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    <p>This post is about static dependency - for dynamic dependency and the difference, see Patrick Smacchia's answer.</p> <p>In an easy to understand way: an entity (class or package) A depends on an entity B when A cannot be used standalone without B.</p> <p>Inheritance, aggregation, composition, all of them introduces dependency between related entities.</p> <blockquote> <p>so there exists no dependency on an interface?</p> </blockquote> <p>there is, but interface only serves as the glue.</p> <blockquote> <p>what about inheritance?</p> </blockquote> <p>see above.</p> <blockquote> <p>so dependency is a transitive relationship not just on packages, but also on class level?</p> </blockquote> <p>yep.</p> <blockquote> <p>but how do you define "relies"?</p> </blockquote> <p>see above "easy to understand" definition. also related to the 3rd definition you posted.</p> <hr> <p>Update:</p> <p>So if you have interface A in Package P1, and class C in Package P2 uses A as </p> <ul> <li>method parameter, or </li> <li>local variable woven into C via AOP, or </li> <li><code>class C implements A</code>, or </li> <li><code>class C&lt;E extends A&gt;</code>, </li> </ul> <p>then C depends on A and P2 depends on P1.</p> <p>But if interface A is implemented by class B and class C programs against the interface A and only uses B via dependency injection, then C still (statically!) only depends on A, not on B, because the point of dependency injection is that it doesn't make glued components dependent. </p>
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    1. POWhen exactly does a class/package depend on another?
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    1. POWhen exactly does a class/package depend on another?
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    1. COIf dependency is transitive, that means most classes depend on each other. If `A` depends `B`, and `B` depends on `C`, `A` also depends on `C`? In most cases, I think not.
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      1. USBryan Menard
    2. CO@Bryan: I find transitivity on class level strange, too. So maybe Martin Fowler's definition should rather be "Class A depends on class B iff changes to the definition of B may cause **direct** changes to A, i.e. not through a ripple effect".
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    3. CODo you think that dependency should be transitive at package level?
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