is substring of string

Check if string 2 is a substring of string 1. If so return position of start string

ex: " a b c d e" , " c d e" returns 2

Solution: iterative check

1. Check for first char of string 2

2. if found, check string 2. length amount of chars

3. if all chars match, return position

public int isSubstring(String one, String two){
    if (one == null || two == null || two.length() > one.length()) return -1;
    if (two.length() == 0) return 0;
    
    //check all possible positions of first char string two
    for (int i = 0; i <= (one.length - two.length); i++){
        int j = 0;
        while (j < two.length() && one.charAt(i + j) == two.charAt(j){
            j++;
        }
        if (j = two.length()) return i;
    }
    return -1;
}