class  JTextFieldFilter  extends  PlainDocument{
   public   static   final  String LOWERCASE =
      "abcdefghijklmnopqrstuvwxyz";
   public   static   final  String UPPERCASE =
      "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
   public   static   final  String ALPHA =
      LOWERCASE + UPPERCASE;
   public   static   final  String NUMERIC =
      "0123456789";

   public   static   final  String B_NUMERIC =
      "123456789";

   public   static   final  String  FLOAT  =
      NUMERIC + ".";
   public   static   final  String ALPHA_NUMERIC =
     ALPHA + NUMERIC;
   protected  String acceptedChars =  null ;
   protected   boolean  negativeAccepted = false;

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

   public   void  setNegativeAccepted( boolean  negativeaccepted){
     if (acceptedChars.equals(NUMERIC) ||
      acceptedChars.equals( FLOAT ) ||
      acceptedChars.equals(ALPHA_NUMERIC)){
      negativeAccepted = negativeaccepted;
      acceptedChars += "-";
  }
  }
   public   void  insertString
      ( int  offset, String str, AttributeSet attr)
       throws  BadLocationException {
     if (str ==  null )  return ;
     if (acceptedChars.equals(UPPERCASE))
      str = str.toUpperCase();
       else   if  (acceptedChars.equals(LOWERCASE))
        str = str.toLowerCase();

     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);
  }
}

jTextField1.setDocument( new  JTextFieldFilter(JTextFieldFilter.NUMERIC));