java - 如何消除 JButton 行边缘和父 JPanel 边缘之间的间隙？

Question

我正在用 Swing 制作一个计算器作为练习，我不明白为什么 JButton 行的边缘和父 JPanel (buttonPanel) 的边缘之间有这个间隙。我查看了我的代码并更改了一些东西，但我仍然无法弄清楚为什么会有差距。如果我删除 MatteBorder，将边框保留为默认值，间隙就会消失，但按钮之间会有双边框。如果我将 JTextField 的列大小减少到 7，那么间隙就会消失。如何在不求助于双边框或缩小我的应用程序的情况下修复差距？

(我将面板的背景设置为红色，以帮助确定是哪个组件导致了间隙。)

public class CalculatorView extends JFrame {
    // NOTE: Not recommended to extend JFrame, recommended to use composition over inheritance
    private JTextField display = new JTextField("0", 17);

    // Number buttons
    private JButton zeroButton = new JButton("0");
    private JButton oneButton = new JButton("1");
    private JButton twoButton = new JButton("2");
    private JButton threeButton = new JButton("3");
    private JButton fourButton = new JButton("4");
    private JButton fiveButton = new JButton("5");
    private JButton sixButton = new JButton("6");
    private JButton sevenButton = new JButton("7");
    private JButton eightButton = new JButton("8");
    private JButton nineButton = new JButton("9");
    private JButton periodButton = new JButton(".");

    // Operation buttons
    private JButton additionButton = new JButton("+");
    private JButton subtractionButton = new JButton("-");
    private JButton divisionButton = new JButton("÷");
    private JButton multiplicationButton = new JButton("x");
    private JButton signChangeButton = new JButton("+/-");
    private JButton percentButton = new JButton("%");
    private JButton clearButton = new JButton("AC");
    private JButton equalButton = new JButton("=");

    CalculatorView() {
        // Set the look and feel to the cross-platform look and feel,
        // otherwise mac os will have quirks like gaps between jbuttons
        try {
            UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
        } catch (Exception e) {
            System.err.println("Unsupported look and feel.");
            e.printStackTrace();
        }
        // Let the OS set location, prevent user from resizing window, and exit app on close
        this.setLocationByPlatform(true);
        this.setResizable(false);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setUndecorated(true);  // must be undecorated for setbackground to work

        // Create the main panel, which by default covers the entire frame
        // NOTE: Good practice. Never put components directly onto a JFrame.
        JPanel gui = new JPanel();
        // Set the main panel's layout manager to BorderLayout
        gui.setLayout(new BorderLayout());

        // Create the button panel 
        JPanel buttonPanel = new JPanel();
        // Set button panel's layout manager to GridBagLayout
        buttonPanel.setLayout(new GridBagLayout());
        // Create a GridBagConstraints object to control the layout of components
        GridBagConstraints c = new GridBagConstraints();
        c.insets = new Insets(0, 0, 0, 0);
        c.fill = GridBagConstraints.HORIZONTAL;

