Sozsun sayı çarpma basamakta sınıır yok (Java)

import javax.swing.*;
public class Carpma
{
public static void main (String[] args)
{
int atHand=0, digit1=0, digit2=0, digitNew=0;
String Number1;
String Number2;
Number1=JOptionPane.showInputDialog("enter a number");
Number2=JOptionPane.showInputDialog("enter a number");

int length1=Number1.length();
int length2=Number2.length();

for(;length1>length2
{
Number2="0"+Number2;
length2++;
}
for(;length1<length2
{
Number1="0"+Number1;
length1++;
}
int k=0;
int i=length2-1;

String satir[]=new String[length2];
for(int h=0;h<length2;h++)
satir[h]="";

while (i>=0) {

digit2=((new Integer(Number2.charAt(i))).intValue()) -48;
atHand=0;
for(int j=length2-1;j>=0;j--){
digit1=((new Integer(Number1.charAt(j))).intValue())-48 ;
digitNew=(atHand+(digit1*digit2)) %10;
atHand=(digit1*digit2+atHand) / 10 ;
satir[k]=digitNew+satir[k];
if(atHand!=0 && j==0){
satir[k]=atHand+satir[k];
}

}//for

i--; k++;

} //while
for(int b=0;b<length2;b++)
{
if(b!=0)
for(int m=0;m<b;m++)
satir[b]=satir[b]+"0";
}

for(int b=0;b<length2-1;b++)
satir[b+1]=topla(satir[b],satir[b+1]);

JOptionPane.showMessageDialog(null,satir[length2-1]);


}

static String topla (String Number1 , String Number2){
int atHand=0, digit1=0, digit2=0, digitNew=0;
int length1=Number1.length();
int length2=Number2.length();

for(;length1>length2
{
Number2="0"+Number2;
length2++;
}
for(;length1<length2
{
Number1="0"+Number1;
length1++;
}

int i=length2-1;

String sum="";
while (i>=0) {
digit1=((new Integer(Number1.charAt(i))).intValue()) -48;
digit2=((new Integer(Number2.charAt(i))).intValue())-48 ;
digitNew=(atHand+digit1+digit2) %10;
atHand=(digit1+digit2+atHand) / 10 ;
sum=digitNew+sum;
--i;
}
if(atHand!=0)
sum=atHand+sum ;
return sum;
}

}//class