How can I justify text in a JTextPane?

You will have to use the following classes -

• javax.swing.text.StyledDocument
• javax.swing.text.Style
• javax.swing.text.StyleConstants
• javax.swing.text.StyleConstants
• javax.swing.text.SimpleAttributeSet
• javax.swing.JTextPane

Specifically you will have to use the StyleConstants.Alignment attribute with a value of StyleConstants.ALIGN_JUSTIFY constant for the paragraph.

Here is an article which explains how to use the AttributeSets and Styles

http://java.sun.com/products/jfc/tsc/articles/text/attributes/

The actual code will look something like this

SimpleAttributeSet sa = new SimpleAttributeSet();
StyleConstants.setAlignment(sa, StyleConstants.ALIGN_JUSTIFY);

textPane.getStyledDocument().setParagraphAttributes(0,/*document length*/,sa,false);