Calculator GUI Java NetBeans

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package main;

/**
 *
 * @author markjasongalang
 */
import java.awt.Color;
import java.awt.Font;
import java.awt.GridLayout;
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;

class Calculator implements ActionListener {

    JFrame frame;
    JTextField textfield;
    JButton[] number_buttons = new JButton[10];
    JButton[] function_buttons = new JButton[8];
    JButton add_button, sub_button, mul_button, div_button;
    JButton dec_button, equ_button, del_button, clr_button;
    JPanel panel;

    Font my_front = new Font("Arial", Font.BOLD, 30);

    double num1 = 0;
    double num2 = 0;
    double result = 0;
    char operator;

    Calculator() {
        frame = new JFrame("Calculator");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(420, 550);
        frame.setLayout(null);

        textfield = new JTextField();
        textfield.setBounds(50, 25, 300, 50);
        textfield.setFont(my_front);
        textfield.setEditable(false);

        add_button = new JButton("+");
        sub_button = new JButton("-");
        mul_button = new JButton("*");
        div_button = new JButton("/");
        dec_button = new JButton(".");
        equ_button = new JButton("=");
        del_button = new JButton("Delete");
        clr_button = new JButton("Clear");

        function_buttons[0] = add_button;
        function_buttons[1] = sub_button;
        function_buttons[2] = mul_button;
        function_buttons[3] = div_button;
        function_buttons[4] = dec_button;
        function_buttons[5] = equ_button;
        function_buttons[6] = del_button;
        function_buttons[7] = clr_button;

        for (int i = 0; i < 8; i++) {
            function_buttons[i].addActionListener(this);
            function_buttons[i].setFont(my_front);
            function_buttons[i].setFocusable(false);
        }

        for (int i = 0; i < 10; i++) {
            number_buttons[i] = new JButton(String.valueOf(i));
            number_buttons[i].addActionListener(this);
            number_buttons[i].setFont(my_front);
            number_buttons[i].setFocusable(false);
        }

        del_button.setBounds(50, 430, 145, 50);
        clr_button.setBounds(205, 430, 145, 50);

        panel = new JPanel();
        panel.setBounds(50, 100, 300, 300);
        panel.setLayout(new GridLayout(4, 4, 10, 10));

        panel.add(number_buttons[1]);
        panel.add(number_buttons[2]);
        panel.add(number_buttons[3]);
        panel.add(add_button);
        panel.add(number_buttons[4]);
        panel.add(number_buttons[5]);
        panel.add(number_buttons[6]);
        panel.add(sub_button);
        panel.add(number_buttons[7]);
        panel.add(number_buttons[8]);
        panel.add(number_buttons[9]);
        panel.add(mul_button);
        panel.add(dec_button);
        panel.add(number_buttons[0]);
        panel.add(equ_button);
        panel.add(div_button);

        frame.add(panel);
        frame.add(del_button);
        frame.add(clr_button);
        frame.add(textfield);
        frame.setVisible(true);
    }

    public void actionPerformed(ActionEvent e) {

    }
}

public class Main {
    public static void main(String[] args) {
        Calculator calc = new Calculator();
    }
}