编辑框加上行号

运行效果如下所示：

示例代码如下：

public class LineNumberHeaderView extends javax.swing.JComponent {    private final  Font DEFAULT_FONT = new Font(Font.MONOSPACED, Font.PLAIN, 13);    public final Color DEFAULT_BACKGROUD = new Color(228, 228, 228);    public final Color DEFAULT_FOREGROUD = Color.red;    public final int nHEIGHT = Integer.MAX_VALUE - 1000000;    public final int MARGIN = 5;    private int lineHeight;    private int fontLineHeight;    private int currentRowWidth;    private FontMetrics fontMetrics;    public LineNumberHeaderView() {        setFont(DEFAULT_FONT);        setForeground(DEFAULT_FOREGROUD);        setBackground(DEFAULT_BACKGROUD);        setPreferredSize(9999);    }    public void setPreferredSize(int row) {        int width = fontMetrics.stringWidth(String.valueOf(row));        if (currentRowWidth < width) {            currentRowWidth = width;            setPreferredSize(new Dimension(2 * MARGIN + width + 1, nHEIGHT));        }    }    @Override    public void setFont(Font font) {        super.setFont(font);        fontMetrics = getFontMetrics(getFont());        fontLineHeight = fontMetrics.getHeight();    }    public int getLineHeight() {        if (lineHeight == 0) {            return fontLineHeight;        }        return lineHeight;    }    public void setLineHeight(int lineHeight) {        if (lineHeight > 0) {            this.lineHeight = lineHeight;        }    }    public int getStartOffset() {        return 4;    }    @Override    protected void paintComponent(Graphics g) {        int nlineHeight = getLineHeight();        int startOffset = getStartOffset();        Rectangle drawHere = g.getClipBounds();        g.setColor(getBackground());        g.fillRect(drawHere.x, drawHere.y, drawHere.width, drawHere.height);        g.setColor(getForeground());        int startLineNum = (drawHere.y / nlineHeight) + 1;        int endLineNum = startLineNum + (drawHere.height / nlineHeight);        int start = (drawHere.y / nlineHeight) * nlineHeight + nlineHeight - startOffset;        for (int i = startLineNum; i <= endLineNum; ++i) {            String lineNum = String.valueOf(i);            int width = fontMetrics.stringWidth(lineNum);            g.drawString(lineNum + " ", MARGIN + currentRowWidth - width - 1, start);            start += nlineHeight;        }        setPreferredSize(endLineNum);    }        public static void main(String[] args){//    try {//UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());//} catch (Exception e) {//e.printStackTrace();//}    JFrame frame = new JFrame("带行号的TextArea Demo");    JLabel label = new JLabel("  编辑区域");    LineNumberHeaderView view = new LineNumberHeaderView();    JScrollPane scrollPane = new JScrollPane(new JTextArea());    scrollPane.setRowHeaderView(view);    frame.getContentPane().add(label, "North");    frame.getContentPane().add(scrollPane,"Center");    frame.setPreferredSize(new Dimension(300, 200));    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);    frame.setBounds(200, 200, 300, 200);    frame.setVisible(true);    }}