Is this the correct way to use an IllegalArgumentException?

Question

I'm trying to work on a Java assignment. This is what it asks:

Write a class named TestScores. The class constructor should accept an array of the test scores as its argument. The class should have a method that returns the average of the test scores. If an test score in the array is negative or greater than 100, the class should throw an IllegalArgumentException. Demonstrate. I need a file named TestScores and TestScoresDemo.

This is what I have so far. I know some of it is wrong and I need help fixing it:

class TestScores {
    public static void checkscore(int s) {
        if (s<0) throw new IllegalArgumentException("Error: score is negative.");
        else if (s>100) throw new IllegalArgumentException("Error Score is higher then 100");
        else if (s>89)throw new IllegalArgumentException("Your grade is an A");
        else if (s>79 && s<90)throw new IllegalArgumentException("Your grade is an B");
        else if (s>69 && s<80)throw new IllegalArgumentException("Your grade is an C");
        else if (s>59 && s<70)throw new IllegalArgumentException("Your grade is an D");
        else if (s<60)throw new IllegalArgumentException("Your grade is an F");

        {
            int sum = 0; //all elements together
            for (int i = 0; i < a.length; i++)
                sum += a[i];
        }
        return sum / a.length;
    }
}

class TestScoresDemo {
    public static void main(String[] args) {
        int score = 0;
        Scanner scanner = new Scanner(System.in);
        System.out.print(" Enter a Grade number: ");
        String input = scanner.nextLine();
        score = Integer.parseInt(input);
        TestScores.checkscore(score);
        System.out.print("Test score average is" + sum);
    }
}

I know the assignment calls for a try statement because in my book that's what I see with the IllegalArgumentException. Can anyone help me? I'm using Eclipse as an IDE.

Solution

Your TestScores class should have two members: a constructor that accepts an array of scores and a method that returns the average of the scores. The assignment isn't totally clear as to which of these should throw an IllegalArgumentException if a test score is out of range, but I'd make it the constructor (since that's what has the argument).

public class TestScores {
    public TestScores(int[] scores) throws IllegalArgumentException {
        // test the scores for validity and throw an exception if appropriate
        // otherwise stash the scores in a field for later use
    }

    public float getAverageScore() {
        // compute the average score and return it
    }
}

You're on the right track with your TestScoresDemo class. It will first need to collect a set of scores into an array. Then it should construct a TestScores object. This is what needs to be inside a try/catch block because it can throw an exception. Then you just need to call getAverageScore() and do something with the result.