Я создаю программу с графическим интерфейсом, но получаю странную ошибку ⇐ JAVA

[Anonymous]

Я создал эту программу в качестве домашнего задания, мне нужно добавить один вариант графического интерфейса к другому.

Сумма двух вариантов не отображается в общей сумме расходов за кнопка семестра.

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

/**

The Main class creates the GUI for the Dorm and
Meal charges.
*/

public class Main extends JFrame
{
private JPanel dormPanel;
private JComboBox dormBox;
private JPanel mealPanel;
private JComboBox mealBox;
private JPanel totalChargesPanel;
private JPanel selectedMealPanel;
private JPanel buttonPanel;
private JButton calcButton;
private JLabel label1;
private JTextField totalCharges;

private String[] dorm = { "Allen Hall: $1,500 per semester",
"Pike Hall: $1,600 per semester",
"Farthing Hall: $1,200 per semester",
"University Suites: $1,800 pe r semester"};

private String[] meal = { "7 meals per week: $650 per semester",
"14 meals per week: $1,095 per semester",
"Unlimited meals: $1,500 per semester"};

/**
Constructor
*/
public Main()
{
super("Dormitory and Meal Plan");

// Specify an action for the close button.
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

// Create a BorderLayout manager.
setLayout(new BorderLayout());

// Create the dorm and meal panel.
buildDormPanel();
buildMealPanel();
buildSelectedTotalChargesPanel();
buildButtonPanel();

// Add the components to the content pane.
add(dormPanel, BorderLayout.WEST);
add(mealPanel, BorderLayout.EAST);
add(totalChargesPanel, BorderLayout.SOUTH);
add(buttonPanel, BorderLayout.NORTH);

// Pack the contents of the window and display it.
pack();
setVisible(true);
}

// The buildDormPanel method builds the dorm panel.
private void buildDormPanel()
{
// Create the dorm panel.
dormPanel = new JPanel();
dormBox = new JComboBox(dorm);

// Register the action listener.
dormBox.addActionListener(new ComboBoxListener());

// Add the dorm panel to the panel.
dormPanel.add(dormBox);
}

// The buildMealPanel method builds the meal panel.
private void buildMealPanel()
{
// Create the meal panel.
mealPanel = new JPanel();
mealBox = new JComboBox(meal);

// Register the action listener.
mealBox.addActionListener(new ComboBoxListener());

// Add the meal panel to the panel.
mealPanel.add(mealBox);
}

// The buttonPanel method builds the bottun panel.
private void buildButtonPanel()
{
// Create a panel.
buttonPanel = new JPanel();

// Create a button.
calcButton = new JButton("Calculate");

// Register an action listener with the button.
calcButton.addActionListener(new ButtonListener());

// Add the button to the panel.
buttonPanel.add(calcButton);
}

// The buildSelectedDormPanel builds the selected totalCharges panel.
private void buildSelectedTotalChargesPanel()
{
// Create the totalChargesPanel for the label.
totalChargesPanel = new JPanel();
label1 = new JLabel("Total charges per semester: ");

// Create the totalCharges textfield.
totalCharges = new JTextField (25);
totalCharges.setEditable(false);

// Add the totalChargesPanel to the panel.
totalChargesPanel.add(label1);
totalChargesPanel.add(totalCharges);
}

/** Private inner class that handles the event when the user
selects the dorm and meal boxes.
*/
private class ComboBoxListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
// Variables to hold the dorm, meal, and total charges.
String dorm = (String) dormBox.getSelectedItem();
String meal = (String) mealBox.getSelectedItem();

// Calculates the total.
totalCharges.setText(meal + dorm);
}
}

private class ButtonListener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
// Add code below
}
}

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


Подробнее здесь: https://stackoverflow.com/questions/496 ... eird-error