0028. Find the Index of the First Occurrence in a String

class Solution {
    public int strStr(String haystack, String needle) {
        int len1 = haystack.length();
        int len2 = needle.length();
        if(len1 < len2) return -1;
        for(int i = 0; i <= len1 - len2; i++){
            if(haystack.charAt(i) == needle.charAt(0)){
                if(haystack.substring(i, i + len2).equals(needle))
                    return i;
            }
        }
        return -1;
    }
}