Java Exception Message

Question

I have a few custom exception classes that I created simply for the sake of having my own exception message:

public class DivideByZeroException extends Exception 
{   
   @Override
   public String toString()
   {
       return "ERROR: Expression cannot divide by 0";
   }
}

I realize that this can be done by throwing an exception like:

throw new Exception("ERROR: Expression cannot divide by 0");

but this contains a prefix of java.lang.Exception: in the string.

I think it's a bit ugly to remove the unwanted prefix from the string, but creating a class just for the exception message seems a bit excessive to me. Is there a different way to do this?

Answer 1

If you want all your exceptions to follow this style, then you could have one parent Exception class, then make your application exceptions extend it, like so:

class MyException extends Exception
{
    MyException(String message)
    {
        super(message);
    }

    @Override
    public String toString()
    {
        return "ERROR: " + this.getMessage();
    }
}

class DivideByZeroException extends MyException
{
    DivideByZeroException()
    {
        super("Expression cannot divide by 0");
    }
}

This way, you don't have to override toString() every time, and you also don't have to put "ERROR" into the actual text of the exception.

Answer 2

Don't throw raw Exceptions, use always a subclass (as explained by MrLore). If you are throwing raw exceptions you also have to catch them, which might mask other exception or runtime exceptions. You would add a instanceof to any of your catch blocks.

Answer 3

Even if it is already cleared by RHS, I am not quite sure what the original idea behind this question is.

I think it's a bit ugly to remove the unwanted prefix from the string, but creating a class just for the exception message seems a bit excessive to me. Is there a different way to do this?

Well, you do not have to remove the prefix? This is just some formating from Java. You could catch an Exception e and use e.getMessage() to see the message and do whatever you want with it.

Nevertheless, you should avoid implementing your own Exception class. And you should not throw a generic "Exception" (See for example Effective Java Programming Language Guide). Just use the current available exceptions from Java, most likely IllegalArgumentException or IllegalStateExceptions. This covers about 90% of all valid cases in typical code (own experience).