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POHow to use Composition instead of Inheritance?
 

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  1. POHow to use Composition instead of Inheritance?
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    <p>I have a <code>Configurator</code> class that, in order to do its work, must receive a <code>Context</code> object via an interface:</p> <pre><code>public class Configurator : IContextAware { private IContext _context; //property setter defined by the IContextAware interface public IContext Context { set { _context = value; } } // use _context object in other methods here ... } </code></pre> <p>I plan to write MANY <code>Configurator</code> types, which will all need to initalize their <code>_context</code> fields the same way. My first thought was "Why not anger every programmer on the Internet and use inheritance"?</p> <pre><code>public class ContextInitializer: IContextAware { // field to hold the injected context protected IContext _context; //property setter defined by the IContextAware interface public IContext Context { set { _context = value; } } } public class Configurator : ContextInitializer { // use _context object here ... } </code></pre> <p>This way, any other classes I write that need a Context object can just inherit from ContextInitializer, and use the Context object straight away.</p> <p>Pros: guaranteed consistent initialization, no distracting, duplicated initialize code in every class</p> <p>Cons: Implementation inheritance is evil!,.. um, Configurator cannot inherit from any other class.. ??</p> <p>I should clarify that one way or another, Configurator MUST implement the <code>IContextAware</code> interface. ALSO, the class must have a default/no-arg constructor, so constructor dependency injection is not an option here.</p> <p>Can someone suggest how Composition can be used to achieve the same benefits, but with more flexible design, and/or describe the real short-comings of the Inheritance solution above? Thanks!</p> <p>P.S. This is adapted from Spring.Net CodeConfig, if anyone is curious.</p>
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    1. COInheritance is evil? I missed that memo.
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    2. CO@Rik http://blogs.msdn.com/b/steverowe/archive/2008/04/28/prefer-composition-over-inheritance.aspx http://www.javaworld.com/javaworld/jw-08-2003/jw-0801-toolbox.html http://c2.com/cgi/wiki?ImplementationInheritanceIsEvil In truth, If I were asked "Is `Configurator` a `ContextInitializer`? I would have to say no, but USES the `ContextInitalizer`, thus suggesting inheritance is not quite right here.
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    3. COAbsolute statements are *always* wrong. Inheritance isn't inherently evil. It's a tool. you can use a hammer to put a nail into a wall or to crack open someones skull. It's how you use it, that could be evil.
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