i'm a new java user i'm having problems with my program using the try and catch

Dear someone who can help
This program is supposed to calculate the average of N integers. The
program should prompt the user to enter the value for N and then
afterword ,ist emter all N numbers. If the user enters a non- positive
value for N then an exception should be thrown (and caught) with the
message "N must be positive. If there is any exceptiono as the user is
entering the N numbers an error message should be displayed and the
user prompted to enter the number again.

The problem that i'm having is that when a negative number is entered
the message displays that it must be greater than 0 but the compiler
goes back to the c prompt instead of asking for another number. and if
anything other than a number is entered such as a letter the compiler
puts out the message null and goes to the c prompt and it's supposed to
say please enter a number again

I would appreciate some help or suggestions

// Author: Judith Spurlock
// Course: ITSE 2417
// Program No: 3
// Due Date: 10/20/2006
//
// Program Name: program3JS.java

import java.util.Scanner;

public class program3JS
{
    public static void main (String[] args)
    {

    // Variable declarations
    int n = 0;
    boolean error = true;
    double average;
    int sum = 0;
    int num;
    char ch;
    Scanner keyboard = new Scanner(System.in);

    // Loop until there is no error
    do
    {
        try
        {
            error = false;
            System.out.println("How many numbers do you want to enter?");
            n = keyboard.nextInt();
            if (n < 0 )
            throw new Exception ("Number must be greater than 0.");
        }
        catch (Exception e)
        {
        String message = e.getMessage();
        System.out.println(message);
        System.exit(0);
        }
    }
    while (error);

    // Loop through each number and calculate the average
    int i;
    for (i = 0; i < n; i ++)
    {
        // Repeat input as long as there is an error
        do
        {
            try
            {
                error = false;
                System.out.println("Enter number " + (i+1));
                num = keyboard.nextInt();
                sum += num;
                if (num != ch)
                throw new Exception ("Error, please enter number again");
            }
            catch (Exception e)
            {
            String message = e.getMessage();
            System.out.println(message);

            }
        }
        while (error);
    }
    average = sum/i;
    System.out.println ("The average is " + average);
    }
}