在arraylists的arraylist中出界

Question

Whenever i try to add an element to the 1st internal array it gives an out of bounds exception. I'm trying to create a 2d arraylist to store a name in the 1st slot then 30 grades. I cannot figure out how to add anything to the internal arraylist. The error is being thrown ((ArrayList)matrix.get(i)).set(0, nameIn); on that line.

import java.util.*;
/**
 * 
 * @author mitchell castaldini 
 * @version 5/5/2016
 */
public class gradebookTester
{
    public static void main(String args[])
    {
        Scanner in = new Scanner(System.in);
        /*List<List<String>> lists = new ArrayList<List<String>>();
        for(int i = 0; i < numNames;i++)
        {
            lists.add(new ArrayList<String>());
        }*/

        ArrayList matrix = new ArrayList();

        //matrix.add(row1);
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        matrix.add(new ArrayList());
        makeArrays(matrix);
    }
    private static void makeArrays(ArrayList matrix)
    {
        ArrayList row1 = new ArrayList();
        matrix.add(row1);
        Scanner in = new Scanner(System.in);
        int a = 2;

        for(int i = 0; i < a; i++)
        {
            System.out.println("Add name");
            String nameIn = in.next();
            row1.set(0, nameIn);
            //((ArrayList)matrix.get(i)).set(0, nameIn);
            for(int d = 0; d < 10;d++)
            {
                System.out.println("Add the " + (d+1) + " grade");
                int gradeIn = in.nextInt();

                ((ArrayList)matrix.get(i)).set(d, gradeIn);
            }
        }
    }
}

Answer 1

Take a look at the documentation for the ArrayList.set method. You will see that it replaces the element at the given position. Because your lists are empty it will throw an error at this point.

You could use add to add the name to the list at this point.

There are serious flaws in your code that you would be wise to address. You intend to store strings (name) and integers (grades) in a single list. That's almost always a sign of the design flaw of storing multiple domains of data in a single collection.

Two options to address this are:

• use a Map<String,List<Integer>> grades to store your grades. Then use grades.put(name, new ArrayList<>()) and grades.get(name).add(grade) .

• Better would be to define a StudentGrade class that stores the name and list of grades. You could then have a List<StudentGrade> or Map<String,StudentGrade>> to store the list of student grades. This would make your code much clearer.