Java: Proper way to throw and catch an exception to validate input

Question

Im writing a program for homework where I need to validate user input and then create an object and add it to an array list. i have included just what i think is the relevant code, but im a beginner for sure so let me know if there is something else you need to see.

I have the user enter a string, then check to see if its a double. if its not a double, i throw an exception that i created

     try{
        price = Double.parseDouble(strPrice);
     }
     catch(NumberFormatException nfe){
        CDException cde = new CDException();
        cde.setMessage("Price must be a number,\nCannot create CD");
        throw cde;
     }

after making sure that it is a number, in my other class i check to make sure it is in the range that i want. (non negative in this case) then create an object with the value

public void setPrice(double newPrice)throws Exception{
    if(newPrice >= 0){
        this.price = newPrice;
    }
    else{
        CDException cde = new CDException();
        cde.setMessage("CD Price cannot be negative,\nCannot create CD");
        throw cde;
    }
}

so my question is...

is there a way to do this in one step, check both that the user entered a number and that the number is non negative. one other thing is that if the input is blank, that is the instruction to end the input loop.

Your code looks great. The only suggestion I'd offer is to pass the message argument to the exception constructor. — erickson
– erickson, Commented Nov 30, 2012 at 4:35
thanks for the input and suggestion. we are using an exception class provided for us by the teacher — Kailua Bum
– Kailua Bum, Commented Nov 30, 2012 at 5:23

Isaac · Accepted Answer · 2012-11-30 04:30:18Z

2

Well, you could change your code to read:

try {
    price = Double.parseDouble(strPrice);
    if (price < 0) {
        throw new CDException("Can't be negative");
    }
} catch (NumberFormatException ex) {
     ...
}

But the question is whether you'd really like to do that. From a design perspective, it might make more sense to do the "negative" check inside setPrice, rather than doing it as part of the parsing logic.

answered Nov 30, 2012 at 4:30

Isaac

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Comments

Perception · Accepted Answer · 2012-11-30 04:31:40Z

Having it split out into separate functions is better, in my opinion. You have better separation of concerns, which promotes code reuse. It's a pretty straightforward 'mashup' though, if you insist on combining the functions (which I don't think you should):

public void setPrice(String newPriceStr) throws CDException {
    if(newPriceStr == null) throw new CDException("Null value given for price");

    double newPrice = -1;

    try {
        newPrice = Double.valueOf(newPriceStr);
    } catch(final NumberFormatException nfe) {
        throw new CDException("Price must be a number,\nCannot create CD");
    }
    if(newPrice >= 0){
        this.price = newPrice;
    }
    else{
        throw new CDException("CD Price cannot be negative,\nCannot create CD");
    }
}

Note the shortened form for creating and throwing exceptions.

Arjan Tijms · Accepted Answer · 2012-11-30 11:00:23Z

I know this is for homework and this answer might not be accepted by your class instructor, but in the "real world" I would use a validation framework rather than reinvent the wheel, for example the built in one in Java EE 6: http://en.wikipedia.org/wiki/Bean_Validation

Here is a tutorial using them: http://docs.oracle.com/javaee/6/tutorial/doc/gircz.html

Some examples:

public class CD {
    @NotNull
    @Size(min=1, max=16)
    private String CDName;

    @Digits(integer=6, fraction=2)
    BigDecimal price;

}

Benefits

DRY (don't repeat yourself, e.g. declare validation once)
Readable (it's clear and documented)

Drawbacks

Java EE has some tendency for over complicating things, but it's still a valid choice
Not everyone likes the "Magic" of using annotations and like to see the code in their eyes

I'm sure there are more pros / cons, but that's out of the scope of this answer / question.

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