Notes

package swing_grupi;

import java.awt.BorderLayout;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

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

public class Calculator implements ActionListener {
private static JTextField inputBox;

Calculator(){}
public static void main(String[] args) {
createWindow();
}

private static void createWindow() {
JFrame frame = new JFrame("Calculator");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

createUI(frame);
frame.setSize(200, 200);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}

private static void createUI(JFrame frame) {
JPanel panel = new JPanel();
Calculator calculator = new Calculator();
GridBagLayout layout = new GridBagLayout();
GridBagConstraints gbc = new GridBagConstraints();
panel.setLayout(layout);

inputBox = new JTextField(10);
inputBox.setEditable(false);

JButton button0 = new JButton("0");JButton button1 = new JButton("1");
JButton button2 = new JButton("2");JButton button3 = new JButton("3");
JButton button4 = new JButton("4");JButton button5 = new JButton("5");
JButton button6 = new JButton("6");JButton button7 = new JButton("7");
JButton button8 = new JButton("8");JButton button9 = new JButton("9");

JButton buttonPlus = new JButton("+");JButton buttonMinus = new JButton("-");
JButton buttonDivide = new JButton("/");JButton buttonMultiply = new JButton("x");
JButton buttonClear = new JButton("C");JButton buttonDot = new JButton(".");
JButton buttonEquals = new JButton("=");

button1.addActionListener(calculator);button2.addActionListener(calculator);
button3.addActionListener(calculator);button4.addActionListener(calculator);
button5.addActionListener(calculator);button6.addActionListener(calculator);
button7.addActionListener(calculator);button8.addActionListener(calculator);
button9.addActionListener(calculator);button0.addActionListener(calculator);

buttonPlus.addActionListener(calculator);buttonMinus.addActionListener(calculator);
buttonDivide.addActionListener(calculator);buttonMultiply.addActionListener(calculator);
buttonClear.addActionListener(calculator);buttonDot.addActionListener(calculator);
buttonEquals.addActionListener(calculator);

gbc.fill = GridBagConstraints.HORIZONTAL;
gbc.gridx = 0; gbc.gridy = 0; panel.add(button1, gbc);
gbc.gridx = 1; gbc.gridy = 0; panel.add(button2, gbc);
gbc.gridx = 2; gbc.gridy = 0; panel.add(button3, gbc);
gbc.gridx = 3; gbc.gridy = 0; panel.add(buttonPlus, gbc);
gbc.gridx = 0; gbc.gridy = 1; panel.add(button4, gbc);
gbc.gridx = 1; gbc.gridy = 1; panel.add(button5, gbc);
gbc.gridx = 2; gbc.gridy = 1; panel.add(button6, gbc);
gbc.gridx = 3; gbc.gridy = 1; panel.add(buttonMinus, gbc);
gbc.gridx = 0; gbc.gridy = 2; panel.add(button7, gbc);
gbc.gridx = 1; gbc.gridy = 2; panel.add(button8, gbc);
gbc.gridx = 2; gbc.gridy = 2; panel.add(button9, gbc);
gbc.gridx = 3; gbc.gridy = 2; panel.add(buttonDivide, gbc);
gbc.gridx = 0; gbc.gridy = 3; panel.add(buttonDot, gbc);
gbc.gridx = 1; gbc.gridy = 3; panel.add(button0, gbc);
gbc.gridx = 2; gbc.gridy = 3; panel.add(buttonClear, gbc);
gbc.gridx = 3; gbc.gridy = 3; panel.add(buttonMultiply, gbc);
gbc.gridwidth = 3;

gbc.gridx = 0; gbc.gridy = 4; panel.add(inputBox, gbc);
gbc.gridx = 3; gbc.gridy = 4; panel.add(buttonEquals, gbc);
frame.getContentPane().add(panel, BorderLayout.CENTER);
}

public void actionPerformed(ActionEvent e) {
String command = e.getActionCommand();
if (command.charAt(0) == 'C') {
inputBox.setText("");
}else if (command.charAt(0) == '=') {
inputBox.setText(evaluate(inputBox.getText()));
}else {
inputBox.setText(inputBox.getText() + command);
}
}

public static String evaluate(String expression) {
char[] arr = expression.toCharArray();
String operand1 = "";String operand2 = "";String operator = "";
double result = 0;

for (int i = 0; i < arr.length; i++) {
if (arr[i] >= '0' && arr[i] <= '9' || arr[i] == '.') {
if(operator.isEmpty()){
operand1 += arr[i];
}else{
operand2 += arr[i];
}
}

if(arr[i] == '+' || arr[i] == '-' || arr[i] == '/' || arr[i] == '*') {
operator += arr[i];
}
}

if (operator.equals("+"))
result = (Double.parseDouble(operand1) + Double.parseDouble(operand2));
else if (operator.equals("-"))
result = (Double.parseDouble(operand1) - Double.parseDouble(operand2));
else if (operator.equals("/"))
result = (Double.parseDouble(operand1) / Double.parseDouble(operand2));
else
result = (Double.parseDouble(operand1) * Double.parseDouble(operand2));
return operand1 + operator + operand2 + "=" +result;
}
}