JScrollPane doesn't work for my JPanel

问题:

first of all I must say that I have checked these questions and didn't find my answer :

1 , 2 , 3 , 4 , 5 , 6 , 7

and many other questions like so

also I have checked these tutorials and examples:

1 , 9 , 10 , 11

and many other sites. but I couldn't fix my problem.

and this is the simple kind of my code:

public class Question extends JFrame {
public Question() {
    Dimension d = Toolkit.getDefaultToolkit().getScreenSize();
    setLayout(new BorderLayout());
    setSize(d.width, d.height);
    setResizable(false);
    setDefaultCloseOperation(EXIT_ON_CLOSE);

    JPanel panel = new JPanel();
    panel.setPreferredSize(new Dimension(d.width, d.height));
    panel.setBorder(BorderFactory.createLineBorder(Color.red));
    panel.setLayout(new BoxLayout(panel, 1));
    for (int i = 0; i < 100; i++) {
        panel.add(new JButton("kjdh"));
    }
    JScrollPane scrollPane = new JScrollPane(panel);
    scrollPane.setPreferredSize(new Dimension(500, 500));
    getContentPane().add(scrollPane);
    getContentPane().add(panel);
    setVisible(true);

}

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

but the JScrollPane doesn't appear. I have tested many things. I have changed the way adding panel and scrollPane to my frame but it didn't work. can any one help me plz?

回答1:

1. Don't set a preferred size on the panel. See Should I avoid the use of setPreferred/Maximum/MinimumSize methods in Java Swing? for the reasons why.
2. Add only the scroll pane to the content pane.
   1. A content pane using the default layout (BorderLayout) will default to putting the component in the CENTER constraint if none is supplied, and the CENTER area can only accept a single component.
   2. Besides that, the panel has already been added to the scroll pane, it will already appear inside it, and can only appear in a single container.
3. Don't extend frame, just use an instance of one.
4. Don't setSize, but setExtendedState.
5. GUIs should be constructed and updated on the EDT.
6. A better close operation is DISPOSE_ON_CLOSE.

---

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

public class Question {

    public Question() {
        JFrame f = new JFrame();
        f.setLayout(new BorderLayout());
        f.setResizable(false);
        f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        JPanel panel = new JPanel();
        panel.setBorder(BorderFactory.createLineBorder(Color.red));
        panel.setLayout(new BoxLayout(panel, 1));
        for (int i = 0; i < 100; i++) {
            panel.add(new JButton("kjdh"));
        }
        JScrollPane scrollPane = new JScrollPane(panel);
        f.getContentPane().add(scrollPane);
        f.pack();
        f.setExtendedState(JFrame.MAXIMIZED_BOTH);
        f.setVisible(true);
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {
            @Override
            public void run() {
                new Question();
            }
        };
        SwingUtilities.invokeLater(r);
    }
}

回答2:

You've added an unecessary duplicate panel on the context pane. Instead of:

getContentPane().add(scrollPane);
getContentPane().add(panel);

use only

getContentPane().add(scrollPane);

It makes sense as a scrool pane is a container for a panel, so it's enough to add a container on the context pane.