Mempelajari coding orang lain

[Darkc0der]

Gan saya lagi coba mempelajari code program dari orang lain coding y seperti ini :

yang mau saya tanyakan adalah coding ini :


Code:

private String op = "=";

kira2 fungsinya buat apa gan, makasih gan atas bantuannya.....



<div style="margin:20px; margin-top:5px">
Code:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Calculator extends JFrame {

private JTextField display;
private double arg = 0;
private String op = "=";
private boolean start = true;
TextFieldHandler handler = new TextFieldHandler();

public Calculator() {
setLayout(new BorderLayout());
display = new JTextField("0");
display.setEditable(false);
add(display, "North");
JPanel p = new JPanel();
p.setLayout(new GridLayout(4, 4));
String buttons = "789/456*123-0.=+";
for (int i = 0; i < buttons.length(); i++)
addButton(p, buttons.substring(i, i + 1));
add(p, "Center");
}

private void addButton(Container c, String s) {
JButton b = new JButton(s);
c.add(b);
b.addActionListener(handler);
}

private class TextFieldHandler implements ActionListener {

public void actionPerformed( ActionEvent event ) {
String s = event.getActionCommand();
if (s.charAt(0) >= '0' && s.charAt(0)