Testing a class

I am trying to test the class by making a program. I don't really understand how to. All I know is that it has something to do with prime numbers, but that is all I know. Can someone please help me? I would really appreciate it if you do. Thank you.

import java.util.Scanner;
public class Android
{
  public static int tag = 1; 
  private String name;
  public static int n = 0;
  Android ()
  {
    name = "Bob";
    changeTag();
  }

  String getName(Object input)
  {
    return input.getClass().getName();
  }

  private static boolean isPrime(int n)
  {
    for(int i = n - 1; i > 0; i--)
    {
      if (n % i == 0)
      {
        return false;
      }
    }
    return true;
  }

  public static void changeTag()
  {
    do
    {
      tag++;
    } while (!isPrime(n));
  }
}