Calculator in Java using JFrame

Question

Today, I made a calculator in JFrame. The code is a little long but I'm still a beginner to Java. In the future I'm going to do some improvements on it, but here it is right now.
This is the meat of the calculator:

@SuppressWarnings("unused")
public class CalcSubClass extends JFrame {
    private static final long serialVersionUID = 1L;

JTextField numDisplay;
JButton zero;
JButton one;
JButton two;
JButton three;
JButton four;
JButton five;
JButton six;
JButton seven;
JButton eight;
JButton nine;
JButton add;
JButton subtract;
JButton multiply;
JButton divide;
JButton percent;
JButton equals;
JButton clear;
String addString;
String subString;
String multiString;
String divString;
String finalString;
double addDouble;
double subDouble;
double multiDouble;
double divDouble;
double finalDouble;
double answer;
String answerAsString;
String percentAsString;
double percentAsDouble;
double prcntAnswerAsDouble;
String prcntAnswerAsString;
JButton decimalPoint;

public static double add(double num1, double num2) {

    return num1 + num2;
}

public static double subtract(double num1, double num2) {

    return num1 - num2;
}

public static double multiply(double num1, double num2) {

    return num1 * num2;
}

public static double divide(double num1, double num2) {

    return num1 / num2;
}

public CalcSubClass() {
    super("Calculator");
    setLayout(new GridBagLayout());

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setVisible(true);
    setResizable(false);

    GridBagConstraints gbc = new GridBagConstraints();

    numDisplay = new JTextField();
    numDisplay.setPreferredSize(new Dimension(130, 30));
    numDisplay.setEditable(false);
    gbc.gridx = 1;
    gbc.gridy = 0;
    gbc.gridwidth = 3;
    gbc.fill = GridBagConstraints.HORIZONTAL;
    add(numDisplay, gbc);

    zero = new JButton("0");
    gbc.gridx = 1;
    gbc.gridy = 5;
    gbc.gridwidth = 2;
    gbc.fill = GridBagConstraints.HORIZONTAL;
    add(zero, gbc);

    one = new JButton("1");
    gbc.gridx = 1;
    gbc.gridy = 2;
    gbc.gridwidth = 1;
    add(one, gbc);

    two = new JButton("2");
    gbc.gridx = 2;
    gbc.gridy = 2;
    add(two, gbc);

    three = new JButton("3");
    gbc.gridx = 3;
    gbc.gridy = 2;
    add(three, gbc);

    four = new JButton("4");
    gbc.gridx = 1;
    gbc.gridy = 3;
    add(four, gbc);

    five = new JButton("5");
    gbc.gridx = 2;
    gbc.gridy = 3;
    add(five, gbc);

    six = new JButton("6");
    gbc.gridx = 3;
    gbc.gridy = 3;
    add(six, gbc);

    seven = new JButton("7");
    gbc.gridx = 1;
    gbc.gridy = 4;
    add(seven, gbc);

    eight = new JButton("8");
    gbc.gridx = 2;
    gbc.gridy = 4;
    add(eight, gbc);

    nine = new JButton("9");
    gbc.gridx = 3;
    gbc.gridy = 4;
    add(nine, gbc);

    add = new JButton("+");
    gbc.gridx = 1;
    gbc.gridy = 1;
    add(add, gbc);

    subtract = new JButton("-");
    gbc.gridx = 2;
    gbc.gridy = 1;
    add(subtract, gbc);

    multiply = new JButton("x");
    gbc.gridx = 3;
    gbc.gridy = 1;
    add(multiply, gbc);

    divide = new JButton("÷");
    gbc.gridx = 4;
    gbc.gridy = 2;
    add(divide, gbc);

    percent = new JButton("%");
    gbc.gridx = 4;
    gbc.gridy = 1;
    add(percent, gbc);

    decimalPoint = new JButton(".");
    gbc.gridx = 3;
    gbc.gridy = 5;
    add(decimalPoint, gbc);

    equals = new JButton("=");
    gbc.gridx = 4;
    gbc.gridy = 3;
    gbc.gridheight = 3;
    gbc.fill = GridBagConstraints.VERTICAL;
    add(equals, gbc);

    clear = new JButton("AC");
    gbc.gridx = 4;
    gbc.gridy = 0;
    gbc.gridheight = 1;
    add(clear, gbc);

    zero.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + "0");

        }

    });

    one.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + "1");

        }

    });

    two.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + "2");

        }

    });

    three.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + "3");

        }

    });

    four.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + "4");

        }

    });

    five.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + "5");

        }

    });

    six.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + "6");

        }

    });

    seven.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + "7");

        }

    });

    eight.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + "8");

        }

    });

    nine.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + "9");

        }

    });

    add.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            addString = numDisplay.getText();
            addDouble = Double.parseDouble(addString);
            numDisplay.setText("");

        }

    });

    subtract.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            subString = numDisplay.getText();
            subDouble = Double.parseDouble(subString);
            numDisplay.setText("");
            decimalPoint.setEnabled(true);

