Class Calculator takes InputStream.
To start calculation run start() method. It will works while InputStream hasNext or until an exit keyword is inputed that you can change using setExitWord(String exitWord) method() (standard is "/exit").
We can define variables: a = 100 (a = b if b exists doesn't work, maybe will add later)
intput: a = 50
intput: b = 100
intput: a + b - 10
output: 140
intput: -b* a +1
output: -4999
intput: 1 - a
output: -49
intput: b/a
output: 2
intput: c = 15
intput: b^c
output: 10
output: Bye!