Apropreate Throwable on contract violation?

Question

I'm just wondering, can anyone suggest how to deal with an implementation returning an incorrect value.
Here's a simple example to show what I mean. Imagine I wanted to make an abstract class which allows you to roll dice and return the sum of the rolls, but the action of rolling one die is implemented by subclass.
public abstract class DiceRoller {
     * Returns a dice roll (a number randomly chosen from 1 to 6)
     * @return a number from 1 to 6
    protected abstract int rollOneDie();
     * Returns a the sum of two dice rolls.
     * @return a number from 2 to 12
    public final int rollTwoDice() {
        int die1 = rollOneDie();
        int die2 = rollOneDie();
        if (die1 < 1 || die1 > 6 || die2 < 1 || die2 > 6) {
            throw new AssertionError("implementation of rollOneDie() did something bad!");
        return die1 + die2;
}Is AssertionError apropreate here? I don't think IllegalArgumentException would be right because that would imply the programmer made a mistake in calling rollTwoDice(). Does anyone know what the convension should be in this case?
This isn't actually what I'm trying to code, but I thought I'd come up with a simpleexample rather than trying to explain my program.

Answer 1

I may not have explained correctly what I'm asking. I'm asking about this from an API point of view, where you would compile the abstract class DiceRoller withuot a concrete implementation, and then release it for clients to implement their own rollOneDie(). I want rollTwoDice() to check incase they screw up the implementation it's better to throw an exception at that point than let the program continue. (fast-fail) Yeh I could just miss out the test and hope the client reads the javadoc for the abstract method they are implementing, and if not that they spot their mistake.
Imagine for instance the client implemented rollOnedie() as such:
@Override
protected int rollOneDie() {
    return (new Random()).nextInt(6); // OOPS! That return 0 to 5!!! Should be 1 to 6.
}Then they unit test rollOneDie() a few times and it returns 3, 4, 2, 2, 4 and assume it to be okay, they they put it into their real application and it sometimes returns 0, meaning that rollTwoDice() (through no fault of it's own) can return 0 or 1 and can't return 11 or 12! But if I have the assertion in place, then when their implementation returns 0, they get an assertion error, find a stack trace and friendly message explaining that their rollOneDie() returned 0, outside of the expected range 1-6. e.g. a nicer version of what I suggested might look like this:
int die1 = rollOneDie();
int die2 = rollOneDie();
if (die1 < 1 || die1 > 6) {
    throw new AssertionError("A call to rollOneDie() returned "
            + die1 + ", expected number in the range 1 to 6!");
} else if (die2 < 1 || die2 > 6) {
    throw new AssertionError("A call to rollOneDie() returned "
            + die2 + ", expected number in the range 1 to 6!");
} else {
    return die1 + die2;
}- My unit testing rollTwoDice() wont be applicable here as the real implementation of the abstract method is, yes you can unit test rollTwoDice() to see how it behaves with a dummy implementation of rollOneDie(), but no ammount of testing rollTwoDice() can prevent the API client from writing an incorrect implementation of rollOneDie();
- I wouldn't want to use the assert keyword here because that's strictly for checking returns of private implimentations and cases which can never happen. Plus it can be disabled, which I don't want.
- IllegalArgumentException is defined as +"Thrown to indicate that a method has been passed an illegal or inappropriate argument."+ and and illegal arguments have been passed to anything here.
- enums are a good idea for the dice problem but the real problem returns a number between 0L to Long.MAX_VALUE, not just numbers 1 to 6, besides even if this were the real problem what would you do if rollOneDie() returned null?
Anyway, my question is, is AssertionError apropreate to use here?