OK, I admit it I’m a simpleton when it comes to scoping things in Java. Out of all four possible keywords for scoping access I manage to use two:
- public
- private
I realize there’s probably a good reason where I’d want to use protected or default scope, but I just haven’t run across one. I find sticking to private and public simplifies my designs. It is very clear whether the variable or method was supposed to be encapsulated by the class. Since I don’t do APIs if I need access then it generally might as well be public.
When I do run across a protected or default in example code I often wonder if there was an explicit reason for it. It turns out that IBM’s RAD 6.0 writes all its JSF backing bean methods as protected scope, though I still don’t get why.
Of course I may also just be missing some blindingly obvious OO concept that makes protected really cool.
i don’t think you’re missing out on much. i save protected for special occasions, like when i have a class hierarchy and intend that a method should be called only by its subclasses.
There’s an interesting (esoteric?) example of an explicit use of default scope in the Java SDK source code. Open up your JAVA_HOME/src.zip, and pull up the WindowsPreferences.java. If you look at Preferences.java, it uses “java.util.prefs.WindowsPreferences” as the default implementation if you’re on Windows. You can’t instantiate that class directly, though. Only the classes in java.util.prefs can. Like you said, you’re not writing API, so it’s mainly an academic example. It is an interesting technique for providing a default, without letting anyone else use (or extend?) it. I don’t think I’ve ever seen anyone else use it, and I don’t recall using it at all myself. I guess it’s only something for guys like Josh Bloch writing widely published APIs.
>Of course I may also just be missing some
>blindingly obvious OO concept that makes
>protected really cool.
Protected is there solely for inheritance. As long as you don’t use inheritance, protected won’t do anything useful for you.
Recently, I had to extend a library class AbstractFtpServer
http://mockftpserver.svn.sourceforge.net/viewvc/mockftpserver/tags/2.0-rc1/src/main/java/org/mockftpserver/core/server/AbstractFtpServer.java?view=markup
I needed to replace DefaultSession, but that was impossible in a subclass because everything was private.
So I requested a change to ‘protected mode’ (now implemented):
http://mockftpserver.svn.sourceforge.net/viewvc/mockftpserver/MockFtpServer/src/main/java/org/mockftpserver/core/server/AbstractFtpServer.java?view=markup
Which has
protected Session createSession(Socket socket) {
return new DefaultSession(socket, handlers);
}
Now, I can override the method in a subclass:
protected Session createSession(Socket socket) {
return new BetterSession(socket, handlers);
}
Private would make this impossible, public would allow everyone to call createSession (which is undesirable).