Java GridBagLayout：行重叠

Question

我有一个非常简单的 JFrame。共有三个主要面板：顶部的横幅图像、左侧的按钮列表以及用户输入登录信息以访问应用程序其余部分的主面板。

我正在使用 GridBagLayout，尽管大多数人都像避免瘟疫一样避免它，但它对我来说非常简单，尽管它确实添加了许多代码行。但是我遇到了这个奇怪的问题，顶行（横幅图像）与底行（按钮和登录面板）重叠。我检查并仔细检查并在整个网络上寻找答案，但无法弄清楚我做错了什么。

基本上，底行在整个 JFrame 中垂直居中，而不是在第二个 GridBag 行中。尽管事先已将 BannerPanel 添加到屏幕上，但不知何故，它仍被绘制在上面。我认为这可能与 BannerPanel 的工作方式有关，但我找不到解决方法。

这就是它的样子：

https://sphotos-a.xx.fbcdn.net/hphotos-snc7/375898_3720190211823_1073177291_n.jpg

它应该是这样的：

https://sphotos-a.xx.fbcdn.net/hphotos-ash4/314993_3720190291825_1429407717_n.jpg

这是我的代码：

public class LoginWindow extends JFrame implements ActionListener {
    final static String unlockCode = "unlock";
    ArrayList <User> userlist = new ArrayList <User> ();
    User user = null;

    // The visible parts of the window
    GridBagConstraints gridbag;
    JLabel inputLabel, errorLabel, lockedLabel, unlockLabel;
    JTextField usernameField, unlockField;
    JPasswordField passwordField;
    JPanel inputPanel, usernamePanel, passwordPanel, unlockPanel;

    public static void main(String[] args) {
        LoginWindow win = new LoginWindow ();
        win.userlist.add(new User ("username", "password", true));
    }

    public LoginWindow () {
        setTitle("Login");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        getContentPane().setBackground(Color.GRAY);
        setSize(640, 480);
        setResizable(false);
        resetGridbag();

        // This is where I declare all the JLabels, JPanels, etc
        inputLabel = new JLabel ("Secure Login");
        inputLabel.setFont(new Font ("SansSerif", Font.BOLD, 24));
        inputLabel.setForeground(Color.WHITE);

        JLabel usernameLabel = new JLabel ("Username  ");
        usernameLabel.setForeground(Color.WHITE);
        usernameField = new JTextField (10);
        usernameField.setActionCommand("Login");
        usernameField.addActionListener(this);
        usernamePanel = new JPanel ();
        usernamePanel.setBackground(Color.GRAY);
        usernamePanel.add(usernameLabel);
        usernamePanel.add(usernameField);

        JLabel passwordLabel = new JLabel ("Password  ");
        passwordLabel.setForeground(Color.WHITE);
        passwordField = new JPasswordField (10);
        passwordField.setActionCommand("Login");
        passwordField.addActionListener(this);
        passwordPanel = new JPanel ();
        passwordPanel.setBackground(Color.GRAY);
        passwordPanel.add(passwordLabel);
        passwordPanel.add(passwordField);

        errorLabel = new JLabel ("");
        errorLabel.setForeground(Color.WHITE);

        lockedLabel = new JLabel ("You've been locked out!");
        lockedLabel.setForeground(Color.WHITE);

        unlockLabel = new JLabel ("Unlock Code");
        unlockLabel.setForeground(Color.WHITE);
        unlockField = new JTextField (10);
        unlockField.setActionCommand("Unlock");
        unlockField.addActionListener(this);
        unlockPanel = new JPanel ();
        unlockPanel.setBackground(Color.GRAY);
        unlockPanel.add(unlockLabel);
        unlockPanel.add(unlockField);

        JLabel newPassword = new JLabel ("Request a new password");
        newPassword.setForeground(Color.WHITE);
        JPanel optionPanel = new JPanel ();
        optionPanel.setBackground(Color.GRAY);
        optionPanel.add(newPassword);

        inputPanel = new JPanel ();
        inputPanel.setBackground(Color.GRAY);
        inputPanel.setLayout(new GridBagLayout ());

        // Now I'm going to add them all to the screen
        GridBagLayout gbl = new GridBagLayout ();
        gbl.columnWeights = new double [] {0.0f, 1.0f};
        gbl.rowWeights = new double [] {0.0f, 1.0f};
        setLayout(gbl);

        gridbag.gridwidth = 2;
        gridbag.gridy = 0;
        gridbag.fill = GridBagConstraints.HORIZONTAL;
        add(new BannerPanel (), gridbag);
        gridbag.gridy = 1;
        gridbag.gridwidth = 1;
        gridbag.anchor = GridBagConstraints.NORTHWEST;
        add(optionPanel, gridbag);
        gridbag.gridx++;
        gridbag.anchor = GridBagConstraints.CENTER;
        add(inputPanel, gridbag);

        redraw();
        setVisible(true);
    }

    public void resetGridbag () {
        gridbag = new GridBagConstraints ();
        gridbag.anchor = GridBagConstraints.CENTER;
        gridbag.gridx = gridbag.gridy = 0;
    }

    public void reset () {
        inputPanel.removeAll();
        resetGridbag();
        validate();
        repaint();
    }

