Calculator class:

• input(c)
• T=0
• n=0
• changeState(c,this)

Subclass the State class. Each state is a singleton.

S0:

• getInstance()
• changeState(c, this)
  • if(c == 0-9)
    • update(c)
    • updateCurrentState(s1, s1.getInstance)

S1:

• if(c == 0-9)
  • updateCurrentState(s2, s2.getInstance)
• if(+)
  • update T
• if(-)
  • do something.

S2, S3...

Do calculation in the calculator object. Do state transition in state machine. Separate them. First, get input: input(c) Current state: getInstance() Change state: changeState(c, this)