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General <strong>Java</strong> Questions I<br />
efforts to create a socket on that same port would throw an exception.<br />
Answer 2: Actually a better way is to (on launch):<br />
1) Check if the file exists. If not, create it, open it and run. Leave it open until you quit,<br />
upon which time you close it.<br />
2) If the file _does_ exist, try to delete it. If the delete fails, then someone else has it<br />
open, which means another copy of your app is running. Inform the user and quit.<br />
3) If you succeeded in deleting it, then you are the first. Now create, open and run.<br />
Doing the above prevents the problem of having the semaphore file left around when<br />
the system crashes. I implemented it recently in one of our apps, and it works like a<br />
charm.<br />
--<br />
Burt Johnson<br />
MindStorm Productions, Inc.<br />
http://www.mindstorm-inc.com<br />
Q: Can you call a class static abstract method from an abstract class or does it<br />
need to be extended and then called from its concrete class?<br />
I've been told that abstract classes do not actually have any code in them cause they<br />
are like a placeholder, so I guess you wouldn't bother calling a static method in an<br />
abstract class cause it wouldn't have any code to begin with....?<br />
Answer: You have been told wrong. Abstract classes can and do have code in them.<br />
See, for example, java.awt.Component, an abstract class with a lot of code and no<br />
abstract methods at all. If a class has any abstract method member, directly declared<br />
or inherited, it is required to be declared abstract. If not, it is the programmer's<br />
decision and should be based on whether it would make sense to have an instance of<br />
that class.<br />
Perhaps whoever told you was confusing abstract classes with interfaces, which don't<br />
contain implementation, only abstract method and constant declarations.<br />
You cannot declare a method to be both static and abstract. Abstract requires the<br />
method to be overridden before you can have a concrete class, static prevents<br />
overriding. You can have a static method in an abstract class - such a method could<br />
be called without creating an instance of the class, the only thing that is prohibited for<br />
abstract classes.<br />
And when a subclass of an abstract method has been instantiated, all the methods<br />
from the original class will keep the same code in the instance. Most of the time an<br />
abstract class will have abstract methods.<br />
However, there are several examples of abstract classes that don't have any abstract<br />
methods at all. Some examples are Component and FontMetrics from the AWT. It<br />
doesn't make sense to have just a Component that's not a specific type of<br />
component. It doesn't make sense to have a FontMetrics that doesn't measure any<br />
specific kind of Font.<br />
Also being abstract never prevents overriding, it just requires overriding in order to<br />
derive a non-abstract subclass. And if a class is a subclass of an abstract class, it<br />
only MUST override those methods declared abstract. The other methods do not<br />
require overriding.<br />
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