Y-Axis 上 Java BoxLayout 中单个字符的对齐方式是 Off-Center

Question

将某些字符沿 Java 中的 y-axis 与 BoxLayout 的中心对齐似乎存在问题。我不知道是什么导致了这种情况，我已经创建了一个 SSCCE 来演示这种效果。在这个例子中，我只使用了字符 'a'，并且我在每个 JPanel 的正中间画了一条线来展示每个案例离中心有多远。粗体文本的情况似乎排列得很好，但正常格式和斜体都非常 off-center，尽管同时使用了 setAlignmentX 和 setHorizontalAlignment。感谢任何帮助理解这种效果的帮助。

如果问题出在我的特定计算机上 Java，这是我运行 SSCCE 时屏幕上显示的图像，它加载了三个不同的 JPanel BoxLayouts 沿着 y-axis & 放置一个居中的 JLabel，每个 JLabel 中只有字符 'a'：

& 这是 SSCCE 的代码：

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

public class AlignmentTest extends JPanel
{
    public AlignmentTest(char label, int style)
    {
        JLabel l = new JLabel(Character.toString(label));
        setBorder(BorderFactory.createLineBorder(Color.BLACK,1));
        setBackground(Color.WHITE);
        setLayout(new BoxLayout(this,BoxLayout.Y_AXIS));
        setPreferredSize(new Dimension(300,50));
        add(Box.createVerticalGlue());
        add(l);
            l.setFont(l.getFont().deriveFont(style));
            l.setAlignmentX(CENTER_ALIGNMENT);
            l.setHorizontalAlignment(JLabel.CENTER);
        add(Box.createVerticalGlue());
    }
    public static void main(String[] args)
    {
        JFrame f = new JFrame("Alignment Test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setLayout(new GridLayout(1,0,5,5));
        f.add(new AlignmentTest('a',Font.PLAIN));
        f.add(new AlignmentTest('a',Font.BOLD));
        f.add(new AlignmentTest('a',Font.ITALIC));
        f.pack();
        f.setVisible(true);
    }
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        g.drawLine(getWidth()/2,0,getWidth()/2,getHeight());
    }
}

Answer 1

在 Windows 7 none 个字符上使用 JDK7 居中对齐。

我进行了一些更改以显示 JTextField，并使用了 JTextField 的列 (1、3、5)。随着列的增加，中心对齐得到改善，并且在第 5 列及以上是合理的。所以这个问题在某种程度上与组件的宽度有关。

我猜布局中存在一些奇怪的舍入错误。这对我来说似乎是一个错误。

如果您对提供与 BoxLayout 类似功能的布局感兴趣，您可以查看 Relative Layout。对您的示例所做的更改很小：

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

public class AlignmentTest extends JPanel
{
    public AlignmentTest(char label, int style)
    {
        JLabel l = new JLabel(Character.toString(label));
        setBorder(BorderFactory.createLineBorder(Color.BLACK,1));
        setBackground(Color.WHITE);
//        setLayout(new BoxLayout(this,BoxLayout.Y_AXIS));
        setLayout(new RelativeLayout(RelativeLayout.Y_AXIS));
        setPreferredSize(new Dimension(300,50));
//        add(Box.createVerticalGlue());
        add(Box.createVerticalGlue(), new Float(1));
        add(l);
            l.setFont(l.getFont().deriveFont(style));
            l.setAlignmentX(CENTER_ALIGNMENT);
            l.setHorizontalAlignment(JLabel.CENTER);
//        add(Box.createVerticalGlue());
        add(Box.createVerticalGlue(), new Float(1));
    }
    public static void main(String[] args)
    {
        JFrame f = new JFrame("Alignment Test");
        JScrollPane scroller = new JScrollPane();
            JPanel panel = new JPanel(new GridLayout(1,0,5,5));
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setLayout(new GridLayout(1,0,5,5));
        f.add(new AlignmentTest('a',Font.PLAIN));
        f.add(new AlignmentTest('a',Font.BOLD));
        f.add(new AlignmentTest('a',Font.ITALIC));
        f.pack();
        f.setVisible(true);
    }
    public void paintComponent(Graphics g)
    {
        super.paintComponent(g);
        g.drawLine(getWidth()/2,0,getWidth()/2,getHeight());
    }
}

Answer 2

您观察到的效果似乎是 BoxLayout works. Interpolating from How to Use BoxLayout: Box Layout Features、"When a BoxLayout lays out components from left to right, … Any extra space appears at the right of the container." 当封闭容器的初始尺寸是标签（固定）尺寸的小倍数时，如下所示，异常是最小的；水平拉伸框架以查看其增长情况。一个 work-around 是最小化封闭容器的首选尺寸被人为放大的程度。

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

public class AlignmentTest extends JPanel {
    private final JLabel l;
    public AlignmentTest(String label, int style) {
        setBorder(BorderFactory.createLineBorder(Color.BLACK, 1));
        setBackground(Color.WHITE);
        setLayout(new BoxLayout(this, BoxLayout.Y_AXIS));
        l = new JLabel(label, JLabel.CENTER);
        l.setFont(l.getFont().deriveFont(style));
        l.setAlignmentX(CENTER_ALIGNMENT);
        l.setOpaque(true);
        l.setBackground(Color.cyan);
        add(Box.createVerticalGlue());
        add(l);
        add(Box.createVerticalGlue());
    }

