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    <p><code>@interface Foo()</code> creates a class extension (I stand corrected, props to bbum) on interface Foo which is like additional methods added to the interface. Some people also use <code>@interafce Foo(Private)</code> (category) instead of a class extension with <code>()</code>. It's more like "injecting" new methods into a class from outside the class.</p> <p>Placing this in the .m file just keeps other things from "seeing it" in the .h file, but that's it. Basically people normally use categories or class extensions in .m files to specify private interfaces, but they are also used for things like UIKit uses categories to add <code>row</code> and <code>section</code> public methods to NSIndexPath. (This can be confusing.)</p> <p>You don't really need to define private methods this way, but if you have a method called bar that calls method foo before foo is defined in the source file you'll get a compiler warning something like "object self may not respond to foo". You can get rid of that by defining foo before you define bar or any other foo-calling code. It's the same with plain C and functions.</p> <p>Like Ole says this doesn't stop anyone from calling the private methods, it just declares your intention that they be private and causes the compiler to generate the "may not respond to" warnings even if they import the .h file.</p> <p>EDIT</p> <p>Also see <a href="http://www.friday.com/bbum/2009/09/11/class-extensions-explained/" rel="noreferrer">http://www.friday.com/bbum/2009/09/11/class-extensions-explained/</a> for some explanation of categories vs. class extensions. Looks like class extensions should be more correct for defining private methods, from a compiler warning perspective, because category methods are optional. Wish my book would have explained this!</p>
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    1. COThere is a huge difference between named categories `(foo)` and a class extension `()`.
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    2. CODo you have a reference to this? I don't see anything in the book "Programming in Objective-C 2.0" about this. It seems like the category name is just optional from everything I can tell. Not that I'm a major expert or anything but it would be helpful if you could provide a link explaining the difference between (Private) and ().
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    3. COnevermind, I seem to have found your blog http://www.friday.com/bbum/2009/09/11/class-extensions-explained/ Wonder why my book doesn't talk about this?
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