        // Position buttons on the grid
        c.gridx = 0;
        c.gridy = 0;
        c.ipady = 30;  // adjust vertical height of buttons
        c.weightx = 1; // needed or buttons will cluster at center w/ gap on sides
        buttonPanel.add(clearButton, c);
        clearButton.setBackground(Color.gray);
        clearButton.setOpaque(true);
        clearButton.setBorder(new MatteBorder(0, 1, 0, 1, Color.BLACK));
        c.gridx = 1;
        buttonPanel.add(signChangeButton, c);
        signChangeButton.setBackground(Color.gray);
        signChangeButton.setOpaque(true);
        signChangeButton.setBorder(new MatteBorder(0, 0, 0, 1, Color.BLACK));
        c.gridx = 2; 
        buttonPanel.add(percentButton, c);
        percentButton.setBackground(Color.gray);
        percentButton.setOpaque(true);
        percentButton.setBorder(new MatteBorder(0, 0, 0, 1, Color.BLACK));
        c.gridx = 3;
        buttonPanel.add(divisionButton, c);
        divisionButton.setBackground(Color.ORANGE);
        divisionButton.setOpaque(true);
        divisionButton.setBorder(new MatteBorder(0, 0, 0, 1, Color.BLACK));
        c.gridx = 0;
        c.gridy = 1;
        buttonPanel.add(sevenButton, c);
        sevenButton.setBorder(new MatteBorder(1, 1, 0, 1, Color.BLACK));
        c.gridx = 1;
        buttonPanel.add(eightButton, c);
        eightButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 2;
        buttonPanel.add(nineButton, c);
        nineButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 3;
        buttonPanel.add(multiplicationButton, c);
        multiplicationButton.setBackground(Color.ORANGE);
        multiplicationButton.setOpaque(true);
        multiplicationButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 0;
        c.gridy = 2;
        buttonPanel.add(fourButton, c);
        fourButton.setBorder(new MatteBorder(1, 1, 0, 1, Color.BLACK));
        c.gridx = 1;
        buttonPanel.add(fiveButton, c);
        fiveButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 2;
        buttonPanel.add(sixButton, c);
        sixButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 3;
        buttonPanel.add(subtractionButton, c);
        subtractionButton.setBackground(Color.ORANGE);
        subtractionButton.setOpaque(true);
        subtractionButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 0;
        c.gridy = 3;
        buttonPanel.add(oneButton, c);
        oneButton.setBorder(new MatteBorder(1, 1, 0, 1, Color.BLACK));
        c.gridx = 1;
        buttonPanel.add(twoButton, c);
        twoButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 2;
        buttonPanel.add(threeButton, c);
        threeButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 3;
        buttonPanel.add(additionButton, c);
        additionButton.setBackground(Color.ORANGE);
        additionButton.setOpaque(true);
        additionButton.setBorder(new MatteBorder(1, 0, 0, 1, Color.BLACK));
        c.gridx = 0;
        c.gridy = 4;
        c.gridwidth = 2; // spans 2 cells
        buttonPanel.add(zeroButton, c);
        zeroButton.setBorder(new MatteBorder(1, 1, 1, 1, Color.BLACK));
        c.gridx = 2;
        c.gridwidth = 1; 
        buttonPanel.add(periodButton, c);
        periodButton.setBorder(new MatteBorder(1, 0, 1, 1, Color.BLACK));
        c.gridx = 3;
        buttonPanel.add(equalButton, c);
        equalButton.setBackground(Color.ORANGE);
        equalButton.setOpaque(true);
        equalButton.setBorder(new MatteBorder(1, 0, 1, 1, Color.BLACK));

        // Customize display field
        display.setHorizontalAlignment(JTextField.RIGHT);
        display.setFont(new Font("Arial", Font.PLAIN, 40));
        display.setBorder(new EmptyBorder(30, 0, 0, 0));
        display.setForeground(Color.WHITE);
        // Make display translucent but leave button panel opaque
        this.setBackground(new Color(0, 0, 0, 100));
        gui.setBackground(new Color(0,0,0,100));
        display.setBackground(new Color(0, 0, 0, 100));

        // I need to fix the gap between the edge of the buttonPanel and the buttons... 
        buttonPanel.setBackground(Color.red);

        // Add display and button panel to main panel, then main panel to frame
        gui.add(display, BorderLayout.NORTH);
        gui.add(buttonPanel, BorderLayout.CENTER);
        this.add(gui);
        this.pack();
    }
}

Answer 1

我不知道为什么会有这个空隙，但是如果你设置下面的空边框，背景就没有了:

JPanel buttonPanel = new JPanel();
// Set button panel's layout manager to GridBagLayout
buttonPanel.setLayout(new GridBagLayout());
buttonPanel.setBorder(new EmptyBorder(0, -1, 0, -1));