remove all special characters in java

use [\\W+] or "[^a-zA-Z0-9]" as regex to match any special characters and also use String.replaceAll(regex, String) to replace the spl charecter with an empty string. remember as the first arg of String.replaceAll is a regex you have to escape it with a backslash to treat em as a literal charcter.

          String c= "hjdg$h&jk8^i0ssh6";
        Pattern pt = Pattern.compile("[^a-zA-Z0-9]");
        Matcher match= pt.matcher(c);
        while(match.find())
        {
            String s= match.group();
        c=c.replaceAll("\\"+s, "");
        }
        System.out.println(c);

You can read the lines and replace all special characters safely this way.
Keep in mind that if you use \\W you will not replace underscores.

Scanner scan = new Scanner(System.in);

while(scan.hasNextLine()){
    System.out.println(scan.nextLine().replaceAll("[^a-zA-Z0-9]", ""));
}

Your problem is that the indices returned by match.start() correspond to the position of the character as it appeared in the original string when you matched it; however, as you rewrite the string c every time, these indices become incorrect.

The best approach to solve this is to use replaceAll, for example:

        System.out.println(c.replaceAll("[^a-zA-Z0-9]", ""));