code: public void append(String s) {
          int length = s.length();
          if (this.length + length > this.ch.length) {
              int newLength = this.ch.length*2;
              if (newLength < this.length + length + DEFAULT_SIZE)
                  newLength = this.ch.length + length + DEFAULT_SIZE;
              char[] newch = new char[newLength];            
              System.arraycopy(this.ch, 0, newch, 0, this.length);
              this.ch = newch;
          }
          s.getChars(0, length, this.ch, this.length);
          this.length += length;
      }
docs: append
  
  @param s