如何修改此程序以包含八个 colors 的 JList，可用于更改此小程序的背景颜色？

Question

I am trying to add a JList of eight items so a user can change the background color of the applet here. I am not sure in which section I should include these added details, or where exactly to turn to. The book I am using is outdated and I would just like to get some advice or a solution as to how to alter this code to carry out a background change function.

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

public class Ch12_PrExercise1 extends JApplet {

    int number;

    @Override
    public void init() {
        String input;

        input = JOptionPane.showInputDialog("Enter a digit");

        number = Integer.parseInt(input);
    }

    @Override
    public void paint(Graphics g) {
        super.paint(g);

        switch (number) {
            case 0:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 175);
                g.fillRect(50, 200, 125, 25);
                g.fillRect(150, 25, 25, 175);
                break;

            case 1:
                g.fillRect(75, 25, 75, 25);
                g.fillRect(100, 50, 50, 125);
                g.fillRect(50, 175, 150, 25);
                break;

            case 2:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(150, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(50, 125, 25, 50);
                break;

            case 3:
                g.fillRect(150, 50, 25, 175);
                g.fillRect(50, 50, 100, 25);
                g.fillRect(50, 125, 100, 25);
                g.fillRect(50, 200, 100, 25);
                break;

            case 4:
                g.fillRect(50, 25, 25, 75);
                g.fillRect(50, 100, 100, 25);
                g.fillRect(150, 25, 25, 175);
                break;

            case 5:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                break;

            case 6:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                g.fillRect(50, 125, 25, 50);
                break;

            case 7:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(150, 50, 25, 150);
                break;

            case 8:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                g.fillRect(50, 125, 25, 50);
                g.fillRect(150, 50, 25, 50);
                break;

            case 9:
            default:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                g.fillRect(150, 50, 25, 50);
                break;
        }
    }
}

Answer 1

Here is an implementation with a few tweaks:

1. It paint to a panel rather than a top-level container such as an applet as the custom painted component.
2. It uses a frame rather than an applet to display the panel.
3. It defines a NamedColor to encapsulate a name and a color. This is handy for the list, as it means it can contain named colors rather than using strings or indexes to map to a color.
4. The custom painted component ( ColoredNumberPanel ) defines a preferred size, so the top level container can be packed to fit it properly.

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

public class Ch12_PrExercise1_Panel {

    private JComponent ui = null;
    private final NamedColor[] namedColors = {
        new NamedColor("CYAN", Color.CYAN),
        new NamedColor("PINK", Color.PINK),
        new NamedColor("GREEN", Color.GREEN),
        new NamedColor("MAGENTA", Color.MAGENTA),
        new NamedColor("ORANGE", Color.ORANGE),
        new NamedColor("RED", Color.RED),
        new NamedColor("WHITE", Color.WHITE),
        new NamedColor("YELLOW", Color.YELLOW)
    };

    Ch12_PrExercise1_Panel() {
        initUI();
    }

    public final void initUI() {
        if (ui != null) {
            return;
        }
        SpinnerNumberModel numberModel = new SpinnerNumberModel(0, 0, 9, 1);
        JSpinner spinner = new JSpinner(numberModel);
        JOptionPane.showMessageDialog(null, spinner, "Which Number?", JOptionPane.QUESTION_MESSAGE);

        ui = new JPanel(new BorderLayout(4, 4));
        ui.setBorder(new EmptyBorder(4, 4, 4, 4));

        DefaultListModel listModel = new DefaultListModel();
        for (NamedColor namedColor : namedColors) {
            listModel.addElement(namedColor);
        }
        final JList list = new JList(listModel);
        list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        list.setVisibleRowCount(8);
        ui.add(new JScrollPane(list), BorderLayout.LINE_START);

        final ColoredNumberPanel coloredNumberPanel = 
                new ColoredNumberPanel(numberModel.getNumber().intValue());
        ui.add(coloredNumberPanel);

        ListSelectionListener listSelectionListener = new ListSelectionListener() {

            @Override
            public void valueChanged(ListSelectionEvent e) {
                if (!e.getValueIsAdjusting()) {
                    NamedColor namedColor = (NamedColor)list.getSelectedValue();
                    Color color = namedColor.color;
                    coloredNumberPanel.setBackground(color);
                }
            }
        };
        list.addListSelectionListener(listSelectionListener);
    }

    public JComponent getUI() {
        return ui;
    }

    public static void main(String[] args) {
        Runnable r = () -> {
            try {
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
            } catch (Exception useDefault) {
            }
            Ch12_PrExercise1_Panel o = new Ch12_PrExercise1_Panel();

            JFrame f = new JFrame(o.getClass().getSimpleName());
            f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
            f.setLocationByPlatform(true);

            f.setContentPane(o.getUI());
            f.pack();
            f.setMinimumSize(f.getSize());

            f.setVisible(true);
        };
        SwingUtilities.invokeLater(r);
    }
}

class ColoredNumberPanel extends JPanel {

    int number;
    Dimension preferredSize = new Dimension(200, 250);

