To learn more java programs
https://java4school.com/string-programs-part-5

Happy Number:
The happy number is an important number program can be defined as a number that will result in one when it is replaced by the sum of the square of its digits repeatedly.

import java.io.*;
class Happy
{
public static void main()throws IOException
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.println("Enter a number");
int num=Integer.parseInt(br.readLine());
int sum=0;
double a=0;
while(num>=0)
{
a=num%10;
a=Math.pow(a,2);
sum+=a;
num=num/10;
if(num==0)
{
if(sum<10)
break;
else
{
num=sum;
sum=0;
continue;
}
}
}
if(sum==1)
System.out.println("It is a happy number");
else
System.out.println("It is a happy number"); 
}
}

Keith number:
An n-digit number Num is a Keith Number if it forms a Fibonacci-like sequence that begins with the digits of the number itself.  For example, 197 is a Keith number since it generates the sequence 1, 9, 7, 1+9+7=17, 9+7+17=33, 7+17+33=57, +17+33+57=107,33+57+107=197
The first few keith numbers are 14, 19, 28, 47, 61, 75, 197

import java.io.*; 
class KEITH 
{ 
public static void main()throws IOException 
{ 
BufferedReader br=new BufferedReader(new InputStreamReader(System.in)); 
System.out.println("Enter a number"); 
int num=Integer.parseInt(br.readLine()); 
int len=n.length(); 
int count=0,rem=0,num=n; 
int arr[]=new int[num]; 
do 
{ 
rem=num%10; 
num=num/10; 
arr[i]=rem; 
} while(num!=0); 
for(int i=0;i<num;i++) 
{ 
arr[i+len]=arr[i]+arr[i+1]; 
count++; 
if(arr[i+len]==num || arr[i+len]>num) 
break; 
} 
if(arr[count]==num) 
System.out.println("It is a Keith number"); 
else  
System.out.println("It is a not Keith number"); } } 

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