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    <p>It looks like you can use the <a href="http://en.wikipedia.org/wiki/Decorator_pattern" rel="nofollow noreferrer">decorator pattern</a> and write e.g. a <code>ThrottledEmail</code>. It simply decorates another instance of <code>Email</code> (it can be <em>ANY</em> <code>Email</code> subclass) and it can <code>@Override</code> the <code>send</code> method to enforce some throttling. All other methods are simply delegated to the underlying <code>Email</code> instance.</p> <p>This is similar to how a <a href="http://java.sun.com/javase/6/docs/api/java/io/BufferedReader.html" rel="nofollow noreferrer"><code>java.io.BufferedReader</code></a> works, for example. It can decorate any <code>Reader</code> to give it a buffering feature. Other examples include <a href="http://java.sun.com/javase/6/docs/api/java/util/Collections.html" rel="nofollow noreferrer"><code>java.util.Collections</code></a> that provides utility methods such as <a href="http://download.oracle.com/javase/6/docs/api/java/util/Collections.html#synchronizedCollection%28java.util.Collection%29" rel="nofollow noreferrer"><code>Collection&lt;T&gt; synchronizedCollection(Collection&lt;T&gt;)</code></a> which wraps <em>ANY</em> <code>Collection&lt;T&gt;</code> and decorates it with synchronization features.</p> <p>Unless the base class is clearly documented to facilitate subclasses to <code>@Override</code> certain methods, you should generally favor composition (<em>has-a</em>) over inheritance (<em>is-a</em>) relationship.</p> <h3>See also</h3> <ul> <li><em>Effective Java 2nd Edition, Item 16: Favor composition over inheritance</em></li> <li><em>Effective Java 2nd Edition, Item 17: Design and document for inheritance, or else prohibit it</em></li> </ul> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/3477962/when-do-we-need-decorator-pattern">when do we need Decorator Pattern?</a></li> <li><a href="https://stackoverflow.com/questions/49002/prefer-composition-over-inheritance">Prefer composition over inheritance?</a></li> <li><a href="https://stackoverflow.com/questions/1673841/examples-of-gof-design-patterns">Examples of GoF Design Patterns</a></li> <li><a href="https://stackoverflow.com/questions/3502066/guava-forwardinglist-usage-example">Guava <code>ForwardingList</code> usage example</a></li> </ul>
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    1. CO+1 for using composition instead of inheritance.
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      1. USVineet Reynolds
    2. COThis is great, I've read a number of tutorials on this pattern. But I don't quite grasp it fully: If I need an HtmlEmail (it extends Email), and all I need is to add functionality to Email.send(), do I also need a ConcreteDecorator for HtmlEmail (and in turn for all other subclasses of Email - SimpleEmail & MultiPartEmail, implementing all respective methods to forward on to the concrete object)?
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      1. USDavid Parks
    3. CO@David: You have concrete decorators for each decoration. So you may have a `ThrottledEmail` which throttles the send of any `Email`, and you may later have further decorations on other aspects of `Email` (e.g. `AutosignedEmail` or whatever), and you can stack these on top of each other. It shouldn't really matter whether you're decorating a `HtmlEmail` or a `SimpleEmail`.
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