If we implement these generalization and specialization relationships between the classes of different layers, using the program language means, that is, abstract class, inheritance, polymorphism, and dynamic binding, then we can observe an interesting effect: Objects that are instances of classes from a higher layer are used by objects of a lower layer. Its required specializations are then moved to the higher product domain layer by inheritance. This means that we can provide a generic concept, such as Account in the business domain, in the form of a “closed” class. This requirement can be met in an object-oriented layer architecture if we allow inheritance between layers. But what does it mean for superclasses and their subclasses in different layers? EXAMPLE This is obvious for the classes within one layer. We know that generalization and specialization are expressed by the inheritance relationship in the object-oriented world. We will discusss in the next section how we can transfer the generalization and specialization concepts to an object-oriented layer architecture, and how this is compatible with a protocol-based layer architecture.
#Layers of the mac os architecture full
Since they are known only by the type of their abstract mechanism, there will be no undesired dependence between layers.Ī protocol-based layer architecture does not make full use of object orientation. Only the objects that implement this pattern at runtime can then belong to different layers. The concepts of the observer mechanism (e.g., the Observer or Observed interfaces) have to be located within one layer. If we need to implement the interaction of elements of a layer to its next higher layer, then we have to use an abstract interface, as in the case of the observer mechanism (see Section 8.6). The following sections will explain why it is reasonable for object-oriented layers not to limit inheritance and use to the next lower layer. Elements of one layer may also inherit form elements from lower layers. The inheritance relationship is valid within one layer.
The classes or interfaces of a layer may use only the classes or interfaces of their own or lower layers. The microstructures of their elements are classes or interfaces. The layers are organized hierarchically by the principles of generalization and specialization.
#Layers of the mac os architecture software
An object-oriented layer architecture is composed of layers, forming a software or domain unit.
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