    @Override
    public Dimension getPreferredSize() {
        int w = l.getPreferredSize().width;
        int h = l.getPreferredSize().height;
        return new Dimension(w * 3, h * 3);
    }

    public static void main(String[] args) {
        JFrame f = new JFrame("Alignment Test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setLayout(new GridLayout(1, 0, 5, 5));
        f.add(new AlignmentTest("aMa", Font.PLAIN));
        f.add(new AlignmentTest("aMa", Font.BOLD));
        f.add(new AlignmentTest("aMa", Font.ITALIC));
        f.pack();
        f.setVisible(true);
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawLine(getWidth() / 2, 0, getWidth() / 2, getHeight());
    }
}

Answer 3

另一种避免 "Box Layout Features: … Any extra space appears at the right of the container" 的方法，您需要将 JLabel#getMinimumSize() 方法重写为 return 与 JLabel#getPreferredSize() 相同的 Dimension ].

对不起，我误会了。

正如@camickr 已经说过的，

I would guess there is some weird rounding error in the layout. This seems like a bug to me.

非常正确。

固定示例：

//MinimumSize checkbox
//selected true: set min width = 100px
//selected false: set min width = 7px(default "a" width)
//Here's my attempt(I am running JDK 1.7.0_72 on Windows 7):
import java.awt.*;
import java.awt.event.*;
import java.util.*;
import javax.swing.*;

public class AlignmentTest4 extends JPanel {
  private static boolean FLAG = false;
  @Override public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.drawLine(getWidth() / 2, 0, getWidth() / 2, getHeight());
  }
  @Override public Dimension getPreferredSize() {
    return new Dimension(300, 80);
  }
  public static JLabel makeLabel(String label, int style) {
    JLabel l = new JLabel(label) {
      @Override public Dimension getPreferredSize() {
        return new Dimension(120, 30);
      }
      @Override public Dimension getMinimumSize() {
        Dimension d = super.getMinimumSize();
        if (FLAG) {
          d.width = 100;
        } else {
          d.width = 7;
        }
        return d;
        //if (FLAG) {
        //  return this.getPreferredSize();
        //} else {
        //  return super.getMinimumSize();
        //}
      }
    };
    l.setOpaque(true);
    l.setBackground(Color.ORANGE);
    l.setFont(l.getFont().deriveFont(style));
    l.setAlignmentX(Component.CENTER_ALIGNMENT);
    l.setAlignmentY(Component.CENTER_ALIGNMENT);
    l.setVerticalAlignment(SwingConstants.CENTER);
    l.setVerticalTextPosition(SwingConstants.CENTER);
    l.setHorizontalAlignment(SwingConstants.CENTER);
    l.setHorizontalTextPosition(SwingConstants.CENTER);
    return l;
  }
  public static JComponent makePanel() {
    JPanel p = new JPanel(new GridLayout(0, 1, 5, 5));

    JPanel p1 = new AlignmentTest4();
    p1.setBorder(BorderFactory.createTitledBorder("BoxLayout.X_AXIS"));
    p1.setLayout(new BoxLayout(p1, BoxLayout.X_AXIS));
    p1.add(Box.createHorizontalGlue());
    p1.add(makeLabel("a", Font.PLAIN));
    p1.add(Box.createHorizontalGlue());

    JPanel p2 = new AlignmentTest4();
    p2.setBorder(BorderFactory.createTitledBorder("BoxLayout.Y_AXIS"));
    p2.setLayout(new BoxLayout(p2, BoxLayout.Y_AXIS));
    p2.add(Box.createVerticalGlue());
    p2.add(makeLabel("a", Font.PLAIN));
    p2.add(Box.createVerticalGlue());

    for (JPanel c : Arrays.asList(p1, p2)) {
      c.setBackground(Color.WHITE);
      p.add(c);
    }
    return p;
  }
  public static JComponent makeUI() {
    final JPanel p = new JPanel(new BorderLayout());
    p.add(makePanel());
    p.add(new JCheckBox(new AbstractAction("MinimumSize") {
      @Override public void actionPerformed(ActionEvent e) {
        FLAG = ((JCheckBox) e.getSource()).isSelected();
        SwingUtilities.updateComponentTreeUI(p);
      }
    }), BorderLayout.SOUTH);
    return p;
  }
  public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
      @Override public void run() {
        createAndShowGUI();
      }
    });
  }
  public static void createAndShowGUI() {
    JFrame f = new JFrame("Alignment Test");
    f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    f.getContentPane().add(makeUI());
    f.pack();
    f.setLocationRelativeTo(null);
    f.setVisible(true);
  }
}

Y-Axis 上 Java BoxLayout 中单个字符的对齐方式是 Off-Center

Alignment of Single Characters in Java BoxLayout on Y-Axis Is Off-Center

java

swing

char

jlabel

boxlayout