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POUse case generalization versus extension
 

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    <p>UML Use Case Diagrams allow for two seemingly equivalent ways to show that a given use case might be realised in several different ways namely <a href="https://i.stack.imgur.com/VThNF.png">use case generalizations</a> as opposed to <a href="https://i.stack.imgur.com/Xc0hU.png">use case extensions</a>. I have seen the following basically example modelled using either approach with equal frequency, sometimes within a single source.</p> <p><img src="https://i.stack.imgur.com/VThNF.png" alt="Use case generalisation image"></p> <p><img src="https://i.stack.imgur.com/Xc0hU.png" alt="Use case extension image"></p> <p>To my mind an extension is a weaker relationship than generalization as a direct substitution of the specialised use case for the base case must be possible in generalization but not necessarily in extensions.</p> <p>It seems to me that generalisation implies a polymorphic implementation is desired while extension implies some branching structure is to be used. </p> <pre><code>void makePayment(const PaymentDetails* pd) { pd-&gt;pay(); } </code></pre> <p>as opposed to</p> <pre><code>void makePayment(const PaymentDetails* pd) { switch(pd-&gt;type) { case EFT: payViaEFT(pd); break; case PAYPAL: payViaPayPal(pd); break; case CREDITCARD: payViaCreditCard(pd); break; } } </code></pre> <p>Isn't the Use Case stage far too early for such implementation specific concerns to be modelled? There are much more appropriate UML diagrams for that. Is there a hard and fast rule regarding which of the two to use and if so what is it?</p>
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