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    <p><strong>Dependency :</strong> </p> <blockquote> <p>Indicates that a <strong>client</strong> element(of any kind, including classes, packages, use cases, etc) has knowledge of another <strong>supplier</strong> element, and a change in supplier can effect the client. </p> </blockquote> <p><strong>So "dependency" is very broad relationship</strong>.Suppose that if a class-object(client) has another class-object(supplier) as a member,if a class-object send a message to another class-object,if a class-object takes another class-object as an parameter from its methods, even if a class(client) is subclass of another class(supplier) there will be dependency since change from supplier will effect clients.</p> <p><strong>Technically all of those relationships can be shown by "Dependency" line. But some of above relationships already has special notations</strong>: such as for superclass-subclass relationship we have generalization relationship.No need to show also "dependency" line because if they have generalization relationship, they have dependency. And we have "association" relationship for class-object(client) who has another class-object as a member [attribute]. So also no need to show extra dependency line in this situation.</p> <p><strong>Actually "Dependency" is badly defined relationship for class diagrams</strong>. But it can be usefull for showing dependency in which UML has no special notation such as :</p> <ul> <li>if you has another class-object(supplier) as a parameter in one of your class(client) methods</li> <li>if you have dependency to global variables</li> <li>when you call static methods on another classes.</li> <li>local variables (which you think you have important dependency)</li> </ul> <p><img src="https://i.stack.imgur.com/LPNxn.jpg" alt="enter image description here"></p> <pre><code>public class RepositoryManager { public UpdatePriceFor(ProductDescription description) { Time date = Clock::GetTime(); Money oldPrice =description.GetPrice(); ... } private IList&lt;Item&gt; itemsList = new List&lt;Item&gt;(); } </code></pre> <blockquote> <p>So all "associations" are also shows "dependency".But "dependency" is broad-general-weak relationship.<strong>As a rule if there is a special relationship which is more specific-stronger than dependency relationship than use it.</strong> And lastly use all your relationship "economically". Show only important ones based on modeler-model reader perspectives.</p> </blockquote> <p>[ Source : Adapted from Craig Larman's Applying UML and Patterns book ]</p> <p>Check Fowlers bliki for further information <a href="http://martinfowler.com/bliki/DependencyAndAssociation.html" rel="nofollow noreferrer">DependencyAndAssociation</a></p>
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    1. POdifference between unidirectional association and dependency
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    1. COwell according to @vainolo if one class calls method of other class then it should be modeled as unidirectional association.you say that if getTime() is called it is a dependency?
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    2. COIf the object which we called its methods[supplier], is our class'(client) attribute, then it is proper to use association.But if it was not client's attribute, i.e. it is local variable or static method call then it is best to show it as dependency. In my example getTime() is static call. Even it is not static, such that we create local Clock variable and call its getTime() method it is still best to show it as dependency. But suppose that Clock object is a member-attribute of RepositoryManager than it is better show it with association.
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    3. COAlso Money class is local variable in UpdatePriceFor method. Since it is not attribute of RepositoryManager, it is best to show it with dependency.But even I chose not to show it since I do not want to make my class diagram "dirty" with lots of dependencies
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