public static String maskLeft(String s,int len,char mask_ch){
  if (len <= 0) {
    return s;
  }
  len=Math.min(len,s.length());
  StringBuilder sb=new StringBuilder();
  for (int i=0; i < len; i++) {
    sb.append(mask_ch);
  }
  sb.append(s.substring(len));
  return sb.toString();
}

public static String maskLeft ( String s , int len , char mask_ch ) { if ( len <= 0 ) { return s ; } len = Math . min ( len , s . length ( ) ) ; StringBuilder sb = new StringBuilder ( ) ; for ( int i = 0 ; i < len ; i ++ ) { sb . append ( mask_ch ) ; } sb . append ( s . substring ( len ) ) ; return sb . toString ( ) ; }
returns a string consisting of " s " , with each of the first " len " characters replaced by " mask_ch " character .
