Question

Ok so I am going to try this again. I have a very basic calculator application. I am currently having trouble figuring out how to get my plus() and minus() methods to work. Whenever I run the code, the plus() method is adding currentValue + currentValue. I want it to act like a calculator and add two different integers together. Any suggestions?

digitButton(buttons, 0);
JButton plus = new JButton("+");
plus.addActionListener(new ActionListener() {
  public void actionPerformed(ActionEvent e)
  {
    myAccumulator.plus();
    updateDisplay();
  }
});
buttons.add(plus);
return buttons;

this is where it is getting called.

And this is my class that I wrote with my plus () method.

public class BasicAccumulator implements Accumulator {

    private int digit;
    private int currentValue;

    public void BasicAccumulator(int digit, int currentValue)
    {
        this.digit = digit;
        this.currentValue = currentValue;
    }

    public void addDigit(int digit)
    {
        currentValue = currentValue * 10 + digit;
    }

    public void plus()
    {
        if (currentValue != 0)
            digit = currentValue;
            currentValue = currentValue + digit;

    }

    public void minus()
    {
        currentValue = currentValue - digit;
    }

    public void clear()
    {
        currentValue = 0;
    }

    public int displayValue()
    {
        return currentValue;
    }

}

Answer 1

Think about it. At the time you press +, you presumably have not yet entered the second number. That is, if your calculator is intended to work like most calculators out there…

Presumably you have the following sequence of events..

1. Type in digits: digits go in the current accumulator to create a number
2. Press +: digits are stored away
3. Type in digits, digits are accumulated as in 1
4. Press =: current accumulated number is added to previous one.

For this, I would conceptually store 3 things in the accumulator:

• The ‘left’ value (left-hand side of the equation) – ie the number that was stored after + was pressed
• The current operation – nothing initially, but will be set to a value that indicates whether + or -.
• The current accumulated value (i’ll just call it ‘right’)

So… Initially, left is zero and operation is nothing (ie ‘clear’).

State: left(0), operation(empty), right(0)

Type in some digits… These are built up in ‘right’.

State: left(0), operation(empty), right(22)

Press +. Now, you apply any pending operation to left. In this case the operation is empty, so you just copy the value and clear ‘right’. Then you set the operation to ‘+’

State: left(22), operation(+), right(0)

You type in more digits, once again building up right.

State: left(22), operation(+), right(20)

You hit + again. Now the operation is applied. You add right to left and clear right.

State: left(42), operation(+), right(0)

etc etc…