reusable components that are reusable in similar circumstances.

Layers are often arranged in a tree-form hierarchy, with dependency relationships as links between the layers. Dependency relationships between layers are often either inheritance, composition or aggregation relationships, but other kinds of dependencies can also be used.