Combinations of Numbers that add up to given Sum

Write a program in Java to allow the user to enter a natural number.

Now generate and display all the possible combinations of natural numbers that add up to the given natural number.

Display a suitable error message and terminate the program if a valid natural number is not entered.

Example 1:
INPUT: N = 5
OUTPUT:
1 1 1 1 1
1 1 1 2
1 1 3
1 2 2
1 4
2 3
5

Example 2:
N = 0
OUTPUT:
Please enter a natural number.

import java.io.*;
class Generate{
    public static void main(String[] args)throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        System.out.print("N = ");
        int n = Integer.parseInt(br.readLine());
        if(n < 1){
            System.out.println("Please enter a natural number.");
            return;
        }
        find(n);
    }
    public static void generate(int a[], int index, int n, int reduced){
        if(reduced < 0)
            return;
        if(reduced == 0){
            for (int i = 0; i < index; i++)
                System.out.print (a[i] + " ");
            System.out.println();
            return;
        }
        int prev = (index == 0)? 1 : a[index - 1];
        for(int k = prev; k <= n ; k++){
            a[index] = k;
            generate(a, index + 1, n, reduced - k);
        }
    }
    public static void find(int n){
        int arr[] = new int [n];
        generate(arr, 0, n, n);
    }
}