    ColoredNumberPanel(int number) {
        this.number = number;
    }

    /**
     * Note: The correct method to do custom painting in a JComponent (like
     * JPanel) is paintComponent(..) rather than paint(..) as used for an
     * applet/frame/window..
     */
    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g); // this also needs to be changed

        switch (number) {
            case 0:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 175);
                g.fillRect(50, 200, 125, 25);
                g.fillRect(150, 25, 25, 175);
                break;

            case 1:
                g.fillRect(75, 25, 75, 25);
                g.fillRect(100, 50, 50, 125);
                g.fillRect(50, 175, 150, 25);
                break;

            case 2:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(150, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(50, 125, 25, 50);
                break;

            case 3:
                g.fillRect(150, 50, 25, 175);
                g.fillRect(50, 50, 100, 25);
                g.fillRect(50, 125, 100, 25);
                g.fillRect(50, 200, 100, 25);
                break;

            case 4:
                g.fillRect(50, 25, 25, 75);
                g.fillRect(50, 100, 100, 25);
                g.fillRect(150, 25, 25, 175);
                break;

            case 5:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                break;

            case 6:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                g.fillRect(50, 125, 25, 50);
                break;

            case 7:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(150, 50, 25, 150);
                break;

            case 8:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                g.fillRect(50, 125, 25, 50);
                g.fillRect(150, 50, 25, 50);
                break;

            case 9:
            default:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                g.fillRect(150, 50, 25, 50);
                break;
        }
    }

    @Override
    public Dimension getPreferredSize() {
        return preferredSize;
    }
}

class NamedColor {

    String name;
    Color color;

    public NamedColor(String name, Color color) {
        this.name = name;
        this.color = color;
    }

    @Override 
    public String toString() {
        return name;
    }
}

General Tips

1. Why code an applet? If it is due to the teacher specifying it, please refer them to Why CS teachers should stop teaching Java applets .
2. Applets have become completely unworkable for sites dealing to 'all comers' on the internet. Safari and FF both block applets by default, and Chrome and IE are set to completely remove support for them. It is best to transition away from applets to pure JS/HTML sooner rather than later.
3. Use a logical and consistent form of indenting code lines and blocks. The indentation is intended to make the flow of the code easier to follow. Most IDEs have a keyboard shortcut specifically for formatting code.

Answer 2

Here is a sample of how you should be doing graphics. There is much more to it than this. I also provide:

• A JList of colors so you could see one way of doing it.
• An inputPanel with a JLabel and JFormattedTextField to accept input instead of a JOptionPane as it is it more easily positioned and allows format control.

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JFormattedTextField;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.border.LineBorder;
import javax.swing.text.MaskFormatter;

public class Ch12_PrExercise1 extends JPanel {
    int number;

    JFormattedTextField textField = null;
    JFrame frame = new JFrame("Ch12 PrExercise1");

    // add more if you want.
    MyColor[] colors = { new MyColor(Color.red, "Red"),
            new MyColor(Color.blue, "Blue"),
            new MyColor(Color.green, "Green"),
            new MyColor(Color.orange, "Orange"),
            new MyColor(Color.magenta, "Magenta"),
            new MyColor(new Color(155, 0, 155), "Purple"),
            new MyColor(Color.yellow, "Yellow"),
            new MyColor(new Color(128, 128, 128), "Gray") };

    Color color = Color.red;

    class MyColor {
        Color color;
        String name;

        public MyColor(Color color, String name) {
            this.color = color;
            this.name = name;
        }

        public String toString() {
            return name;
        }
    }

    public Ch12_PrExercise1() {
        setPreferredSize(new Dimension(250, 300));
        frame.add(this);
        // create JList here
        JList<MyColor> colorList = new JList<>(colors);
        colorList.setBorder(new LineBorder(Color.black, 1));
        try {
            textField =
                    new JFormattedTextField(new MaskFormatter("#"));
        } catch (Exception e) {
        }
        textField.setColumns(3);
        setLayout(new BorderLayout());
        add(colorList, BorderLayout.EAST);
        JPanel inputPanel = new JPanel();
        inputPanel.add(new JLabel("Please enter a digit: "));
        inputPanel.add(textField);
        add(inputPanel, BorderLayout.SOUTH);

        textField.addActionListener((ae) -> {
            String text = textField.getText();
            number = Integer.parseInt(text);
            textField.setText("");
            repaint();
        });

        // gets the selected color in the JList, sets color to that
        // color and issues a repaint.
        colorList.addListSelectionListener((lde) -> {
            color = ((JList<MyColor>) lde.getSource())
                    .getSelectedValue().color;
            repaint();
        });

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities
                .invokeLater(() -> new Ch12_PrExercise1().start());
    }

    public void start() {
        String input;

    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        // from the list of colors.
        g.setColor(color);
        switch (number) {
            case 0:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 175);
                g.fillRect(50, 200, 125, 25);
                g.fillRect(150, 25, 25, 175);
                break;

            case 1:
                g.fillRect(75, 25, 75, 25);
                g.fillRect(100, 50, 50, 125);
                g.fillRect(50, 175, 150, 25);
                break;

            case 2:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(150, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(50, 125, 25, 50);
                break;

            case 3:
                g.fillRect(150, 50, 25, 175);
                g.fillRect(50, 50, 100, 25);
                g.fillRect(50, 125, 100, 25);
                g.fillRect(50, 200, 100, 25);
                break;

            case 4:
                g.fillRect(50, 25, 25, 75);
                g.fillRect(50, 100, 100, 25);
                g.fillRect(150, 25, 25, 175);
                break;

            case 5:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                break;

            case 6:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                g.fillRect(50, 125, 25, 50);
                break;

            case 7:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(150, 50, 25, 150);
                break;

            case 8:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                g.fillRect(50, 125, 25, 50);
                g.fillRect(150, 50, 25, 50);
                break;

            case 9:
            default:
                g.fillRect(50, 25, 125, 25);
                g.fillRect(50, 50, 25, 50);
                g.fillRect(50, 100, 125, 25);
                g.fillRect(50, 175, 125, 25);
                g.fillRect(150, 125, 25, 50);
                g.fillRect(150, 50, 25, 50);
                break;
        }
    }
}