int string2Int(String s) throws NumberFormatException
{
if(s==null)
throw new NumberFormatException();
int index=1;
int result=0;
int len=s.length();
char zero='0';
for(int i=len;i>0;i--)
{
char c=s.charAt(i-1);
int digit=c-zero;
if(digit<0||digit>9)
{
throw new NumberFormatException();
}
result = result+ digit*index;
index = index*10;
}
return result;
}