JPanel 未在 JFrame 中显示，但 JFrame 仍会更改大小

Question

I don't know what I did, or what went wrong, but a change I made at some point in the last while has made my JPanel completely invisible. The JFrame it's nested in still changes in size to house it, and I can still toggle the content in the combobox.

In my desperation, I tried replacing the content of the SnakeSettingsPanel class with a single button, but the same thing happened - completely invisible, yet I can still interact with it. I figured it might be a computer error, so I tried restarting, but still nothing. When I tried adding a button to the frame outside of the JPanel, it worked just fine. What am I doing wrong?

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

public class SnakeSettingsPanel extends JPanel {
    public boolean quit = false;
    public boolean play = false;
    public int width = 20;
    public int height = 15;
    public Speed speed = Speed.SLOW;

    public JTextField wField;
    public JTextField hField;
    public JComboBox<Speed> sField;

    public static void main(String[] args) {
        JFrame jf = new JFrame();
        jf.setTitle("Snake");
        SnakeSettingsPanel settings = new SnakeSettingsPanel();
        jf.add(settings);
        jf.pack();
        jf.setVisible(true);
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public SnakeSettingsPanel() {
        setLayout(new GridBagLayout());

        // @author Create our labels.
        JLabel wLabel = new JLabel("Width:");
        JLabel hLabel = new JLabel("Height:");
        JLabel sLabel = new JLabel("Speed:");
        
        GridBagConstraints p = new GridBagConstraints();
            p.gridx = 0;
            p.gridy = 0;
            p.insets = new Insets(10, 10, 10, 10);

        // @author Create the buttons, and add listeners
        JButton y = new JButton("Play");
        JButton n = new JButton("Quit");
        y.addActionListener(new PlayListener());
        n.addActionListener(new QuitListener());

        // @author Create text fields for height/width
        wField = new JTextField(15);
        wField.setText("20");
        hField = new JTextField(15);
        hField.setText("15");

        // @author Creates a combobox for selecting speed.
        Speed[] speeds = {Speed.SLOW, Speed.MEDIUM, Speed.FAST};
        sField = new JComboBox<Speed>(speeds);

        // @author Stitch everything into the panel.
        add(wLabel, p);
        p.gridx = 1;
        add(wField, p);
        p.gridx = 0;
        p.gridy = 1;
        add(hLabel, p);
        p.gridx = 1;
        add(hField, p);
        p.gridx = 0;
        p.gridy = 2;
        add(sLabel, p);
        p.gridx = 1;
        add(sField, p);
        p.gridx = 0;
        p.gridy = 3;
        add(y, p);
        p.gridx = 1;
        add(n, p);

        setVisible(true);
    }

    public boolean getPlay() {
        return play;
    }

    public boolean getQuit() {
        return quit;
    }

    // @author Returns all settings as a SnakeSettings object
    public SnakeSettings getSettings() {
        return new SnakeSettings(width, height, speed);
    }

    public int getWidth() {
        return width;
    }

    public int getHeight() {
        return height;
    }

    public Speed getSpeed() {
        return speed;
    }

    // @author Sends out the word to start a new game.
    public class PlayListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            quit = false;
            play = true;
            width = Integer.parseInt(wField.getText());
            height = Integer.parseInt(hField.getText());
            speed = (Speed) sField.getSelectedItem();
        }
    }

    // @author Sends out the word to shut down the program.
    public class QuitListener implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            quit = true;
            play = false;
        }       
    }
}

Answer 1

Let this be a lesson on why you should avoid mixing Model (your application's data) with View (how it is displayed). Your SnakeSettingsPanel is currently both.

• As Model, it contains 3 important fields: width , height , and speed .
• As View, it is a full JPanel. JPanels have a lot of fields which you should avoid touching directly. Including width and height , usually accessed via getHeight and getWidth -- which you are overwriting with a version that always returns the same built-in values of 20 and 15 (until the user changes those values through a UI that they cannot see).

The fast fix is to rename your current getWidth() and getHeight() to avoid clashing with the built-in getWidth() and getHeight() methods of the parent JPanel class. Call them getMyWidth() , getMyHeight() , and suddenly everything works.

The better fix is to remove those fields and methods entirely, and store your own model attributes in a SnakeSettings attribute. Update it when the user clicks on play, and return it when it is requested via getSettings() . Less code for you, less chance of accidental name clashes with your parent JPanel class. This would look like:

    // SnakeSettingsPanel, before
    public int width = 20;
    public int height = 15;
    public Speed speed = Speed.SLOW;

    public int getWidth() {    // <-- clashes with superclass
        return width;
    }
    public int getHeight() {   // <-- clashes with superclass
        return height;
    }
    public Speed getSpeed() {
        return speed;
    }
    public SnakeSettings getSettings() {
        return new SnakeSettings(width, height, speed);
    }

    // SnakeSettingsPanel, after
    SnakeSettings settings = new SnakeSettings();

    public SnakeSettings getSettings() {
        return settings;
    }