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