Here's what that doco page currently says (from https://www.mathworks.com/help/matlab/matlab_external/handling-data-returned-from-java-methods.html):

------------------------

Convert to MATLAB Strings

To convert java.lang.String objects and arrays to MATLAB strings or character vectors, use the MATLAB string or char function.

If the object specified in the MATLAB function is not an instance of the java.lang.String class, then MATLAB checks its class definition for a toString or a toChar method.

------------------------

So, the pseudocode is, I think:

if (x instanceof java.lang.String)
  return toOctaveChar(x);
else if (x hasmethod toChar)
  return toOctaveChar(x.toChar());
else
  return toOctaveChar(x.toString());

There is no "else throw error", because every Java object has a toString method: it's defined in the root superclass java.lang.Object. Subclasses can just provide customizations of it.

My testing agrees with this. In Matlab, `char(jobj)` succeeds on things that clearly do not have a toChar method. In fact, it succeeds on every Java object I could think of.

>> m = java.util.HashMap
m =
{}
>> char(m)
ans =
    '{}'
>> class(ans)
ans =
    'char'

That doco page also has a sidebar that says this: "If the class of the object is not java.lang.String and it does not implement a toChar method, then the char function displays an error message." I think they may have made a small mistake, and meant to say "a toString or toChar method". And since every Java object has a toString method, that sidebar is never actually relevant. Perhaps it is left over from an older version of Matlab that did not use toString.

Octave's current behavior seems correct and Matlab-consistent to me. (Except for the UTF-8 vs UTF-16 issue of converting Java char[] arrays, per https://savannah.gnu.org/bugs/index.php?54170, but that's a separate issue.)

I read: if an array of strings, the result is an array of chars.
Well not clear whether String[] is mapped onto char[][] or onto char[].

The rest seems to me as you describe it.

http://de.mathworks.com/help/matlab/matlab_external/handling-data-returned-from-a-java-method.html

shows that unless a java object implements toChar,
method char(m) applied to java object m
shall result in an error.

what it seems to do is to invoke m.toString()
which is wrong.