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();
 }
}

• Legato: Welcome to CodeReview, Ethan. I hope you get some fine answers!

Answer 1

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(/* */));
}

• Ethan Cotler: 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!
• h.j.k.: If I want to be snarky the Operation enum isn't actually an Equation but a binary operator, or DoubleBinaryOperator. :p +1 anyways.