Writing Your Own Exceptions

[Michael Hubele]

I am trying to write my own exception but I am not sure if I am doing it right.

I found a tutorial and basicly it told me to what I have below but I am not sure if there is more too it or what I also am not sure how I can then get this exception to work in other java files I made.

I got like 6 files and there in a package that I called data so do I stick my exception file in the package folder and then go to every file that is is needed and type data.CodeLengthOutOfBoundsException in it?


Tutorial site: http://www.tech-recipes.com/java_programming_tips1199.html

 
package data
 
/*
 * CodeLengthOutOfBoundsException.java
 *
 * Created on April 22, 2006, 1:29 PM
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 */
 
/**
 *
 * @author 
 */
public class CodeLengthOutOfBoundsException extends java.lang.Exception {
    
    /**
     * Creates a new instance of <code>CodeLengthOutOfBoundsException</code> 
     * without detail message.
     */
    public CodeLengthOutOfBoundsException() {
    }
    
    
    /**
     * Constructs an instance of <code>CodeLengthOutOfBounds</code> 
     * with the specified detail message.
     * @param msg the detail message.
     */
    public CodeLengthOutOfBoundsException(String msg) {
        super("Code should be 4characters long");
    }
}

Like to me I am sure there more to it then just that but I don't know what and I don't know what I have written would get me since I have not made a test class for it yet since I don't know yet how to get it into that test class file I am sure I need to import it or do something like that.

[Mark Van Tuyl]

What you've got will indeed define a new type of Exception.

I would suggest, however, that you include the call to super() in both constructors. Otherwise, when creating an instance of the exception using the default constructor, no specific information about the exception will be available.

You might do something like this:

class AnnoyingException extends Exception {
public AnnoyingException() {
super("A really annoying exception has occurred");
}
 
public AnnoyingException(String msg) {
this();
}
}

[Ernest Friedman-Hill]

I agree with Mark to a point; however, the constructor that accepts a String argument should pass that String to the super() call; the other one, the one that doesn't accept a parameter, is the one that should pass the custom message to the superclass constructor. It would be very annoying to the programmer using your exception if it had a constructor that accepted, but then ignored, a parameter.

[Michael Hubele]

Originally posted by Ernest Friedman-Hill:
I agree with Mark to a point; however, the constructor that accepts a String argument should pass that String to the super() call; the other one, the one that doesn't accept a parameter, is the one that should pass the custom message to the superclass constructor. It would be very annoying to the programmer using your exception if it had a constructor that accepted, but then ignored, a parameter.

I don't understand can you give me a example of what going on and how to import it into another file I still don't know how to do that.

Also is there more stuff you can add to it or is that all? like do you have to add stack traces or something like that or is it automatic like I just find that a bit too easy to write an exception.
[ April 22, 2006: Message edited by: Michael Hubele ]

[Paul Clapham]

Yes, it is just that easy to write an Exception class That is all you have to do. (Apart from the problems that the other posters mentioned.) You would import it into your class exactly the same way you import any other class you use. And to use it, you just throw an instance of it when some suitable condition arises. Like for example

if (code.length != 4) {
  throw new CodeLengthOutOfBoundsException();
}

[Michael Hubele]

So what should the exception look like should both constructors have super in it or what?

[Michael Hubele]

Originally posted by Paul Clapham:
Yes, it is just that easy to write an Exception class That is all you have to do. (Apart from the problems that the other posters mentioned.) You would import it into your class exactly the same way you import any other class you use. And to use it, you just throw an instance of it when some suitable condition arises. Like for example

if (code.length != 4) {
  throw new CodeLengthOutOfBoundsException();
}

So since I wrote something in my exception right I don't need to put this:

if (code.length != 4) {
throw new CodeLengthOutOfBoundsException("stuff here right");

since that would be kind of redundent

[John Simpson]

Having a constructor in the exception that accepts a message string, as well as a default constructor gives greater flexibility for future use.
For instance if you want to check a field length of 5 in another scenario, you can re-use the same exception, and just pass a different message. Also if you need to change the code.length for any reason, you don't have to change the exception.

John

[Ernest Friedman-Hill]

This is what I was talking about:

public class CodeLengthOutOfBoundsException extends Exception {
    
    public CodeLengthOutOfBoundsException() {
        super("Code should be 4 characters long");
    }
        
    public CodeLengthOutOfBoundsException(String msg) {
        super(msg);
    }
}

So if the programmer supplies a message, it's used; if they don't the default message is used instead.