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


public class LimitedLengthDocument extends PlainDocument
{
    protected int maxLength;

    public LimitedLengthDocument(int maxLength)
    {
         this.maxLength = maxLength;
    }

    public int maxLength()
    {
        return maxLength;
    }

    public void insertString(int offs, String str, AttributeSet a)
         throws BadLocationException
    {
        if (str == null) return;
        int strLen = str.length();
        if (strLen == 0) return;

        int len = getLength();
        if (strLen + len > maxLength)
        {
            Toolkit.getDefaultToolkit().beep();
            str = str.substring(0, maxLength - len); // or return;
        }

        super.insertString(offs, str, a);
   }
}



JTextField f = new JTextField(new LimitedLengthDocument(20), "", 20);