https://qiita.com/roadto93ds/items/5b2bd3be7b5d1d36c589#comment-12f4e4465c9aa3519351

Calculator.java

//package myProgram;

import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.math.BigDecimal;
import java.math.MathContext;
import java.math.RoundingMode;
import java.text.DecimalFormat;

import javax.swing.JComponent;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.WindowConstants;

public class Calculator extends JFrame {

    public static void main(String[] args) {
        Calculator calculator = new Calculator("my電卓");
        calculator.setBounds(100, 100, 400, 250);
        calculator.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        calculator.setVisible(true);
    }

    // 数字入力や計算結果はentryに代入
    BigDecimal entry = BigDecimal.ZERO;
    // "1+2=" 等の二項演算は operand=1 operator="+" entry=2 にして calculate()
    BigDecimal operand = BigDecimal.ZERO;
    final BinaryOperator NOP = () -> {};
    BinaryOperator operator = NOP;
    String point = "";                    // 浮動小数点入力なら"."、数値入力
    boolean enteringNumber = false;       // "1+2√34" を "1+34" にするため
    boolean replaceableOperator = false;  // "1+-/*2" を "1*2" にするため
    final DecimalFormat DISPLAY_FORMAT = new DecimalFormat("0.#########");

    JTextField display = new JTextField("0", 16);
    NumberButton bt0 = new NumberButton("0");
    NumberButton bt1 = new NumberButton("1");
    NumberButton bt2 = new NumberButton("2");
    NumberButton bt3 = new NumberButton("3");
    NumberButton bt4 = new NumberButton("4");
    NumberButton bt5 = new NumberButton("5");
    NumberButton bt6 = new NumberButton("6");
    NumberButton bt7 = new NumberButton("7");
    NumberButton bt8 = new NumberButton("8");
    NumberButton bt9 = new NumberButton("9");
    BinaryButton btAdd = new BinaryButton("+", () -> {
        setEntry(operand.add(entry));
    });
    BinaryButton btMultply = new BinaryButton("×", () -> {
        setEntry(operand.multiply(entry));
    });
    BinaryButton btSubtract = new BinaryButton("-", () -> {
        setEntry(operand.subtract(entry));
    });
    BinaryButton btDivide = new BinaryButton("÷", () -> {
        if (entry.equals(BigDecimal.ZERO)) {
            setEntry(BigDecimal.ZERO);  //0除算時
        } else {
            setEntry(operand.divide(entry, 9, RoundingMode.HALF_EVEN));
        }
    });
    UnaryButton btPoint = new UnaryButton(".", () -> {
        if (!enteringNumber) {
            setEntry(BigDecimal.ZERO);
            enteringNumber = true;
        }
        point = ".";
    });
    UnaryButton btClear = new UnaryButton("C", () -> {
        operand = BigDecimal.ZERO;
        operator = NOP;
        setEntry(BigDecimal.ZERO);
    });
    UnaryButton btNegative = new UnaryButton("+/-", () -> {
        setEntry(entry.multiply(new BigDecimal("-1")));
    });
    UnaryButton btSqrt = new UnaryButton("√", () -> {
        try {
            setEntry(entry.sqrt(new MathContext(9)));
        } catch (ArithmeticException e) {
            setEntry(BigDecimal.ZERO);  // 負数時
        }
    });
    UnaryButton btEqual = new UnaryButton("=", () -> {
        calculate();
    });

    Calculator(String title) {
        super(title);
        buildComponents();
    }

    void buildComponents() {
        getContentPane().add(buildDisplay(), BorderLayout.NORTH);
        getContentPane().add(buildButtons(), BorderLayout.CENTER);
    }

    JComponent buildDisplay() {
        JPanel p = new JPanel();
        display.setHorizontalAlignment(JTextField.RIGHT);
        display.setEditable(false);
        p.add(display);
        return p;
    }

    JComponent buildButtons() {
        JPanel p = new JPanel();
        p.add(buildNumbers(), BorderLayout.CENTER);
        p.add(buildFunctions(), BorderLayout.EAST);
        return p;
    }

    JComponent buildNumbers() {
        JPanel p = new JPanel();
        p.setLayout(new GridLayout(5, 3));
        p.add(btClear); p.add(btNegative); p.add(btSqrt);
        p.add(bt7);     p.add(bt8);        p.add(bt9);
        p.add(bt4);     p.add(bt5);        p.add(bt6);
        p.add(bt1);     p.add(bt2);        p.add(bt3);
        p.add(bt0);     p.add(btPoint);
        return p;
    }

    JComponent buildFunctions() {
        JPanel p = new JPanel();
        p.setLayout(new GridLayout(5, 1));
        p.add(btDivide);
        p.add(btMultply);
        p.add(btSubtract);
        p.add(btAdd);
        p.add(btEqual);
        return p;
    }

    void setEntry(BigDecimal value) {
        entry = value;
        display.setText(DISPLAY_FORMAT.format(entry));
        enteringNumber = false;
        replaceableOperator = false;
    }

    void calculate() {
        operator.calculate();
        operator = NOP;
        enteringNumber = false;
        replaceableOperator = false;
    }

    interface UnaryOperator {
        void operate();
    }

    interface BinaryOperator {
        void calculate();
    }

    class NumberButton extends JButton {

        public NumberButton(final String number) {
            super(number);
            addActionListener((ActionEvent event) -> {
                if (!enteringNumber) {
                    setEntry(BigDecimal.ZERO);
                }
                try {
                    setEntry(new BigDecimal(entry + point + number));
                } catch (NumberFormatException e) {
                    // 小数点が複数回入力された場合例外発生、無視する
                }
                point = "";
                enteringNumber = true;
                replaceableOperator = false;
            });
        }
    }

    class UnaryButton extends JButton {

        UnaryButton(String label, final UnaryOperator unary) {
            super(label);
            addActionListener((ActionEvent event) -> {
                unary.operate();
            });
        }
    }

    class BinaryButton extends JButton {

        BinaryButton(String label, final BinaryOperator binary) {
            super(label);
            addActionListener((ActionEvent event) -> {
                if (replaceableOperator) {
                    operator = binary;
                    return;
                }
                calculate();
                operand = entry;
                operator = binary;
                enteringNumber = false;
                replaceableOperator = true;
            });
        }
    }
}