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    <p>Check this document out: <a href="https://drive.google.com/file/d/0BwhCYaYDn8EgMjdlMWIzNGUtZTQ0NC00ZjQ5LTkwYzQtZjRhMDRlNTQ3ZGMz/view" rel="noreferrer">The Dependency Inversion Principle</a>.</p> <p>It basically says:</p> <ul> <li>High level modules should not depend upon low-level modules. Both should depend upon abstractions.</li> <li>Abstractions should never depend upon details. Details should depend upon abstractions.</li> </ul> <p>As to why it is important, in short: changes are risky, and by depending on a concept instead of on an implementation, you reduce the need for change at call sites.</p> <p>Effectively, the DIP reduces coupling between different pieces of code. The idea is that although there are many ways of implementing, say, a logging facility, the way you would use it should be relatively stable in time. If you can extract an interface that represents the concept of logging, this interface should be much more stable in time than its implementation, and call sites should be much less affected by changes you could make while maintaining or extending that logging mechanism.</p> <p>By also making the implementation depend on an interface, you get the possibility to choose at run-time which implementation is better suited for your particular environment. Depending on the cases, this may be interesting too.</p>
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    1. POWhat is the dependency inversion principle and why is it important?
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    1. USNick Weaver
    1. USCarl Seleborg
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    1. POWhat is the dependency inversion principle and why is it important?
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    1. COThis answer does not say why DIP is important, or even what DIP is. I have read the official DIP document, and think this is really a poor and unjustified "principle", because it's based on a flawed assumption: that high-level modules are reusable.
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      1. USRogério
    2. COConsider a dependancy graph for some objects. Apply DIP to the objects. Now any object will be indpendent of the implementation of the other objects. Unit testing is now simple. Later refactoring for reuse is possible. Design changes have very limited change scopes. Design problems don not cascade. See also the AI pattern "Blackboard" for data dependaecy inversion. Together, very powerful tools fo making the software understandable, maintainable and reliable. Ignore dependency injection in this context. It is unrelated.
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      1. USTim Williscroft
    3. COA class A using a class B does not mean A "depends" on the implementation of B; it depends on the public interface of B only. Adding a separate abstraction on which both A and B now depend only means that A will no longer have a compile-time dependency on the public interface of B. Isolation between units can be achieved easily without these extra abstractions; there are specific mocking tools for that, in Java and .NET, that deal with all situations (static methods, constructors, etc.). Applying DIP tends to make software more complex and less maintainable, and no more testable.
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      1. USRogério
 

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