Java Project - UPC Calculator / Calling ToString Method?

Our goal is to write a pretty simple program, one that takes the 12 digit UPC code entered by a user and to not only spit it back out in a format with dashes using toString, and also returns the first digit, a 2 more groups of digits numbering 2-6 and 7-11, and finally display the 12th digit. It then performs an equation to check the last digit and make sure the UPC code is correct.

However, being so new to java (I only learned visual basic before), with this I was introduced to two new concepts that for some reason I simply cannot grasp for the life of me: Using and calling the toString method, and calling on methods that are created in a completely different class file.

The first section of code is my UPC class, which is meant to contain all my methods as well as the toString to be called on:

public class UPC
{
// Instance variables
private int itemType; // digit 1
private int manufacturer; // digits 2,3,4,5,6
private int product; // digits 7,8,9,10,11
private int checkDigit; // digit 12

[Code] .....