much simple soln I guess:

public static void main(String[] args) 
{
  Scanner scan = new Scanner(System.in);
  String s=scan.nextLine();
//Complete the code
    if (s.trim().length()==0 || s.trim().length()>400000)
    {
        System.out.println(0);
        return;
    }

    String words[]=s.trim().split("[ !,?.\\_'@]+");
    System.out.println(words.length);
    for (String word:words)
        System.out.println(word);
}