Thread: Help with understanding simple GUI Code

Hi, I am fairly new to java and was wondering if someone could comment this code for me each relevent as if to a new GUI user, my friend suggested this type of feed back could be useful so I am hoping someone could help me out in simple terms lol.

The two files are attached, the first is basically a caller and the second has all the main code thanks.

first

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
 
class SwingCalculator {
 public static void main(String[] args) {
  JFrame frame = new Calculator();
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.setVisible(true);
  }
}

second

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
 
  class Calculatortest extends JFrame {
 
  private JTextField textfield;
  private boolean number = true;
  private String  equalOp  = "=";
  private CalculatorOp op = new CalculatorOp();
 
  public Calculatortest()
  {
  textfield = new JTextField("0", 12);
  textfield.setHorizontalAlignment(JTextField.RIGHT);
 
 
  ActionListener numberListener = new NumberListener();
  String buttonOrder = "1234567890 ";
  JPanel buttonPanel = new JPanel();
  buttonPanel.setLayout(new GridLayout(4, 4, 4, 4));
 
  for (int i = 0; i < buttonOrder.length(); i++)
  {
  String key = buttonOrder.substring(i, i+1);
  if (key.equals(" "))
  {
  buttonPanel.add(new JLabel(""));
  }
  else
  {
  JButton button = new JButton(key);
  button.addActionListener(numberListener);
 
  buttonPanel.add(button);
  }
  }
  ActionListener operatorListener = new OperatorListener();
  JPanel panel = new JPanel();
  panel.setLayout(new GridLayout(4, 4, 4, 4));
 
  String[] opOrder = {"+", "-", "*", "/","=","C"};
 
  for (int i = 0; i < opOrder.length; i++)
  {
  JButton button = new JButton(opOrder[i]);
  button.addActionListener(operatorListener);
 
  panel.add(button);
  }
 
 JPanel pan = new JPanel();
  pan.setLayout(new BorderLayout(4, 4));
  pan.add(textfield, BorderLayout.NORTH );
  pan.add(buttonPanel , BorderLayout.CENTER);
  pan.add(panel , BorderLayout.EAST  );
  this.setContentPane(pan);
  this.pack();
  this.setTitle("Calculator");
  this.setResizable(false);
  }
 
  private void action()
  {
  number = true;
  textfield.setText("0");
  equalOp  = "=";
  op.setTotal("0");
  }
  class OperatorListener implements ActionListener
  	{
  public void actionPerformed(ActionEvent e)
  {
  if (number)
  {
  action();
  textfield.setText("0");
  }
  else
  {
  number = true;
  String displayText = textfield.getText();
  if (equalOp.equals("=")) {
  op.setTotal(displayText);
  } else if (equalOp.equals("+")) {
  op.add(displayText);
  } else if (equalOp.equals("-")) {
  op.subtract(displayText);
  } else if (equalOp.equals("*")) {
  op.multiply(displayText);
  } else if (equalOp.equals("/")) {
  op.divide(displayText);
  }
 
 textfield.setText("" + op.getTotalString());
 equalOp = e.getActionCommand();
  }
 }
 }
 
  class NumberListener implements ActionListener
  	{
  public void actionPerformed(ActionEvent event)
  	{
  String digit = event.getActionCommand();
 
  if (number)
  {
  textfield.setText(digit);
  number = false;
  }
  else
  {
 textfield.setText(textfield.getText() + digit);
  }
  }
  }
  public class CalculatorOp
  	{
 
private int total;
 
public CalculatorOp() {
  total = 0;
  }
 public String getTotalString() {
  return ""+total;
  }
 public void setTotal(String n) {
  total = convertToNumber(n);
  }
 public void add(String n) {
  total += convertToNumber(n);
 }
 public void subtract(String n) {
  total -= convertToNumber(n);
  }
 public void multiply(String n) {
  total *= convertToNumber(n);
  }
   public void divide(String n) {
  total /= convertToNumber(n);
  }
 private int convertToNumber(String n) {
  return Integer.parseInt(n);
  }
 
}
}

I am attempting to learn, on second glance It may seem quite contentious but my intentions are good, A guide in the form of commenting would go a long way in helping me out and if that is not considered learning, I appreciate the effort to tell me your opinion.

Also to clarify I wrote about 50% of the code myself but the finer details in the main files are really lost on me, thanks to anyone who takes the time.

Ok. I have one specefic question, on 11 where it says calculatorop and its apperences as op through the code.

what does this actually do? that seems to be the main problem for me on this. thanks