A linha é para mostrar onde estão 80 caracteres. Há uma boa explicação sobre o fórum do netbeans aqui :
It's there as a guide to line length.
Good style generally avoids lines longer than 80 characters, and only 70 characters for examples intended to be used in documentation.
The red line simply marks 80 characters wide (by default) and is a visual guide to break long expressions over multiple lines.
You can adjust the placement of the red line in (Netbeans 6.7): Tools > Options > Editor > Formatting.
On the left you'll see Right Margin, with a default value of 80.
If you want to remove the red line all together, you can either:
- Increase the margin value to 160 or more; or goto
- Tools > Options > Fonts and Colors, change the color of the Text Limit Line to white
There may be other ways, but these are the easiest ways I know of.
The Java Code Conventions can be found here (and clause 4.1 is on page 9 of the pdf):