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    <p><strong>Dependency Inversion Principle</strong>; One of the five <a href="http://en.wikipedia.org/wiki/SOLID_%28object-oriented_design%29" rel="nofollow noreferrer">Object Oriented Design Principles</a>. Check out the answers to this question; <a href="https://stackoverflow.com/questions/62539/what-is-the-dependency-inversion-principle-and-why-is-it-important">"What is the Dependency Inversion Principle and why is it important?"</a> - there's some really good information there.</p> <blockquote> <p>In conventional application architecture, lower-level components are designed to be consumed by higher-level components which enable increasingly complex systems to be built. In this composition, higher-level components depend directly upon lower-level components to achieve some task. This dependency upon lower-level components limits the reuse opportunities of the higher-level components.</p> </blockquote> <p>In '<em>simple</em>' terms, this means that when you rely upon a concrete instance of an object - you're building a dependency in to your code (albeit without that intention), this then limits the ability to re-use it. </p> <p>Remember that a <strong>Concrete Type</strong> is a type of class that can be instantiated and a <strong>Abstract Type</strong> is a type that cannot; i.e an interface. (See the <a href="http://en.wikipedia.org/wiki/Class_%28computer_programming%29#Abstract_and_Concrete" rel="nofollow noreferrer">Taxonomy of Classes</a>)</p> <p>If you code to a specific concrete class then you will always have the requirement for that class. However, if you code to an interface (an <em>abstraction</em>) then it's possible to adapt your code to work with any number of classes; as long as they implement that common interface.</p> <p>So in Java, it means that where possible you should code to an Interface - and avoid making your code depend on specific items. This is often as simple as passing <code>Interface</code> types as parameters and return types; as opposed to concrete classes. </p> <p>Less dependencies = More ability to re-use code. More dependencies = More requirements to be able to re-use code.</p> <p><strong>This will be a re-occuring theme when you're studying Design Patterns!</strong></p> <p><a href="http://en.wikipedia.org/wiki/Dependency_inversion_principle" rel="nofollow noreferrer">Dependency Inversion Principle</a> on Wikipedia.</p>
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