Implement strStr()- Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

indexOf.java

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