JLabel vertical alignment not working as expected

Font font = Font("Arial", Font.BOLD, 35);

JLabel label = new JLabel("57");
JPanel panel = new JPanel();
panel.setLay开发者_高级运维out(new BoxLayout(panel, BoxLayout.LINE_AXIS));
panel.add(label);

This creates a JLabel with an extra space above and below it. I tried setVerticalAlignment(SwingConstants.TOP) but it not working. Again, I don't want to align JLabel to top but the text inside JLabel should be aligned to top.

here is how my label looks like

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The text in your label is actually aligned to the top already. Even if you set all three of:

label.setVerticalAlignment(JLabel.TOP);
label.setVerticalTextPosition(JLabel.TOP);
panel.setAlignmentY(TOP_ALIGNMENT);

you still would find that gap.

The problem is related to font-metrics. The font leaves space for diacritics, and while English numbers and even letters do not contain diacritics on capital letters, Arial definitely contains a full-breadth of international characters including ones taller than a capital letter, for example, German umlauts (ÄÖÜ) or characters containing Portuguese diacritics (ÁÂÃ).

If you want a quick, easy solution that is a hack, that may not scale well across fonts and platforms, you can use a negative value on a border to compensate for the font metrics.

label.setBorder(BorderFactory.createEmptyBorder( -3 /*top*/, 0, 0, 0 ));

If you want to fix it "right" you should look into learning about the FontMetrics package, as it has many functions that could be useful to calculating the actual height and location of the text being displayed, such that you can move the whole string by the appropriate amount of pixels.

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The arrow in your diagram points to the difference between the glyph's nominal ascent and the maximum ascent, as discussed in FontMetrics. You can tinker with setBorder(null); but for absolute control, you'll have to render the glyphs yourself, as shown here. Fortunately, the digit glyphs of most fonts have a uniform advance and ascent.