1.

public static int indexOf(String mainStr,String subString,int start) {
		if((mainStr.length()<subString.length()) || mainStr==null || subString==null ) {
			return -1;
		}
		for (int i = start; i <= mainStr.length()-subString.length(); i++) {
			int j;
			for (j= i; j < i+subString.length(); j++) {
				if(mainStr.charAt(j)!=subString.charAt(j-i)) {
					break;
				}
			}
			if(j==i+subString.length()) {
				return i;
			}
		}
		return -1;
	}
 

2.

public static int indexOf(String mainString,String subString,int start) {
		int i = start;
		int j = 0;;
		if(mainString==null || subString==null || (mainString.length()<subString.length())) {
			return -1;
		}
		while(i<mainString.length() &&  j<subString.length()) {
			if(mainString.charAt(i)==subString.charAt(j)) {
				i++;
				j++;
			}else {
				i = i-j+1;
				j = 0;
			}
			if((i+subString.length())>mainString.length()) {
				return -1;
			}
		}
		if(j==subString.length()) {
			return i-j;
		}
		return -1;
	}