Java JScrollpane 不可见

Question

我试图在 JScrollpane 中显示一系列按钮。阅读周围，我设法退出此代码，但没有显示任何内容。我不明白可能的错误。谢谢你的帮助

按照建议我做了一些更改，我编辑了但不起作用

已编辑或者我很笨，或者这是其他问题。这是我带有输出图像的完整代码

public class Main extends javax.swing.JFrame {
    private final JPanel gridPanel;

    public Main() {
        initComponents();
        // EXISTING PANEL
        gridPanel = new JPanel();
        JScrollPane scrollPane = new JScrollPane(gridPanel);
        scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        JPanel borderLayoutPanel = new JPanel(new BorderLayout());
        borderLayoutPanel.add(scrollPane, BorderLayout.CENTER);

        this.Avvio();
    }

    private void Avvio() {
        JPanel pane = new JPanel(new GridBagLayout());
        pane.setBorder(BorderFactory.createLineBorder(Color.BLUE));
        pane.setLayout(new GridBagLayout());

        for (int i = 0; i < 10; i++) {
            JButton button;
            GridBagConstraints c = new GridBagConstraints();
            c.fill = GridBagConstraints.HORIZONTAL;
            c.anchor = GridBagConstraints.PAGE_START;

            button = new JButton("Button 1");
            c.weightx = 0.5;
            c.gridx = 0;
            c.gridy = i;
            pane.add(button, c);

            button = new JButton("Button 2");
            c.gridx = 1;
            c.gridy = i;
            pane.add(button, c);

            button = new JButton("Button 3");
            c.gridx = 2;
            c.gridy = i;
            pane.add(button, c);

        }
        gridPanel.add(pane);
        gridPanel.revalidate();
        gridPanel.repaint();

    }
}

Answer 1

好的，从你在另一个答案中的评论:

No problem for compile , simply the Jpanel is empty. The buttons does not appear.

调用 this.Avvio(); 后，您必须调用:

this.add(scrollPane);
this.pack();

这将产生以下输出(在调整大小之前和之后):

但是还是没有JScrollPanel

这至少解决了第一个问题，但是您的代码中有更多错误，其中一些错误已经在其他答案中进行了注释:

1. 您正在扩展 JFrame，这不是必需的，因为您可以创建一个 JFrame 实例/对象并在以后使用它。您永远不会更改 JFrame 的行为，这就是不需要扩展它的原因。参见 Extends JFrame vs. creating it inside the program有关这方面的更多信息。

2. 您没有调用 pack() 或 setSize(...) 这会创建一个小窗口，您需要手动调整它的大小。在使您的 JFrame 可见之前调用推荐的 pack()。 (正如本答案开头所建议的那样)。

3. 您正在调用 .invokeLater() 方法两次。你只需要调用一次，我更喜欢这种方式:

    SwingUtilities.invokeLater(() -> new Main()); //Note there is no call to .setVisible(true); as per point #1. It should go later in the program like:  frame.setVisible(true);
   

4. 您正在调用 gridPanel.revalidate(); 和 gridPanel.repaint() 虽然它不会影响您的程序，但您不需要它GUI 仍然不可见，因此这些调用对您的程序没有影响，您可以安全地删除它们。

5. 您在 for 循环的每次迭代中创建了一个新的 GridBagConstraints 对象，您只需在其中更改其属性并在 外部声明它code>for 循环，这将使您的程序变得更好。

遵循上述建议后，您可以得到如下代码:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.SwingUtilities;

public class Main {
    private final JPanel gridPanel;
    private JFrame frame;

    public Main() {
        // EXISTING PANEL
        gridPanel = new JPanel();
        JScrollPane scrollPane = new JScrollPane(gridPanel);
        scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        JPanel borderLayoutPanel = new JPanel(new BorderLayout());
        borderLayoutPanel.add(scrollPane, BorderLayout.CENTER);

        this.Avvio();
        
        frame.add(scrollPane);
        frame.pack();
        frame.setVisible(true);
    }

    private void Avvio() {
        JPanel pane = new JPanel(new GridBagLayout());
        pane.setBorder(BorderFactory.createLineBorder(Color.BLUE));
        pane.setLayout(new GridBagLayout());

        for (int i = 0; i < 10; i++) {
            JButton button;
            GridBagConstraints c = new GridBagConstraints();
            c.fill = GridBagConstraints.HORIZONTAL;
            c.anchor = GridBagConstraints.PAGE_START;

            button = new JButton("Button 1");
            c.weightx = 0.5;
            c.gridx = 0;
            c.gridy = i;
            pane.add(button, c);

            button = new JButton("Button 2");
            c.gridx = 1;
            c.gridy = i;
            pane.add(button, c);

            button = new JButton("Button 3");
            c.gridx = 2;
            c.gridy = i;
            pane.add(button, c);

        }
        gridPanel.add(pane);
    }

    public static void main(String args[]) {
        /* Create and display the form */
        SwingUtilities.invokeLater(() -> {
            new Main();
        });
    }
}

仍然产生这个输出:

但是......我们还可以再改进一点!

我们可能有两个嵌套的 for 循环，用于 GridBagConstraints 属性以及按钮的生成:

import java.awt.BorderLayout;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.GridLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.SwingUtilities;

public class ScrollablePaneWithButtons {
    private static final int ROWS = 10;
    private static final int COLS = 3;

    private JFrame frame;
    private JPanel pane;
    private JButton[][] buttons;
    private GridBagConstraints gbc;
    private JScrollPane scroll;
    private JButton[] menuButtons;
    private JPanel menuPane;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new ScrollablePaneWithButtons()::createAndShowGui);
    }

    private void createAndShowGui() {
        frame = new JFrame(this.getClass().getSimpleName());

        pane = new JPanel();
        pane.setLayout(new GridBagLayout());
        
        menuPane = new JPanel();
        menuPane.setLayout(new GridLayout(1, 3));

        buttons = new JButton[ROWS][COLS];
        
        menuButtons = new JButton[] {new JButton("Edit"), new JButton("Delete"), new JButton("Sort Fields")};

        gbc = new GridBagConstraints();

        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.anchor = GridBagConstraints.PAGE_START;
        gbc.weightx = 0.5;

        for (int i = 0; i < ROWS; i++) {
            for (int j = 0; j < COLS; j++) {
                buttons[i][j] = new JButton("Button " + (j + 1));

                gbc.gridx = j;
                gbc.gridy = i;

                pane.add(buttons[i][j], gbc);
            }
        }

        scroll = new JScrollPane(pane, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
        
        for (JButton b : menuButtons) {
            menuPane.add(b);
        }
        frame.add(scroll);
        frame.add(menuPane, BorderLayout.SOUTH);

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

而且这个例子(在我看来)更容易阅读和跟进。这是上面代码生成的输出:

您仍然可以选择要使用的代码，可以在此答案的第一部分进行修改，第二部分按照上述建议进行修改，或者最后一个更短。