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Gavin King

Ceylon: Object construction and validation

When porting Java code to Ceylon, I sometimes run into Java classes where the constructor mixes validation with initialization. Let’s illustrate what I mean with a simple but very contrived example.

Some bad code

Consider this Java class. (Try not to write code like this at home, kiddies!)

public class Period {

    private final Date startDate;
    private final Date endDate;

    //returns null if the given String
    //does not represent a valid Date
    private Date parseDate(String date) {
       ...
    }

    public Period(String start, String end) {
        startDate = parseDate(start);
        endDate = parseDate(end);
    }

    public boolean isValid() {
        return startDate!=null && endDate!=null;
    }

    public Date getStartDate() {
        if (startDate==null) 
            throw new IllegalStateException();
        return startDate;
    }

    public Date getEndDate() {
        if (endDate==null)
            throw new IllegalStateException();
        return endDate;
    }
} 

Hey, I warned you it was going to be contrived. But it’s really not uncommon to find stuff like this in real Java code. parseDate() method) fails, we still receive an instance of Period. But the Period we get isn’t actually in a “valid” state. What do I mean by that, precisely?

Well, I would say that an object is in an invalid state if it can’t respond meaningfully to its public operations. In this case, getStartDate() andgetEndDate() can throw an IllegalStateException, which is a condition I would consider not “meaningful”.
Another way to look at this is that what we have here is a failure of type safety in the design of Period. Unchecked exceptions represent a “hole” in the type system. So a more typesafe design for Period would be one which never uses unchecked exceptions—that doesn’t throw IllegalStateException, in this case.

(Actually, in practice, in real code, I’m more likely to encounter a getStartDate() which doesn’t check for null, and actually results in aNullPointerException further down the line, which is even worse.)
We can easily translate the above Period class to Ceylon:

shared class Period(String start, String end) {

    //returns null if the given String
    //does not represent a valid Date
    Date? parseDate(String date) => ... ;

    value maybeStartDate = parseDate(start);
    value maybeEndDate = parseDate(end);

    shared Boolean valid
        => maybeStartDate exists 
        && maybeEndDate exists;

    shared Date startDate {
        assert (exists maybeStartDate);
        return maybeStartDate;
    }

    shared Date endDate {
        assert (exists maybeEndDate);
        return maybeEndDate;
    }
} 

And, of course, this code suffers from the same problem as the original Java code. The two assertions are screaming at us that there is a problem with the typesafety of the code.

Making the Java code better

How could we improve this code in Java. Well, here’s a case where Java’s much-maligned checked exceptions would be a really reasonable solution! We could slightly change Period to throw a checked exception from its constructor:

public class Period {

    private final Date startDate;
    private final Date endDate;

    //throws if the given String
    //does not represent a valid Date
    private Date parseDate(String date)
            throws DateFormatException {
       ...
    }

    public Period(String start, String end) 
            throws DateFormatException {
        startDate = parseDate(start);
        endDate = parseDate(end);
    }

    public Date getStartDate() {
        return startDate;
    }

    public Date getEndDate() {
        return endDate;
    }
} 

Now, with this solution, we can never get a Period in an invalid state, and the code which instantiates Period is obligated by the compiler to handle the case of invalid input by catching the DateFormatException somewhere.

try {
    Period p = new Period(start, end);
    ...
}
catch (DateFormatException dfe) {
    ...
}

This is a good and excellent and righteous use of checked exceptions, and it’s unfortunate that I only rarely find Java code which uses checked exceptions like this.

Making the Ceylon code better

What about Ceylon? Ceylon doesn’t have checked exceptions, so we’ll have to look for a different solution. Typically, in cases where Java would call for use of a function that throws a checked exception, Ceylon would call for the use of a function that returns a union type. Since the initializer of a class can’t return any type other than the class itself, we’ll need to extract some of the mixed initialization/validation logic into a factory function.

//returns DateFormatError if the given 
//String does not represent a valid Date
Date|DateFormatError parseDate(String date) => ... ;

shared Period|DateFormatError parsePeriod
        (String start, String end) {
    value startDate = parseDate(start);
    if (is DateFormatError startDate) {
        return startDate;
    }
    value endDate = parseDate(end);
    if (is DateFormatError endDate)  {
        return endDate;
    }
    return Period(startDate, endDate);
}

shared class Period(startDate, endDate) {
    shared Date startDate;
    shared Date endDate;
} 

The caller is forced by the type system to deal with DateFormatError:

value p = parsePeriod(start, end);
if (is DateFormatError p) {
    ...
}
else {
    ...
}

Or, if we didn’t care about the actual problem with the given date format (probable, given that the initial code we were working from lost that information), we could just use Null instead of DateFormatError:

/returns null if the given String 
//does not represent a valid Date
Date? parseDate(String date) => ... ;

shared Period? parsePeriod(String start, String end)
    => if (exists startDate = parseDate(start), 
           exists endDate = parseDate(end))
       then Period(startDate, endDate)
       else null;

shared class Period(startDate, endDate) {
    shared Date startDate;
    shared Date endDate;
} 

At least arguably, the approach of using a factory function is superior, since in general it obtains better separation between validation logic and object initialization. This is especially useful in Ceylon, where the compiler enforces some quite heavy-handed restrictions on object initialization logic in order to guarantee that all fields of the object are assigned exactly once.

Summary

In conclusion:

  • Try to separate validation from initialization, wherever reasonable.
  • Validation logic doesn’t usually belong in constructors (especially not in Ceylon).
  • Don’t create objects in “invalid” states.
  • An “invalid” state can sometimes be detected by looking for failures of typesafety.
  • In Java, a constructor or factory function that throws a checked exception is a reasonable alternative.
  • In Ceylon, a factory function that returns a union type is a reasonable alternative.
Reference: Ceylon: Object construction and validation from our JCG partner Gavin King at the Ceylon Team blog blog.

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