Creating my own Java Exception subclass

Question

I am creating my own Exception class for a simple command line tool I created. Basically the purpose is to decide when a fatal Exception is thrown and to terminate the program in that case. So my ReconToolException class has a boolean flag to record a fatal Exception.

However, my question is, what is the proper way to "throw" the Exception?

Should I do it this way?:

if(columnNumberForGroupByColumnFile1 == null || columnNumberForGroupByColumnFile2 == null ||
            columnNumberForCountColumnFile1 == null || columnNumberForCountColumnFile2 == null ){

              new ReconToolException("Can't find column number. Likely that there are unexpected headers in the CSV files. Check your CSV configuration enum .java files to see if they match the actual input CSV files.");

    }

or should I do it the following way?:

if(columnNumberForGroupByColumnFile1 == null || columnNumberForGroupByColumnFile2 == null ||
                columnNumberForCountColumnFile1 == null || columnNumberForCountColumnFile2 == null ){
             try {
                 throw new ReconToolException("Can't find column number. Likely that there are unexpected headers in the CSV files. Check your CSV configuration enum .java files to see if they match the actual input CSV files.");
            } catch (ReconToolException e) {
                e.printStackTrace();
            }
        }

the former seems to require that I explicitly code a print out statement in my ReconToolException class to print the error to the command line. The latter will catch itself (which is very strange AND honestly seems incorrect) and will print out the stack trace in the catch block. The former seems to make more sense, but the latter seems to work better. Is there a correct way?

here is the Exception class I created:

public class ReconToolException extends Exception {

    private boolean isFatal;

    public ReconToolException() {
        // TODO Auto-generated constructor stub
    }
    public ReconToolException(boolean isFatalException, String msg) {
        super(msg);
        this.setFatal(isFatalException);
    }

    public ReconToolException(String message) {
        super(message);
        // TODO Auto-generated constructor stub
    }

    public ReconToolException(Throwable cause) {
        super(cause);
        // TODO Auto-generated constructor stub
    }

    public ReconToolException(String message, Throwable cause) {
        super(message, cause);
        // TODO Auto-generated constructor stub
    }

    public ReconToolException(String message, Throwable cause,
            boolean enableSuppression, boolean writableStackTrace) {
        super(message, cause, enableSuppression, writableStackTrace);
        // TODO Auto-generated constructor stub
    }


    public boolean isFatal() {
        return isFatal;
    }

    public void setFatal(boolean isFatal) {
        this.isFatal = isFatal;
    }

}

What makes you think you need to explicitly print out the message? Normally, if you're running a console app and it terminates via an exception, the exception is printed automatically. I would strongly suggest that you consider separating out your fatal and non-fatal exceptions into two types though - the fatal one should probably subclass RuntimeException. — Jon Skeet
– Jon Skeet, Commented Jun 13, 2014 at 16:46

Craig Otis · Accepted Answer · 2014-06-13 17:04:32Z

4

You should simply throw the exception, without catching it yourself in the method that's throwing it, unless you want to catch it there. Normally you don't, if you're creating the exception yourself. Other callers of this method may want to catch it though.

This will require that you modify your method signature to include throws ... indicating to callers what exceptions your method has the potential to throw:

if(columnNumberForGroupByColumnFile1 == null || columnNumberForGroupByColumnFile2 == null ||
                columnNumberForCountColumnFile1 == null || columnNumberForCountColumnFile2 == null ){

        throw new ReconToolException("Can't find column number. Likely that there are unexpected headers in the CSV files. Check your CSV configuration enum .java files to see if they match the actual input CSV files.");
     }
        ...

edited Jun 13, 2014 at 17:04

answered Jun 13, 2014 at 16:46

Craig Otis

32.4k33 gold badges148 silver badges243 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Craig Otis Over a year ago

I just included it as an indication to end his if block, and continue execution if the condition is not met. Answer updated.

Jonathan Drapeau Over a year ago

without catching it yourself : In the if itself, it's pointless but it could be useful to catch it in any of the methods that called this one, or that's been called to call this one, to manage the exception there. It should be managed as needed.

M A · Accepted Answer · 2014-06-13 17:02:47Z

3

If you follow the second approach (i.e. catching the exception), then there is no meaning for the isFatal flag. If you simply throw and immediately catch the exception, then you are just displaying an error stack trace, and not allowing the exception to propagate up the sequence of method calls, and hence it cannot terminate the program in case it is fatal.

You should throw the exception, and let the caller of the method handle it.

edited Jun 13, 2014 at 17:02

answered Jun 13, 2014 at 16:49

M A

73.2k14 gold badges150 silver badges182 bronze badges

Comments

Maarten Bodewes · Accepted Answer · 2022-12-08 13:01:52Z

To me this is a design mistake that is also a common one in languages that don't have exception handling. The problem is that the code that generates the exception gets to define what is fatal and what is not. However, if something is fatal or not depends on the calling code. Your isFatal field (which could simply be called fatal) might indicate that something is fatal when run in a CLI. But you could also be running it as a web-application that reads lines from a text field in HTML, and in that case the web-server should keep running.

It's probably better to encapsulate the meaning of the exception using field or by creating multiple subclasses of Exception. You can indicate how serious the problem is in the documentation. The determination of the severity is then up to the calling classes.

Personally I never use e.printStackTrace(); I even replace it with different code in code templates offered by my IDE:

// TODO create more specific exception handling
throw new RuntimeException("Unhandled exception", e);

If the calling code thinks that the exception thrown is fatal then it can throw a new RuntimeException or one of its subclasses with a clear description. Thus should then propagate all the way back to the start of execution of the module. If it passes the main method then the process will terminate after the stack trace is printed.

Collectives™ on Stack Overflow

Creating my own Java Exception subclass

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related