In methods.m, Octave displays the result of calling

javaMethod ("getMethods", "org.octave.ClassHelper", OBJ);

in which OBJ is either a Java object or the name of a Java class.  The org.octave.ClassHelper method is here:

http://hg.savannah.gnu.org/hgweb/octave/file/9bbe11da0c38/scripts/java/org/octave/ClassHelper.java#l169

If there should be more information about the methods when "-full" is given, can you provide a patch?

Ernst: We have never defined Octave as a "clone" of Matlab.  We aim for compatibility where it makes sense.  You might find it easier to work with us if you treat us as fellow volunteer developers who are doing our best to make Octave better (which we are) instead of as adversaries who are working to break things just to frustrate you (which we are not).

https://www.mathworks.com/help/matlab/ref/methods.html
shows how it is meant:
methods('java.lang.String')
returns just names of constructor(s) and methods.
If two (constructors or) functions have the same names (but differ e.g. in arguments), only one name is given.

In contrast, methods('java.lang.String','full') shows method signatures.
It is ugly, but for that variant,
the result is given somewhere else:
https://www.mathworks.com/help/matlab/matlab_external/java-method-information.html
-verbatim-
Methods for class java.awt.Dimension:
Dimension()
Dimension(java.awt.Dimension)
Dimension(int,int)
java.lang.Class getClass()  % Inherited from java.lang.Object
int hashCode()  % Inherited from java.lang.Object
boolean equals(java.lang.Object)
java.lang.String toString()
void notify()  % Inherited from java.lang.Object
void notifyAll()  % Inherited from java.lang.Object
void wait(long) throws java.lang.InterruptedException
   % Inherited from java.lang.Object
void wait(long,int) throws java.lang.InterruptedException
   % Inherited from java.lang.Object
void wait() throws java.lang.InterruptedException
   % Inherited from java.lang.Object
java.awt.Dimension getSize()
void setSize(java.awt.Dimension)
void setSize(int,int)

You can see, that now the following is added:
- parameter list
- throws clause for checked exceptions,
- return type,
- inheritance information
- I think, also modifiers like static, final, .... (no access)
Again, the constructors are treated as methods.

[Thread drift alert]
That further Java development in Octave is "refused" may be perceived that way but I think there's more nuance behind the scenes.

As an example, we have had issues in the past with Java JVMs apparently fiddling with CPU flags which somehow seemed to affect numerical results elsewhere in Octave's code base. Maybe an extreme example, but bearing such things in mind I can follow the core devs being reluctant to adapt changes to the Java subsystem if the consequences aren't fully understood.
Plus, we're all volunteers and all lack time and as a consequence we pick up the things we're most familiar with. Again, as it happens Java is not in the lucky collection of subjects.

As to javachk and version:

javachk.m (that I wrote) needs to stay Matlab-compatible; same for usejava.m. Matlab's output tmplate for these allows no easy way to return the version.
But I'll post s/th in bug #60058.

@Ernst:
You're not quite the only Java user here, we occasionally get bug reports relating to Java from other users than you and in the io package (that I maintain) there's a lot of Java based code (TBH, how many people would be using that is beyond me:-) ).

One big difference is is that AFAIK the entire Matlab GUI is based on Java (at least, it used to be) so Java is a much more natural choice to fall back on with Matlab.
OTOH in my experience Matlab's dependency on Java can be a PITA as well, as it heavily relies on its own bundled JRE. E.g., it can be hard to invoke methods in newer class libs outside Matlab's bundle, or swap/shadow the bundled ones with other versions with other/more capabilities, as one soon gets drowned in dependency "hell".

