Exam 15: Components Reuse

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The most obvious, reliable, and effective mechanism for reuse is a component. Well-defined and well-designed components have one distinct advantage over any type of reuse: they come as packages or black boxes and are, largely, foolproof. This advantage, though, does not obviate the need for other kinds of reuse.

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The standard UML icon is a box with two tabs, similar to a plug. Components, however, can also be represented by "stereotyped" boxes. A component can be modeled both as a black box that hides its internal structure, and as a white box that exposes that structure.
In black-box modeling, we represent the interfaces that the component supports as lines that end in a circle. The outside world can interact with the component only through these interfaces. In turn, if the component requires an external interface in its environment, the requirement is shown as a line ending in a half-circle. This arrangement is often called "ball and socket" because the interface provided by one component must fit into the "socket" provided by its "client."
In white-box modeling, the internal structure of the component is shown as a diagram inside the stereotyped box that represents the component. The communication between entities inside the box and outside entities takes place through ports, shown as small boxes that are situated on the borders of the component box. An "incoming" port delegates a provided interface to an object, or another component, inside the component. An "outgoing" port conveys requests by internal entities to the required interface of an outside component.