java基础习题，求解答。

public int indexOf(String str, int fromIndex)
Returns the index within this string of the first occurrence of the specified substring, starting at the specified index. 
Parameters: 
str - the substring to search for.
fromIndex - the index from which to start the search.
Returns: 
the index of the first occurrence of the specified substring, starting at the specified index, or -1 if there is no such occurrence.