This program runs in O(N*M) since
str1.contains(s) is O(N) with N = length(str1), M = length(str2)

Using List it takes O(N) only, N = max length of (str1, str2).

I make several classes but compile error shows:
/Calculator.java:24: error: class Square is public, should be declared in a file named Square.java

How to compile multitple classes here?