Calculator

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Calculator extends JFrame{

    JButton add, subtract, multiply, divide;
    JTextField num1, num2;
    JLabel result, enter1, enter2;


    public Calculator(){
        setLayout(new GridBagLayout());
        GridBagConstraints c = new GridBagConstraints();

        enter1 = new JLabel("1st:");
        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 0;
        c.gridy = 0;
        c.gridwidth = 0;
        add(enter1, c);

        num1 = new JTextField(" ");
        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 1;
        c.gridy = 0;
        c.gridwidth = 3;
        add(num1, c);

        enter2 = new JLabel("2nd: ");
        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 0;
        c.gridy = 1;
        c.gridwidth = 1;
        add(enter2, c);

        num2 = new JTextField(" ");
        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 1;
        c.gridy = 1;
        c.gridwidth = 3;
        add(num2, c);

        add = new JButton(" + ");
        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 0;
        c.gridy = 2;
        c.gridwidth = 1;
        add(add, c);

        subtract = new JButton(" - ");
        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 1;
        c.gridy = 2;
        c.gridwidth = 0;
        add(subtract, c);

        multiply = new JButton(" * ");
        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 2;
        c.gridy = 2;
        c.gridwidth = 0;
        add(multiply, c);

        divide = new JButton(" / ");
        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 3;
        c.gridy = 2;
        c.gridwidth = 0;
        add(divide, c);

        result = new JLabel (" ");
        c.fill = GridBagConstraints.HORIZONTAL;
        c.gridx = 0;
        c.gridy = 4;
        c.gridwidth = 4;
        add(result, c);

        event a = new event();
        add.addActionListener(a);
        subtract.addActionListener(a);
        multiply.addActionListener(a);
        divide.addActionListener(a);
    }
    public class event implements ActionListener{
        public void ActionPerformed(ActionEvent a){
            double number1, number2;

            try{
                number1 = Double.parseDouble(num1.getText());
            }catch (NumberFormatException e){
                result.setText("Illegal data in 1st field!");
                return;
                }

            try{
                number2 = Double.parseDouble(num2.getText());
            }catch (NumberFormatException e){
                result.setText("Illegal data in 2nd field!");
                return;
                }

            String operation = a.getActionCommand();

        if(operation.equals(" + ")){
                double sum = number1 + number2;
                result.setText(number1 + "+" + number2 + "=" + sum);
        }else if(operation.equals(" - ")){
            double diff = number1 - number2;
            result.setText(number1 + " - " + number2 + "=" + diff);
        }else if(operation.equals(" * ")){
            double factor = number1 * number2;
            result.setText(number1 + " * " + number2 + "=" + factor);
        }else if(operation.equals(" / ")){
            if(number2 == 0){
                result.setText("Cannot divide by zero");
            }else{
                double quotient = number1/number2;
                result.setText(number1 + "/" + number2 + "=" + quotient);
}
        }
        }
    }
public static void main(String args[]){
    Calculator gui = new Calculator();
    gui.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    gui.setVisible(true);
    gui.setSize(250, 175);
    gui.setTitle("My First Calculator");}
}