How to calculate the number of rows (and columns in each row) a text takes in a JTextArea?

What am I doing wrong, what am I forgetting about?

Nothing, really. I modified your example to use "<=" on the width and to highlight a few features:

1. FontMetrics notes, "the advance of a String is not necessarily the sum of the advances of its characters measured in isolation…"

2. The preferred size of the text component matches the metric bounds pretty well for the widest line. This varies by font due to proportional spacing.

3. TextLayout shows even tighter bounds, but note the "baseline-relative coordinates."

4. The getLineCount() method counts line.separator delimited lines, not wrapped lines.

line: Twas brillig and the slithy toves
line: Did gyre and gimble in the wabe;
line count: 2
preferred: java.awt.Dimension[width=207,height=48]
bounds1: java.awt.geom.Rectangle2D$Float[x=0.0,y=-12.568359,w=205.0,h=15.310547]
layout1: java.awt.geom.Rectangle2D$Float[x=0.0,y=-10.0,w=200.0,h=13.0]
bounds2: java.awt.geom.Rectangle2D$Float[x=0.0,y=-12.568359,w=207.0,h=15.310547]
layout2: java.awt.geom.Rectangle2D$Float[x=1.0,y=-10.0,w=205.0,h=13.0]

import java.awt.Dimension;
import java.awt.FontMetrics;
import java.awt.font.FontRenderContext;
import java.awt.font.TextLayout;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JTextArea;
import javax.swing.SwingUtilities;

/** #see http://stackoverflow.com/questions/5979795 */
public class TextAreaLine {

    private static final String text1 =
        "Twas brillig and the slithy toves\n";
    private static final String text2 =
        "Did gyre and gimble in the wabe;";
    private static final JTextArea ta = new JTextArea(text1 + text2);

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                display();
            }
        });
    }

    static void display() {
        JFrame f = new JFrame();
        ta.setWrapStyleWord(false);
        ta.setLineWrap(false);
        ta.setRows(3);
        f.add(ta);
        f.pack();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setLocationRelativeTo(null);
        f.setVisible(true);
        FontMetrics fm = ta.getFontMetrics(ta.getFont());
        List<String> texts = new ArrayList<String>();
        Dimension d = ta.getPreferredSize();
        String text = ta.getText();
        String line = "";
        for (int i = 0; i < text.length(); i++) {
            char c = text.charAt(i);
            if (c != '\n') {
                if (fm.stringWidth(line + c) <= d.width) {
                    line += c;
                } else {
                    texts.add(line);
                    line = "" + c;
                }
            }
        }
        texts.add(line);
        for (String s : texts) {
            System.out.println("line: " + s);
        }
        System.out.println("line count: " + ta.getLineCount());
        System.out.println("preferred: " + d);
        System.out.println("bounds1: " + fm.getStringBounds(text1, null));
        FontRenderContext frc = new FontRenderContext(null, false, false);
        TextLayout layout = new TextLayout(text1, ta.getFont(), frc);
        System.out.println("layout1: " + layout.getBounds());
        System.out.println("bounds2: " + fm.getStringBounds(text2, null));
        layout = new TextLayout(text2, ta.getFont(), frc);
        System.out.println("layout2: " + layout.getBounds());
    }
}

One thing you can do is use FontMetrics. I wrote some code for splitting JTextAreas up at certain line numbers. The setup code looked like:

Graphics2D g = (Graphics2D) g2;
FontMetrics m = g.getFontMetrics();
int lineHeight = m.getHeight();

This will tell you how tall a line of text is.

Unfortunately, letters have different widths in most fonts. But, you can use the following code to determine the width of a String.

int width = m.getStringBounds("Some String", g).getWidth();

I know this doesn't fully answer your question, but I hope it helps.

If you aren't using word wrap, here is the general algorithm you could use: (in the paint component method)

String text[] = getText().split("\n");
String newText = "";
for (String line: text) {
    newText = line + "| " + line.length() + "\n";
}
setText(newText);

That's the general idea. Not sure how well it would work out. Let me know if you try it.

Not sure if this helps but you need to set the width of the text area so that the view knows when to wrap the text. Once you set the size you can determine the preferred height. When you know the preferred height you can use the font metrice line height to determine the total number of lines including the wrapped lines if any.

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

public class TextAreaPreferredHeight extends JFrame
{

    public TextAreaPreferredHeight()
    {
        JTextArea textArea = new JTextArea();
        textArea.setText("one two three four five six seven eight nine ten");
        textArea.setLineWrap( true );
        textArea.setWrapStyleWord( true );

        FontMetrics fm = textArea.getFontMetrics( textArea.getFont() );
        int height = fm.getHeight();

        System.out.println("000: " + textArea.getPreferredSize());
        textArea.setSize(100, 1);
        System.out.println("100: " + textArea.getPreferredSize());
        System.out.println("lines : " + textArea.getPreferredSize().height / height);

        textArea.setSize(200, 1);
        System.out.println("200: " + textArea.getPreferredSize());
        System.out.println("lines : " + textArea.getPreferredSize().height / height);

        textArea.setSize(300, 1);
        System.out.println("300: " + textArea.getPreferredSize());
        System.out.println("lines : " + textArea.getPreferredSize().height / height);
        add(textArea);
        pack();
        setVisible(true);
}

    public static void main(String[] args)
    {
        new TextAreaPreferredHeight();
    }
}