code: @GwtIncompatible("incompatible method")
      public StrBuilder append(final CharBuffer buf) {
          if (buf == null) {
              return appendNull();
          }
          if (buf.hasArray()) {
              final int length = buf.remaining();
              final int len = length();
              ensureCapacity(len + length);
              System.arraycopy(buf.array(), buf.arrayOffset() + buf.position(), buffer, len, length);
              size += length;
          } else {
              append(buf.toString());
          }
          return this;
      }
docs: Appends the contents of a char buffer to this string builder.
  Appending null will call {@link #appendNull()}.
  
  @param buf  the char buffer to append
  @return this, to enable chaining
  @since 3.4