Passing next character to command

Question

I have a command defined as follows

\documentclass{article}
\makeatletter%
\newcommand{\@withstar}[3]{We have star #1, #2, #3}
\newcommand{\@withoutstar}[3]{We have #1, #2, #3}
\DeclareRobustCommand\mycmd{%
  \@ifnextchar *%
  {\@firstoftwo{\@withstar}} %
  {\@withoutstar}
}%
\makeatother%

\begin{document}
\mycmd{first}{second}{and third}
\mycmd*{first}{second}{and third}
\end{document}

I want to define another command that just passes the next character to \mycmd and sets specific arguments. Something along the lines of

\newcommand\newcmd[1]{\mycmd\nextchar{first}{,second}}

Then I can call it like

 \newcmd{and third}

or

 \newcmd*{and third}

I understand that I can repeat the test for next char (using @ifnextchar) inside the definition of \newcmd, but I would like to avoid that if possible because in my real situation \mycmd tests for many characters, not just *.

Answer 1

Something like

\def\newcmd#1#{\mycmd#1{first}{,second}}

#1 will be all tokens between \newcmd and { which will be empty or * in your examples.

Answer 2

No code duplication with xparse and no auxiliary macros:

\documentclass{article}
\usepackage{xparse}

\NewDocumentCommand{\mycmd}{smmm}{%
  \IfBooleanTF{#1}
    {We have star #2, #3, #4}
    {We have #2, #3, #4}%
}

\NewDocumentCommand{\newcmd}{sm}{%
  \IfBooleanTF{#1}{\mycmd*}{\mycmd}{first}{second}{#2}%
}

\begin{document}

\mycmd{first}{second}{and third}

\mycmd*{first}{second}{and third}

\newcmd{and third}

\newcmd*{and third}

\end{document}