Exception In Thread Main - Cannot Format Given Object

I am working on a program that will allow a user to input grades for a class of four students who have taken two tests. Based on the grades entered, the program will calculate the averages of the two tests for each student and display it along with their respected letter grades.

Now I can get the program to compile successfully, but after inputting the grades in, I get the error message saying that it cannot format given object as a number. I am using 4 arrays to execute this program and maybe that's why I'm having the trouble? I'm not sure because I am still fairly new at this stuff and can't sen to resolve it.

I was having problems for a while and then finally got excited when I got it to compile without any errors and now I'm getting an error inside my program. All I need to do is format the numbers of the grades into something like: 000, and each of the averages as 000.0. I understand how to do it because I have done it in another program I've done in the past. I just don't know how to fix this error that is coming up.

here is my code:

import java.util.Scanner;
import java.text.DecimalFormat;
public class TestAverage
{
/**
* A program that will store and process 2 test scores for a class of 4 students.
* The program will prompt for the test scores as shown above in the sample run.
* After all the data is entered,the program will display the score for test 1, test 2 .
* The average of the 2 tests and the letter grade for the class for each student in a tabular format.
*
*

[code]....