        }

    });

    multiply.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            multiString = numDisplay.getText();
            multiDouble = Double.parseDouble(multiString);
            numDisplay.setText("");
            decimalPoint.setEnabled(true);

        }

    });

    divide.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            divString = numDisplay.getText();
            divDouble = Double.parseDouble(divString);
            numDisplay.setText("");
            decimalPoint.setEnabled(true);
        }

    });

    percent.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            percentAsString = numDisplay.getText();
            percentAsDouble = Double.parseDouble(percentAsString);
            prcntAnswerAsDouble = percentAsDouble * 0.01;
            prcntAnswerAsString = Double.toString(prcntAnswerAsDouble);
            numDisplay.setText(prcntAnswerAsString);
        }

    });

    decimalPoint.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText(numDisplay.getText() + ".");
            String numDisplayContents = numDisplay.getText();

            if (numDisplayContents.contains(".")) {
                decimalPoint.setEnabled(false);
            }

        }

    });
    clear.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            numDisplay.setText("");
            addString = null;
            subString = null;
            multiString = null;
            divString = null;
            finalString = null;
            prcntAnswerAsString = null;
        }

    });

    equals.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            finalString = numDisplay.getText();
            finalDouble = Double.parseDouble(finalString);

            if (addString != null) {
                answer = add(addDouble, finalDouble);
                answerAsString = Double.toString(answer);
                numDisplay.setText(answerAsString);

            } else if (subString != null) {
                answer = subtract(subDouble, finalDouble);
                answerAsString = Double.toString(answer);
                numDisplay.setText(answerAsString);

            } else if (multiString != null) {
                answer = multiply(multiDouble, finalDouble);
                answerAsString = Double.toString(answer);
                numDisplay.setText(answerAsString);

            } else if (divString != null) {
                answer = divide(divDouble, finalDouble);
                answerAsString = Double.toString(answer);
                numDisplay.setText(answerAsString);

            } else {
                addString = null;
                subString = null;
                multiString = null;
                divString = null;
            }
        }

    });

  }

}

This is the main class:

public class Calculator {
    public static void main(String[] args) {

    CalcSubClass calc = new CalcSubClass();
    calc.pack();
   }

}

Welcome to CodeReview, Ethan. I hope you get some fine answers! — Legato, Jul 20 at 3:59

Legato · Accepted Answer · 2015-07-20 04:29:57Z

Calculators seem really popular...

I'll start with what stands out. new JButton("÷"); This likely doesn't work for you and definitely won't work everywhere, to use the Obelus you want to refer to its unicode, so rather instantiate using new JButton("\u00F7");

Instead of having all those variables for your number buttons simple create an array.

JButton[] buttons = new JButton[10];

and instantiate using a loop, which you can also leverage to add the relevant action listeners using a lambda expression

As an example of a Lambda Expression, you have a lot of places where you write this code:

addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent event) {
            // content
        }
    });

this can be shortened to just

addActionListener(e -> {
 // content
});

As we see in the loop:

for (int i = 0; i < buttons.length; i++) {
  String val = Integer.toString(i)
  buttons[i] = new JButton(val);
  buttons[i].addActionListener(e -> numDisplay.setText(numDisplay.getText() + val));
}

If you're worried about positioning consider using a layout manager and placing the buttons within, e.g. JPanel numberPanel = new JPanel(new GridLayout(0, 3)); of course with that in our loop we would also call numberPanel.add(buttons[i]);

Your many static methods are very similar, you can simplify them by using an interface method also using lambda expressions.

First, defining the interface:

We know we always have two inputs with doubles as input and return value, so:

interface Equation {
    double compute(double alpha, double beta);
}

and now an enumeration to use it:

public enum Operation implements Equation {
    ADD("+", (x, y) -> x + y),
    SUBTRACT("-", (x, y) -> x - y),
    MULTIPLY("x",(x, y) -> x * y),
    DIVIDE("\u00F7", (x, y) -> x / y);

    private final Equation equation;
    private final String symbol

    Operation(String symbol, Equation equation) {
        this.symbol = symbol;
        this.equation = equation;
    }

    @Override
    public double compute(double alpha, double beta) {
        return equation.compute(alpha, beta);
    }

    @Override
    public String toString() {
        return symbol;
    }
}

You can make operator buttons like so:

JButton[] operatorButtons = new JButton[Operation.values().length];
for (int i = 0; i < operatorButtons.length; i++) {
  Operation op = Operation.values()[i];
  operatorButtons[i] = new JButton(op.toString)
  operatorButtons[i].addActionListener(e -> numDisplay.setText(op.compute(/* */));
}

The division symbol works for me but your right it probably won't work everywhere. And also thanks for the feedback! Now I can shorten up my code a bit! — Ethan Cotler, Jul 20 at 4:12
If I want to be snarky the Operation enum isn't actually an Equation but a binary operator, or DoubleBinaryOperator. :p +1 anyways. — h.j.k., Jul 20 at 5:58

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