Implement strStr() Java

Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

 Goal: Determine whether needle is substring of haystack or not &
    return the substring(i) i is first occurrence of needle in haystack
    Idea: Use KMP algorithm is an optimal Approach to solve this problem
    but since there is no time complexity requirement in problem.
    Approach:
    we can use brute force approach instead.
    Check match in haystack with needle every time in lengt of needle using
    nested for loop + boolean variable as a checker to skip unmatched
    Time Complexity: O(n*m)
    Space: O(1)

public class Solution {    public String strStr(String haystack, String needle) {          if(needle.length()==0) return haystack;        int hLen=haystack.length();        int nLen=needle.length();       if(nLen>hLen){  return null;}        for(int i=0;i<=hLen-nLen;i++){            boolean checker=true;            //check the subString in length of needle            for(int j=0;j<nLen;j++){                //skip it if they are unmatched                if(haystack.charAt(i+j)!=needle.charAt(j)){                    checker=false;                    break;                }            }            if(checker){                return haystack.substring(i);            }        }        return null;    }}