// Exceptions demo

import java.util.ArrayList;

public class GradeList
{
    public GradeList()
    {
        myGrades = new ArrayList();
    }
    
    public void add(int newGrade)
    {
        if (newGrade < -1)
        {	
            throw new IllegalArgumentException("no negative grades allowed");
    	}
        else
        {
            myGrades.add(new Integer(newGrade));
    	}
    }
    
    public void add(String newGrade)
    {
    	throw new IllegalArgumentException("no Strings allowed");
    }
    
    public double computeAverage()
    {
        int sum = 0;
        
        for (int i = 0; i < myGrades.size(); i++)
        {
            sum += ((Integer) myGrades.get(i)).intValue();
        }
        
        if (myGrades.size() <= 0)
        {
            throw new ArithmeticException("cannot divide by zero, at least one grade must be entered");
        }
        
        return (double) sum/myGrades.size();
    }

    private ArrayList myGrades;
}