为什么事件在我的applet中不起作用？ （它们适用于原始应用程序）

Question

My applet is basically two combo boxes that store values in variables with their events (when the user selects an option). The confirm button generates an event that adds those two values together. The appliction works fine, but when I try to convert it into an applet, the text field doesn't show up, and there seems to be some warning sign that comes whenever I click a combo option Any advice please?

Here is the code for the applet:

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

public class DormPlanApplet extends JApplet
{
    private JPanel selectionPanel;
    private JPanel costPanel;
    private JComboBox dormListBox;
    private JComboBox mealPlanListBox;
    private JLabel costLabel;
    private JTextField costField;
    private JButton confirmButton;
    double dormCost;
    double mealCost;
    double totalCost;
    int checker1;
    int checker2;
    String costString;

    private String[] dormListArray = {"Allen Hall", "Pike Hall", "Farthing Hall", "University Suites" };
    private String[] mealPlanListArray = {"7 Meals", "14 Meals", "Unlimited Meals" };


    public void init()
    {
        //super("College Cost Calculator");

        //setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        setLayout(new BorderLayout());

        buildSelectionPanel();
        buildCostPanel();

        add(selectionPanel, BorderLayout.CENTER);
        add(costPanel, BorderLayout.SOUTH);

        //pack();
        //setVisible(true);
    }


    private void buildSelectionPanel()
    {
        selectionPanel = new JPanel();
        selectionPanel.setLayout(new FlowLayout());

        dormListBox = new JComboBox(dormListArray);
        mealPlanListBox = new JComboBox(mealPlanListArray);

        dormListBox.addActionListener(new dormCostListener());
        mealPlanListBox.addActionListener(new mealCostListener());

        selectionPanel.add(dormListBox);
        selectionPanel.add(mealPlanListBox);

    }

    private void buildCostPanel()
    {
        costPanel = new JPanel();
        costPanel.setLayout(new FlowLayout());

        costLabel = new JLabel("The total cost is:");
        confirmButton = new JButton("Confirm");
        confirmButton.addActionListener(new calcButtonListener());

        costField = new JTextField(12);
        costField.setEditable(false);
        costPanel.add(confirmButton);
        costPanel.add(costLabel);
        costPanel.add(costField);
    }

    private class dormCostListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {

            checker1 = 1;

            switch (dormListBox.getSelectedIndex())
            {
                case 0:
                    dormCost = 1500;
                break;

                case 1:
                    dormCost = 1600;
                break;

                case 2:
                    dormCost = 1200;
                break;


                case 3:
                    dormCost = 1800;
                break;

                default:
                    dormCost = 0;
                break;
            }
        }
    }


    private class mealCostListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            checker2 = 1;

            switch (mealPlanListBox.getSelectedIndex())
            {
                case 0:
                    mealCost = 560;
                break;

                case 1:
                    mealCost = 1095;
                break;

                case 2:
                    mealCost = 1500;
                break;

                default:
                    mealCost = 0;
                break;
            }
        }
    }

    private class calcButtonListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            if ((checker1 == 1) && (checker2 == 1))
            {
                totalCost = dormCost + mealCost;
                costString = Double.toString(totalCost);

                costField.setText(costString);
            } else {
                    costField.setText("It doesn't work!");
                }

        }
    }

}

Here is the code for the oringal appliction:

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

public class DormPlanApp extends JFrame
{
    private JPanel selectionPanel;
    private JPanel costPanel;
    private JComboBox dormListBox;
    private JComboBox mealPlanListBox;
    private JLabel costLabel;
    private JTextField costField;
    private JButton confirmButton;
    double dormCost;
    double mealCost;
    double totalCost;
    int checker1;
    int checker2;
    String costString;

    private String[] dormListArray = {"Allen Hall", "Pike Hall", "Farthing Hall", "University Suites" };
    private String[] mealPlanListArray = {"7 Meals", "14 Meals", "Unlimited Meals" };


    public DormPlanApp()
    {
        super("College Cost Calculator");

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        setLayout(new BorderLayout());

        buildSelectionPanel();
        buildCostPanel();

        add(selectionPanel, BorderLayout.CENTER);
        add(costPanel, BorderLayout.SOUTH);

        pack();
        setVisible(true);
    }


    private void buildSelectionPanel()
    {
        selectionPanel = new JPanel();
        selectionPanel.setLayout(new FlowLayout());

        dormListBox = new JComboBox(dormListArray);
        mealPlanListBox = new JComboBox(mealPlanListArray);

        dormListBox.addActionListener(new dormCostListener());
        mealPlanListBox.addActionListener(new mealCostListener());

        selectionPanel.add(dormListBox);
        selectionPanel.add(mealPlanListBox);

    }

    private void buildCostPanel()
    {
        costPanel = new JPanel();
        costPanel.setLayout(new FlowLayout());

        costLabel = new JLabel("The total cost is:");
        confirmButton = new JButton("Confirm");
        confirmButton.addActionListener(new calcButtonListener());

        costField = new JTextField(12);
        costField.setEditable(false);
        costPanel.add(confirmButton);
        costPanel.add(costLabel);
        costPanel.add(costField);
    }

    private class dormCostListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {

            checker1 = 1;

            switch (dormListBox.getSelectedIndex())
            {
                case 0:
                    dormCost = 1500;
                break;

                case 1:
                    dormCost = 1600;
                break;

                case 2:
                    dormCost = 1200;
                break;


                case 3:
                    dormCost = 1800;
                break;

                default:
                    dormCost = 0;
                break;
            }
        }
    }


    private class mealCostListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            checker2 = 1;

            switch (mealPlanListBox.getSelectedIndex())
            {
                case 0:
                    mealCost = 560;
                break;

                case 1:
                    mealCost = 1095;
                break;

                case 2:
                    mealCost = 1500;
                break;

                default:
                    mealCost = 0;
                break;
            }
        }
    }

    private class calcButtonListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            if ((checker1 == 1) && (checker2 == 1))
            {
                totalCost = dormCost + mealCost;
                costString = Double.toString(totalCost);

                costField.setText(costString);
            } else {
                    costField.setText("It doesn't work!");
                }

        }
    }

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

Answer 1

You need to set the size of your applet. While testing, you can use the setSize method, like this:

public void init()
{
    //super("College Cost Calculator");

    //setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    setLayout(new BorderLayout());

    buildSelectionPanel();
    buildCostPanel();

    add(selectionPanel, BorderLayout.CENTER);
    add(costPanel, BorderLayout.SOUTH);

    setSize(400, 100);

    //pack();
    //setVisible(true);
}

The HTML applet statement will pass the width and height to your applet, so you can set the size through the applet statement parameters.

Edited to add: You set default strings for the combo boxes, but not default values. If someone just presses the Calculate button, accepting the combo box defaults, then your program doesn't calculate the correct value.