Java program to calculate numbers longer than long datatype can hold.

[wiggity]

hello, I am trying to write a java program that will add and multiply numbers that can have up to 100 digits. here is what i have so far.

import java.util.*;

public class Mult {
 	private static Scanner in = new Scanner (System.in);

 public static void main (String[] args) {
	int test = 0;
	do{
  		System.out.print ("Enter the first number:  ");
  		String a = in.nextLine();
		String y = a;
  		boolean check = isValid (y);
  		if(check == true){
			test = 2;
		}
	}while(test < 1);
	System.out.println(a);
	int test2 = 0;
	do{
		System.out.print("Enter the second number: ");
		String b = in.nextLine();
		String x = b;
		boolean check2 = isValid(x);
		if(check2 == true){
			test2 = 2;
		}
 	}while(test2 < 1);
	System.out.println(b);
	String T = add(a, b);
	System.out.println(T);
}
	public static boolean isValid (String num) {
		for (int i = 0; i < num.length(); i++) {
	    		char c = num.charAt(i);
	    		if ((c >= '0') && (c <= '9')){
	    		return true;
			}
	  	}
		return false;
	}
	public static String noLeadingZeroes (String num){
		int count = 0;
		for (int i = 0; i < num.length(); i++) {
	    		char c = num.charAt(i);
	    			if(c == 0){
					count = count + 1;
				}
				else{
					i = num.length();
				}
		}
		return num;
	}
	public static String add (String a, String b){
		while (a.length() < b.length()) a = "0"+a;
		while (b.length() < a.length()) b = "0"+b;
		int c = 0;
		String total = "";
		for (int i=a.length()-1; i>=0; i--) {
			int adig = a.charAt(i)-'0';
  			int bdig = b.charAt(i)-'0';
   			int ddig = 0;
			if (adig+c+bdig > 9) {
    				ddig = adig+c+bdig-10;
    				c = 1;
   			} 
			else {
   		 		ddig = adig+c+bdig;
    				c = 0;
			}
		total = ((char)(ddig + '0'))+total;
		}
	return total;

	}	
}

when i comment out the check isValid method the program runs. and I recieve the correct answer. where are my a and b variables getting lost or corrupted?

thanks for any help...

[Norm]

Try the BigInteger class.

[wiggity]

Try the BigInteger class.

This is for an assignment. The point of the excersice is to use strings. I just need to know where in my code my a and b variables are changing. I know it is somewhere in the isValid method.

[Norm]

where in my code my a and b variables are changing

Try debugging the code by adding printlns that print out the values of all the variable as they are given values and as they are used. The print out will should you where those variables are changing.
Be sure to add an ID String with the println("var=" + var) where var is the name of the variable you are printing so you know what the variable is that is printed and where it was printed.

Your code does not compile without errors. You need to fix the errors in the code BEFORE you try to execute it.