Java program to check for a happy number

A number whose sum of the square of digits is ultimatly equal to 1 is a happy number. By ultimately we mean that starting with any positive number we continue the addition of square of digits till we get a single digit number (0 ~ 9). But the on whose sum is ultimately 1 is a happy number.

Example #1: 13 ; 1² + 3² = 1 + 9 =10 ; 1²+ 0²= 1 (Thus a happy number)
Example #2: 32; 3 2 + 2 2 = 9 + 4 = 13; and in Example#1 13 is already a happy number;
Some of the first happy numbers are: 1, 7, 10, 13, 19, 23, 28, 31, 32, 44, 49, 68, 70, 79, 82, 86, 91, 94, 97, 100, 103, 109, 129, 130.

import java.io.*;

class happynumber

{

public static void main(String args[])throws Exception

{

int i,j;

happyno obj=new happyno();

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

System.out.print("enter a number : ");

i=Integer.parseInt(br.readLine());

while((j=obj.happy(i))/10!=0)       //   You have to check the sum of all digits until a single digit is achieved i.e. sum=1,2,3,..9

{

i=j; // If sum of digits= 19 it then again goes to 1+9 =10 and again 1+0= '1' a single digit to check if 1 or not

}

if ( j==1)

System.out.println("It is a happy number ");

else

System.out.println("Not a happy number");

}

 

int happy(int n)

{

if(n/10==0)

return n*n;

else

return (int)Math.pow(n%10,2)+ happy(n/10);

}

}