Passing Objects As Arguments

HI, THANKS FOR REPLYING, THANKU VERY MUCH FOR TAKING YOUR TIME, to solve my problem.THE PROGRAM WILL WORK IF I CHANGE THE LINE IN MAIN METHOD

LongNum lc=la.add(lb)

int lc=la.add(lb)

BUT WITH OUT CHANGING THE LINE, i want to make this program work

XX

//BLUE COLOR REPRESENTS QUESTION FOR THE PROBLEM

//WORKING WITH Numbers of Unlimited length

//Write the class LongNum so that the following code will produce the sum of the two values 'a' and 'b' input as strings.

SAMPLE RUN:

1
2222222
3333333333333333333333333333333333333333333333
=333333333333333333333333333333333335555555555

2
4567
9999
=14566

import java.util.Scanner;

public class Lab5
{

public static void main(String args[])
{
Scanner sc=new Scanner(System.in);

String a=sc.nextLine();

LongNum.lon(b);

String b=sc.nextLine();

LongNum la=new LongNum(a);

LongNum lb=new LongNum(b);

LongNum lc=la.add(lb);

System.out.println(la.displayWithCommas());

System.out.println(lb.displayWithCommas());

System.out.println("="+lc.displayWithCommas());

System.out.println(" ");

System.out.println(la.toString());

System.out.println(lb.toString());

System.out.println("="+lc.toString());

}

}

---------------
// I SHOULD NOT CHANGE THE QUESTION

//I WROTE THE FOLLOWING

class LongNumno
{
private String st;
//private char a;
//private int c;
public LongNum()
{;}
public LongNum(String s1)
{
st=s1;
}
public void setSt(String s1)
{
st=s1;
}
public char[] displayWithCommas()
{
StringBuffer str = new StringBuffer(st);
String d;
char []t=new char[4];
int []r=new int[4];
for(int i=0;i<=3;i++)
{
t[i]=str.charAt(i);
//System.out.println(t[i]);
//System.out.println(r[i]);
}
return t;
}
public int add(LongNum n)
{
int car=0;
int []r = new int[10];
char[]c = n.displayWithCommas();
StringBuffer strBuf = new StringBuffer();

for(int i=0;i<=3;i++)
{

r[i]=c[i]-'0';

strBuf=strBuf.append(r[i]);

}

String strAdd = strBuf.toString() + st ;

System.out.println("am here in String add : " + strAdd);

int a = Integer.parseInt(strBuf.toString());

int b = Integer.parseInt(st);

int intAdd = a + b ;

System.out.println("am here in integer add : " + intAdd);

return intAdd;
}
}