retaining textbox values

Here is a code that creates calculator using swing components.

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

  class Calculator extends JFrame {
  private final Font BIGGER_FONT = new Font("monspaced",  Font.PLAIN, 20);
  private JTextField textfield; 
  private boolean number = true;  
  private String  equalOp  = "=";  
  private CalculatorOp op = new CalculatorOp(); 

  public Calculator() {
  textfield = new JTextField("0", 12);
  textfield.setHorizontalAlignment(JTextField.RIGHT);
  textfield.setFont(BIGGER_FONT);

  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);
  button.setFont(BIGGER_FONT);
  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);
  button.setFont(BIGGER_FONT);
  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");
  }

continue....

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);
  }
}
}
class SwingCalculator {
 public static void main(String[] args) {
  JFrame frame = new Calculator();
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  frame.setVisible(true);
  }
}