Your STCalculator class is now created, but as you can see in the text pane, there are no variables. The class STCalculator will need variables to keep track of its state. You define class and instance variables in the text pane displayed. We'll also define temporary variables later in the methods that need them.
Now is the time to take your best guess at which class-wide variables you will need. Our calculator object will be created as an instance of the class STCalculator, so the calculator state variables will be instance variables.
The variable textWidget holds the numeric display widget. The variable lastValue is a number memory for the calculator. The variable operation denotes which function has been selected. The variable state lets the calculator remember whether it is in function mode or equals mode.
Your class definition should look like the following: