Find the first index of the substring.

public int findIndex(String mainString, String subString)
{
int result = -1;
 if (mainString == null || subString == null)
                         return result;
int mainLength= mainString.length();
int subLength = subString.length();

                 if(mainLength < subLength)
return result;
for(int i =0 ;i < mainLength; i++)
{
char cur = mainString.charAt(i);
if (cur == subString.charAt(0))
{
if (i + subLength <= mainLength)
{
if ((mainString.substring(i, subLength + i).equals(subString)))
return i;
}
else
return result;

}
}
return result;
}