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.

Answer 1

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.

Answer 2

An exception is something used to define something that didn't go right on the normal flow of an application. You have to throw the IllegalArgumentException when the method checkScore is called and it finds any argument outside the range (between 0 and 100).

Your class should have this structure:

public class TestScore {

    private int scores[]; //With setters and getters.

    public TestScore(int scores[]) {
        //Here, you set the scores array to the one on this class.
    }

    public int getAverage() {
        //You do the average here, and since you need to iterate over the 
        //array to sum each value, you can check the value and if it's not
        //ok you throw the IllegalArgumentException. No throws keyword
        //required since this kind of exception (like NullPointerException
        //and many others) are unchecked exceptions, meaning they can be 
        //thrown by a method and it does not need to specify them.
    }

}

The test class should create a TestScore object with an int array as a parameter of its constructor. Then you make a testAverageScore method that has the try-catch statement on it, since it's required to call the getAverage method.

Hope that helps. Good luck!.

EDIT: IllegalArgumentException is an unchecked exception.

Answer 3

public class TestScores {
private final int[] scores;

public TestScores(int[] scores) {
    this.scores = scores;
}

public int getAverage() {
    int sum = 0;

    if(scores.length == 0) {
        return 0;
    }

    for(int score: scores) {
        if(score < 0 || score > 100) {
            throw new IllegalArgumentException("Score is not valid!");
        }
        sum += score;
    }
    return sum/scores.length;
}

}