    public void redraw () {
        reset();
        if (user == null || !user.locked()) {
            inputPanel.add(inputLabel, gridbag);
            gridbag.gridy++;
            inputPanel.add(new JLabel ("   "), gridbag);
            gridbag.gridy++;
            inputPanel.add(usernamePanel, gridbag);
            gridbag.gridy++;
            inputPanel.add(passwordPanel, gridbag);
            gridbag.gridy++;
            inputPanel.add(new JLabel ("   "), gridbag);
            gridbag.gridy++;
            inputPanel.add(errorLabel, gridbag);
        }
        else {
            inputPanel.add(lockedLabel, gridbag);
            gridbag.gridy++;
            inputPanel.add(unlockPanel, gridbag);
            gridbag.gridy++;
            inputPanel.add(errorLabel, gridbag);
            errorLabel.setText("");
        }
        validate();
        repaint();
    }

    public void actionPerformed (ActionEvent e) {
        String button = e.getActionCommand();
        if (button.equals("Login")) {
            boolean usernameMatch = false;
            boolean passwordMatch = false;

            for (int i = 0; i < userlist.size(); i++) {
                if (usernameField.getText().equals(userlist.get(i).username())) {
                    usernameMatch = true;
                    user = userlist.get(i);
                }
                if (new String (passwordField.getPassword()).equals(userlist.get(i).password()))
                    passwordMatch = true;
            }
            passwordField.setText("");

            if (usernameMatch) {
                if (passwordMatch) {
                    user.unlock();
                    //new MainWindow ();
                    dispose();
                }
                else {
                    user.loginFail();
                    if (!user.locked())
                        errorLabel.setText("Login unsuccessful. " +
                                user.loginAttempts() + " attempts left!");
                    else
                        redraw();
                }
            }
            else
                errorLabel.setText("Login unsuccessful.");

            validate();
        }
        else if (button.equals("Unlock")) {
            if (unlockField.getText().equals(unlockCode)) {
                errorLabel.setText("");
                user.unlock();
                redraw();
            }
            else {
                errorLabel.setText("Invalid unlock code.");
                validate();
            }
            unlockField.setText("");
        }
    }
}

class BannerPanel extends JPanel {
    Image image;
    int width = 0, height = 0;
    double ratio = 0.0;

    public BannerPanel () {
        try {
            image = ImageIO.read(BannerPanel.class
                    .getClassLoader().getResourceAsStream("banner.png"));
        }
        catch (Exception e) { e.printStackTrace(); }
    }

    @Override
    protected void paintComponent (Graphics g) {
        super.paintComponent(g);

        ratio = (double) getWidth() / image.getWidth(null);
        width = getWidth();
        height = getImageHeight();
        setSize(width, height);

        if (image != null) {
            g.drawImage(image, 0, 0, width, height, this);
        }
    }

    public int getImageHeight () {
        return (int) (image.getHeight(null) * ratio);
    }
}

public class User {
    String username = "";
    String password = "";
    boolean superuser = false;
    int loginAttempts = 3;

    public User (String username, String password, boolean superuser) {
        this.username = username;
        this.password = password;
        this.superuser = superuser;
    }

    public String username () {
        return username;
    }

    public String password () {
        return password;
    }

    public boolean superuser () {
        return superuser;
    }

    public int loginAttempts () {
        return loginAttempts;
    }

    public void loginFail () {
            if (loginAttempts > 0)
            loginAttempts--;
    }

    public boolean locked () {
        return (loginAttempts == 0);
    }

    public void lock () {
        loginAttempts = 0;
    }

    public void unlock () {
        loginAttempts = 3;
    }
}

Answer 1

BannerPanel 的高度基于图像的高度 (height = getImageHeight();)。然而，这是BannerPanel 绘制的高度，而不是它要求的高度。您需要覆盖 getPreferredSize() 以根据图像和比例提供正确的所需高度 - 否则布局将假设 BannerPanel 的高度为 0。

编辑

我认为问题在于您试图创建一个具有固定高度和宽度比率的组件，同时将宽度推迟到父容器。这会产生一种情况，您必须等待布局执行一次，然后才能知道组件的首选大小。正如您所经历的那样，在paintComponent 中执行这些计算将通过等待布局、调整大小、绘图、等待布局和再次绘图来工作。这并不理想 - 您确实必须执行两次布局，但没有内在需要进行两次绘图。我认为 Swing 没有给你足够的控制来可靠地避免这种情况，但是有 更惯用的方法来做到这一点！例如，您可以覆盖setBounds，以根据实际宽度更改首选高度：

@Override
public void setBounds(int x, int y, int width, int height) {
    super.setBounds(x, y, width, height);
    int pWidth = getPreferredSize().width;
    setPreferredSize(new Dimension(pWidth, width / 2));
}

Answer 2

Jacob Raihle 的想法是对的，但它对我不起作用。 起作用的是在setSize() 旁边添加对setPreferredSize() 到paintComponent() 的调用。

显然setSize() 允许绘制整个图像，而setPreferredSize() 告诉布局的其余部分它占用了多少空间。无需重写即可将所有内容集中在一种方法中会很棒，但是，您要做什么？

这是我的 BannerPanel 对象的新代码：

class BannerPanel extends JPanel {
    Image image;
    int width = 0, height = 0;
    double ratio = 0.0;

    public BannerPanel () {
        try {
            image = ImageIO.read(BannerPanel.class
                    .getClassLoader().getResourceAsStream("banner.png"));
        }
        catch (Exception e) { e.printStackTrace(); }
    }

    @Override
    protected void paintComponent (Graphics g) {
        super.paintComponent(g);

        ratio = (double) getWidth() / image.getWidth(null);
        width = getWidth();
        height = getImageHeight();
        setPreferredSize(new Dimension (width, height));
        setSize(width, height);

        if (image != null) {
            g.drawImage(image, 0, 0, width, height, this);
        }
    }

    public int getImageHeight () {
        return (int) (image.getHeight(null) * ratio);
    }
}