code: public StrBuilder setCharAt(final int index, final char ch) {
          if (index < 0 || index >= length()) {
              throw new StringIndexOutOfBoundsException(index);
          }
          buffer[index] = ch;
          return this;
      }
docs: Sets the character at the specified index.
  
  @see #charAt(int)
  @see #deleteCharAt(int)
  @param index  the index to set
  @param ch  the new character
  @return this, to enable chaining
  @throws IndexOutOfBoundsException if the index is invalid