JAVA GUI CAL

package com.swing;

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

@SuppressWarnings("serial")
public class InfixToPostfixCalculator extends JFrame implements ActionListener {

    JFrame frame;
    JTextField textField = new JTextField("0 ");
    Font font = new Font("Orbitron", Font.BOLD, 35);//OrbitronRoboto
    Font btnfont = new Font("Arial", Font.BOLD, 35);
    String[] btnText = new String[] { "1", "2", "3", "C", "4", "5", "6", "+", "7", "8", "9", "-", "*", "0", "/", "=", };
    JButton[] calBtn = new JButton[16];
    JPanel panel = new JPanel();

    // Contructor
    public InfixToPostfixCalculator() {
        frame = new JFrame("Calculator");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(380, 470);
        frame.setLocation(450, 130);
        frame.setLayout(null);
        frame.getContentPane().setBackground(Color.BLACK);
        frame.setResizable(false);

        textField.setFont(font);
        textField.setBounds(5, 10, 353, 50);
        textField.setEditable(false);
        textField.setBackground(Color.WHITE);
        textField.setHorizontalAlignment(JTextField.RIGHT);
        textField.setFocusable(false);
        textField.setForeground(Color.BLACK);
        textField.setBorder(BorderFactory.createLineBorder(Color.GRAY, 1, true));
        frame.add(textField);

        for (int i = 0; i < btnText.length; i++) {
            calBtn[i] = new JButton();
            calBtn[i].setText(btnText[i]);
            calBtn[i].setHorizontalTextPosition(SwingConstants.CENTER);
            calBtn[i].setFont(btnfont);
            calBtn[i].setSize(50, 50);
            calBtn[i].setForeground(Color.BLACK);
            calBtn[i].setFocusable(false);
            calBtn[i].addActionListener(this);
            panel.add(calBtn[i]);
        }
        panel.setBackground(Color.DARK_GRAY);
        panel.setBounds(5, 70, 353, 355);
        panel.setLayout(new GridLayout(4, 3, 10, 12));
        frame.add(panel);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        new InfixToPostfixCalculator();

    }

    @Override
    public void actionPerformed(ActionEvent event) {
        String input = event.getActionCommand();
        String result = textField.getText();
        switch(input) {
            case "1":case "2":case "3":case "4":case "5":
            case "6":case "7":case "8":case "9":
                result = (result.equals("0 "))? input:result+input;
                textField.setText(result);
                break;
            case"0":
                result = (!result.equals("0 "))? result+input:input;
                textField.setText(result);
                break;
            case "C": textField.setText("0 ");
                break;
            case"*":case"/":case"+":case"-":
                result = result+input;
                textField.setText(result);
                break;
            case"=":
                MyInfixToPostfix itf = new MyInfixToPostfix(result);
                textField.setText(itf.compute()+"");
                break;
        }
    }

}