Re: [Jython-users] NullPointerException error when calling a python method

[Robert W. B. <rb...@di...>]

Hello Hubert,

On Tue, 16 Oct 2001, Hubert Yoshizaki wrote:
> Hi,
> I'm just experimenting with jythonc (new to Python/Jython and not reall=
y experienced in Java
> either for that matter) and came up with this problem.
>
> My python test code is simply this:
>
> import java
> class tryJythonSample(java.lang.Object):
> =A0=A0=A0=A0=A0=A0=A0 def __init__(self):
> =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 "@sig public tryJython()"
> =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 print "Constructor for tryJython"
> =A0=A0=A0=A0=A0=A0=A0 def makeObject(self):
> =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 "@sig public void makeObject()"
> =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 print "I have just called a python me=
thod!!"

There is a class naming problem.  The class statement defines
"tryJythonSample", but the initializer tries to initialize the
class "tryJython" in its @sig string.

> after doing a jythonc tryJythonSample.py and get the corresponding clas=
s files and java file
> I created a java file that calls the methods.
> My java file.
>
> import tryJythonSample.*;

This import might be a problem.  Unless you used the jythonc --package
(-p) option to specify the tryJythonSample package, you should just use
the following import:

import tryJythonSample

It's easy to forget Jython's 'extra' module layer (package/module/class a=
s
opposed to package/class). Using a jythonc-compiled class from Java acts
as if there were no module.

> import org.python.core.*;
>
> public class tryJythonMain
> {
> public static void main(String[] args)
> {
> tryJythonSample jySample =3D new tryJythonSample();
> jySample.makeObject();
> =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0
> }
> }=A0=A0 =A0=A0=A0=A0
>
> when I run my java file (tryJythonMain) I get the following:
>
> Exception in thread "main" java.lang.NullPointerException
> =A0=A0=A0=A0=A0=A0=A0 at org.python.core.PyJavaClass.lookup(PyJavaClass=
.java:39)
> =A0=A0=A0=A0=A0=A0=A0 at org.python.core.PyObject.<init>(PyObject.java:=
46)
> =A0=A0=A0=A0=A0=A0=A0 at org.python.core.PySingleton.<init>(PySingleton=
.java:8)
> =A0=A0=A0=A0=A0=A0=A0 at org.python.core.PyNone.<init>(PyNone.java:7)
> =A0=A0=A0=A0=A0=A0=A0 at org.python.core.PySystemState.initStaticFields=
(PySystemState.java:341
> )
> =A0=A0=A0=A0=A0=A0=A0 at org.python.core.PySystemState.initialize(PySys=
temState.java:320)
> =A0=A0=A0=A0=A0=A0=A0 at org.python.core.Py.initProperties(Py.java:665)
> =A0=A0=A0=A0=A0=A0=A0 at org.python.core.Py.initProxy(Py.java:709)
> =A0=A0=A0=A0=A0=A0=A0 at tryJythonSample.<init>(tryJythonSample.java:10=
5)
> =A0=A0=A0=A0=A0=A0=A0 at tryJythonMain.main(tryJythonMain.java:9)
>
> Can anyone tell me what I'm doing wrong?

see notes in code samples.

> Is there a problem with dependencies?

no.


-robert