Ah little appendix:

documentation:

I read:
'If the argument is an object, return the underlying struct.'

should be replaced by

'If the argument is an Octave object or a Java object,
return the underlying struct.'

Maybe a word on static fields.

I would like to ask a question on struct of java object:
what about the static fields of the class?
Are they also included in the struct???

We really need someone with a Matlab license to try.

to implement struct,
one has to find out a list of field names as strings.
To that end, get_invoke_list (JNIEnv *jni_env, void *jobj_arg)
returning the list of fields and methods is broken into two halfs,
both implemented by get_mf_list
which return either fields or methods depending on a parameter.

I provided in comments the methods get_field_list and get_method_list which are an alternative to get_mf_list.

Dear reviewer,
take whatever you consider better and drop the rest.

(file #38328)

The attached patch is a reimplementation
of fieldnames, methods and constructors,
or, to be more precise of the according methods in ClassHelper.
Methods getFieldsXXX, getMethodsXXX and getConstructorsXXX:
The return type changed from String to String[],
from ';'-separated list of objects to array of objects.

This is more suitable to implementation of struct of java object.

(file #38287)

yes, java fields are turned into struct fields.
If none present, then the resulting struct is empty.

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

shows that method struct can also be applied to java objects.

If done, Octave outputs
error: struct: additional arguments must occur as "field", VALUE pairs


because the object is not recognized as java object.