java截取字符串，第4位以后的字符串用*代替

public class F {
    public static void main(String[] args) {
        /**截取字符串，第4位以后的字符串用*代替*/
        String s = "Hello Java World";
        System.out.println("直接輸出：" + s);

        String a = s.substring(4,s.length());
        for (int i = 0;i < a.length() ; i++){
            a = a.replace(String.valueOf(a.charAt(i)),"*");
        }
        System.out.println("屏蔽輸出：" + s.substring(0,4)+a);


    }
}