About Components

Software Components are typically business objects that have predefined and reusable behaviors. Component-Based Development is the idea of creating software applications from components. Implementation detail is hidden in the interfaces, which isolate and encapsulate a set of functionality. Interfaces are the means by which components connect. An interface is a set of named operations that can be invoked by clients. More importantly, well-defined interfaces define the component's entry points, and a component's accessibility is done only via its interface. In a component-based approach, providers and clients communicate via the specification of the interface, which becomes the mediating middle that lets the two parties collaborate and work together.

In summarizing, we have the following:

1. A component is an independent, application-level software unit that is developed for a specific purpose and not for a specific application. Components are self-contained, pluggable abstract data types. They are of large grained entities. Component assembly (application development) is concerned with high level domain experts rather than programmers. Programmers will be responsible for component construction and the fabrication of components for use would be accomplished by component assemblers.
2. Components are accessed through well-defined interfaces. A component's interface serve as the access points to its functionality. These interfaces may be implemented by objects internal to the component, if the component developer so desires. But interfaces may be implemented by a non-OO language.
3. Only a single instance of a component exist per system. If there are more than one client accessing the functionality being provided by the component, different object references may be distributed to those clients. An object reference is a handle to an object in a component which implements an interface.
4. Component comprised systems will typically feature components from different vendors. Components will be interoperable by conformance to industry standard interfaces
5. Components are platform-independent
6. Components will be encapsulated and modular. Their internals may utilize inheritance for implementation reasons. This fact will remain as an implementation detail, not visible to the component clients. Components can not be extended by inheritance.

Components and Objects

It is worth considering the relationship existing between components and objects. A component approach to software development builds upon fundamental constructs of object-oriented paradigms. Although the terms "component" and "object" are often used interchangeably, a component is not an object. An object is an instance of a class that gets created at runtime. A component can be a class, but is usually a collection of classes and interfaces. At runtime, a component becomes alive when its classes get instantiated. Therefore, at runtime, a component becomes a web of objects.

Their usage seems synonymous. While objects are well-suited for component construction, a non-OO approach to component construction is perfectly valid. The following explains how objects differ from components.

• Components are large-grained entities, e.g. a payroll module. Objects are fine-grained entities, e.g. a Person.
• Components are accessed via well-defined interfaces. Objects are also accessed via interfaces. Thus the two are similar in this respect, but not identical. Components interfaces are based on industry standards, object interfaces are not. A component with a CORBA IDL interface is easily accessible from any language. An object implemented in C++ is not easily accessible from another language. Components need not use OO approach to implement their interfaces. The same is also true of objects. The objects often act as wrappers to non-OO-code.
• Only single component instances will exist per system whereas multiple objects will exist per system. Components are more akin to Object factories which also delegate functionality to objects within the factory.
• Component based systems will be made up of heterogeneous vendor components. Seamless integration is possible as components will be built to industry standard interfaces. Achieving the same with objects tends to be more difficult as objects are built to proprietary interfaces, thus can not be seamlessly integrate. Such integration is possible if the object interfaces are standard such as Java AWT.
• Components will be runnable out of the box. Only Java objects on a system running a JVM can boast of this property.
• Components can be encapsulated and non-extendable. Objects will be both encapsulated and extendible through inheritance. But inheritance may be misused and hence this is being avoided in components as component assemblers are experts in domain knowledge not on OO techniques.

The aim of software components is to deliver better applications to the market in a shorter time. This is easily achieved through leveraging existing components. Thus the philosophy of component assembly is re-use. The philosophy for component construction remains the same.

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