I compiled Octave 5.2.0 and tried `methods('java.lang.Double','-full')`. That wasn't supported in that version. So I used the following:

>> methods('java.lang.Double')
Methods for class java.lang.Double:
boolean equals(java.lang.Object)
java.lang.String toString(double)
java.lang.String toString()
int hashCode()
int hashCode(double)
double min(double, double)
double max(double, double)
long doubleToRawLongBits(double)
long doubleToLongBits(double)
double longBitsToDouble(long)
int compareTo(java.lang.Object)
int compareTo(java.lang.Double)
byte byteValue()
short shortValue()
int intValue()
long longValue()
float floatValue()
double doubleValue()
java.lang.Double valueOf(java.lang.String) throws java.lang.NumberFormatException
java.lang.Double valueOf(double)
java.lang.String toHexString(double)
int compare(double, double)
boolean isNaN(double)
boolean isNaN()
boolean isInfinite(double)
boolean isInfinite()
boolean isFinite(double)
double sum(double, double)
double parseDouble(java.lang.String) throws java.lang.NumberFormatException
void wait(long) throws java.lang.InterruptedException
void wait(long, int) throws java.lang.InterruptedException
void wait() throws java.lang.InterruptedException
java.lang.Class getClass()
void notify()
void notifyAll()

In Octave 7.0.0 (hg id ca1cc1bca9cb):

Methods for class java.lang.Double:
Double(double)
Double(java.lang.String) throws java.lang.NumberFormatException
boolean equals(java.lang.Object)
java.lang.String toString(double)
java.lang.String toString()
int hashCode()
int hashCode(double)
double min(double, double)
double max(double, double)
long doubleToRawLongBits(double)
long doubleToLongBits(double)
double longBitsToDouble(long)
int compareTo(java.lang.Object)
int compareTo(java.lang.Double)
byte byteValue()
short shortValue()
int intValue()
long longValue()
float floatValue()
double doubleValue()
java.lang.Double valueOf(java.lang.String) throws java.lang.NumberFormatException
java.lang.Double valueOf(double)
java.lang.String toHexString(double)
int compare(double, double)
boolean isNaN(double)
boolean isNaN()
boolean isInfinite(double)
boolean isInfinite()
boolean isFinite(double)
double sum(double, double)
double parseDouble(java.lang.String) throws java.lang.NumberFormatException
void wait(long) throws java.lang.InterruptedException
void wait(long, int) throws java.lang.InterruptedException
void wait() throws java.lang.InterruptedException
java.lang.Class getClass()
void notify()
void notifyAll()

That looks very similar to me (apart from the constructors now being included -- like you already wrote). Should that be including modifiers (static, ...)?
Maybe that depends on the used Java version?

>> version -java
ans = Java 11.0.10+9-Ubuntu-0ubuntu1.20.10 with Ubuntu OpenJDK 64-Bit Server VM mixed mode, sharing

Please, propose better wording for the documentation if you think it is currently incomplete.

In which Octave release was the behavior "good" and in which one "bad" (esp. w.r.t. comment #2)?

I don't think any changes to the Java code were done to cripple the code on purpose; if any it may rather have been unintended side effects of changes elsewhere in the code (e.g., in the interpreter etc.).

In addition we have no real Java developers - that is, not ones with enough time on their hands to fix or improve things.
I'm very sorry for that; I know you've uploaded lots of patches and good ideas for improving the Java subsystem. Unfortunately there's no one around here to pick it up.

also improved: in intermediate versions methods(.)
behaved almost like methods(.,'-full').
This also changed and it is positive.

What is negative:
behavior of  methods(.,'-full') for java object:
Older version had modifiers static, native and final (at least)
and indication of inheritance of a method.
That way one could see which of the methods
were really defined in the class.

I have the impression, the guy who changed back has no clue on java programming and just removed he did not understand.

I would like to have back old interface
and adaption of documentation.

In octave 5.2.0, methods('java.lang.Double')
did not include the constructor, now in 6.2.0 it does.
I think, this shall be added to the documentation,
both manual and help.

The function ismethod was changed accordingly,
and here again, documentation needs clarification.

Also, and I dont know how long this is,
to a complete signature belong
also so called checked exceptions.
These are also given.
Also that shall be noted in the documentation.
Good illustration is really class java.lang.Double.