Java JTextField set document for float value

import java.awt.FlowLayout;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.text.AttributeSet;
import javax.swing.text.BadLocationException;
import javax.swing.text.PlainDocument;

class JTextFieldFilter extends PlainDocument {
   public static final String FLOAT = "0123456789.";

   protected String acceptedChars = null;

   protected boolean negativeAccepted = false;

   public JTextFieldFilter() {
      this(FLOAT);
   }/*  w  w w  . j  a  v  a  2  s .  co m*/

   public JTextFieldFilter(String acceptedchars) {
      acceptedChars = acceptedchars;
   }

   public void setNegativeAccepted(boolean negativeaccepted) {
      if (acceptedChars.equals(FLOAT)) {
         negativeAccepted = negativeaccepted;
         acceptedChars += "-";
      }
   }

   public void insertString(int offset, String str, AttributeSet attr) throws BadLocationException {
      if (str == null)
         return;

      for (int i = 0; i < str.length(); i++) {
         if (acceptedChars.indexOf(str.valueOf(str.charAt(i))) == -1)
            return;
      }
      if (acceptedChars.equals(FLOAT) || (acceptedChars.equals(FLOAT + "-") && negativeAccepted)) {
         if (str.indexOf(".") != -1) {
            if (getText(0, getLength()).indexOf(".") != -1) {
               return;
            }
         }
      }
      if (negativeAccepted && str.indexOf("-") != -1) {
         if (str.indexOf("-") != 0 || offset != 0) {
            return;
         }
      }
      super.insertString(offset, str, attr);
   }
}

public class Main extends JFrame {

   public Main() {
      setLayout(new FlowLayout());
      JLabel label = new JLabel("only float");
      JTextField textField = new JTextField(10);
      getContentPane().add(label);
      getContentPane().add(textField);
      textField.setDocument(new JTextFieldFilter(JTextFieldFilter.FLOAT));
      setSize(300, 300);
      setVisible(true);
   }

   public static void main(String[] argv) throws Exception {
      new Main();
   }
}