Java GridBagLayout : Rows are overlapping

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，尽管大多数人像躲避瘟疫一样避免使用它，但它对我来说非常简单，尽管它确实添加了许多行代码。然而，我遇到了这个奇怪的问题，其中顶行(横幅图像)与底行(按钮和登录面板)重叠。我检查了一遍又一遍，并在网上寻找答案，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，而不是像它应该的那样在第二个 GridBag 行中。尽管事先已将 BannerPanel 添加到屏幕上，但不知何故，它还是在其之上绘制了 BannerPanel。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();)。但是，这是 BannerPanel 绘制时的高度，而不是它要求的高度。您需要重写 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高度和宽度之间具有固定比例的组件，同时将宽度推迟到父容器。这就造成了一种情况，您必须等待布局执行一次才能知道组件的首选大小。正如您所经历的，在 PaintComponent 中执行这些计算将通过等待布局、调整大小、绘制、等待布局和再次绘制来进行。这并不理想 - 您确实必须执行两次布局，但本质上不需要绘制两次。我认为 Swing 没有给您足够的控制来可靠地避免这种情况，但是有更惯用的方法可以做到这一点!例如，您可以重写 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));
}