General Java Questions I

   case 11:

       /* other stuff */

}

Jan Schaumann 

 Q: Is there a general reason why nullpointer exceptions occur?

Answer: Of course there is. A NullPointerException happens when you have a

reference, it s set to null (or is a class or instance variable and has never been

initialized), and you try to perform some operation on the object it points to. Since it

really doesn t point to an object, you can t do that.

Chris Smith

 Q: I am aware that the exceptions in the catch statements must be of the same

type or a super type of the exception thrown in the try block.

My question is this:

Is there any significance in the ordering of the catch statements after a try block?

Answer: It is required to design you catch statement sequences with most derived

exception class type first and progressing to the highest superclass. Else, the code

will not compile. 

For a obvious reason, if the a catch block for a given class precedes a catch block for

a type that is derived from the first, the second catch block will never be executed.

D. Michael Nelson

 Q: I wrote a program that use a few RS232 ports. The operators are unskilled

and often start multiple instances of the program. Will someone please be so kind

and tell me how I can prevent them doing it?

Answer 1: The first instance might write a file. Subsequent instances could check for

the existence of that file, or else check it s contents.

Another method could involve creating a server socket on a specific port. Subsequent

efforts to create a socket on that same port would throw an exception.

Answer 2: Actually a better way is to (on launch):

1) Check if the file exists. If not, create it, open it and run. Leave it open until you quit,

upon which time you close it.

2) If the file _does_ exist, try to delete it. If the delete fails, then someone else has it

open, which means another copy of your app is running. Inform the user and quit.

3) If you succeeded in deleting it, then you are the first. Now create, open and run.

Doing the above prevents the problem of having the semaphore file left around when

the system crashes. I implemented it recently in one of our apps, and it works like a

charm.

Burt Johnson

MindStorm Productions, Inc.

http://www.mindstorm inc.com

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