如何正确处理错误

Question

I'm a beginner in java writing an frontend for a webservice.I have to validate the input to get useful error messages for the user.Currently it works this way:

public Object zipVal(String zip)
{
    try
    {
        if (zip.length() == 5)
        {
            val.setZip(Integer.parseInt(zip));
            return val.getZip();
        } else
        {
            return lengthError;
        }

    } catch (NumberFormatException e)
    {
        return formatError;
    }
}

for zip Codes.Using Objects to declare the return type is not what I want tho(and is afaik discouraged),but I'm not sure how I should handle wrong inputs other than that.Should I just return null for every Exception and invalid input and handle such things in another method?

Edit:Added something to actually throw an Exception...

Answer 1

Yeah, exception handling might be one of the trickier things to consider (if one comes from a C programming background for example, where we used to be happy with < 0 return code for indicating erroneous program flow). Normally you are pretty safe off by catching other API:s you integrate with and encapsulate them in your own exception (sort of masking them away), but by doing so don't forget to chain the original exception into your own with this constructor (and/or derivatives of such):

public MyException(final String message, final Throwable cause) {
        super(message, cause);
}

One surely see alot of:

catch (Exception) {
    return null;
}

and such in code as well, I wouldn't say that this is "good" object orientation, but it is still common and could be used in special occasions.

And also, its usually very important what you do (how to handle) when you catch the exception, someone told me that programing is 90% about error control and 10% about functionality :)

Here are some tutorials/resources:
http://docs.oracle.com/javase/tutorial/essential/exceptions/ http://howtodoinjava.com/2013/04/04/java-exception-handling-best-practices/

Answer 2

If you are returning a value, then there is no need to handle the exception. It is better you declare that the method may throw the exception.

NumberFormatException is a RunTimeException. So if you wish to handle it, then better return an invalid zip (say -1) to let the caller know that something went wrong.

Otherwise, declare that you will throw a Custom Exception if NFE occurs.

This snippet may be useful.

public int setZipVal(String zip) // throws CustomException
{
    try
    {
        if (zip.length() == 5)
        {
            val.setZip(Integer.parseInt(zip));
            return val.getZip();
        } 

    } catch (NumberFormatException e)
    {
       // Log the error and return invalid zip
       return -1;

       // OR throw custom exception
       throw new CustomException("Length Error"));

    }
}