import javax.swing.*; 
import javax.swing.text.*; 

import java.awt.Toolkit;

public class LimitedStyledDocument extends DefaultStyledDocument {

    int maxCharacters;

    public LimitedStyledDocument(int maxChars) {
	maxCharacters = maxChars;
    }

    public void insertString(int offs, String str, AttributeSet a) 
	throws BadLocationException {

	//This rejects the entire insertion if it would make
	//the contents too long. Another option would be
	//to truncate the inserted string so the contents
	//would be exactly maxCharacters in length.
	if ((getLength() + str.length()) <= maxCharacters)
	    super.insertString(offs, str, a);
	else
	    Toolkit.getDefaultToolkit().beep();
    }
}