NullPointerException on calling Jython Class from Java application

[Raj Sikka]

Hi ,
I am getting NullPointerException on calling Jython class from java code.
Following are the source code files:
.py file is :
import java
import sys
from math import *

class CalledByJava(java.lang.Object):
def __init__(self, equation):
"@sig public CalledByJava(String equation)"
self.equation = equation

def solve(self):
"@sig public float solve()"
result = eva(self.equation)
return float(result)


if __name__ == '__main__':
jt = CalledByJava(sys.argv[1])
print jt.solve()
.java file is :
public class CallJython {
public static void main(String[] args) {
System.out.println("Calling Jython function :"+args[0]);
CalledByJava cbj = new CalledByJava(args[0]);
System.out.println(cbj.solve());
}
}
Error i am getting is :
Exception in thread "main" java.lang.NullPointerException
at org.python.core.PyJavaClass.lookup(PyJavaClass.java:43)
at org.python.core.PyObject.<init>(PyObject.java:46)
at org.python.core.PySingleton.<init>(PySingleton.java:8)
at org.python.core.PyNone.<init>(PyNone.java:10)
at org.python.core.PySystemState.initStaticFields(PySystemState.java:396
)
at org.python.core.PySystemState.initialize(PySystemState.java:375)
at org.python.core.Py.initProperties(Py.java:696)
at org.python.core.Py.initProxy(Py.java:738)
at CalledByJava.__initProxy__(CalledByJava.java:135)
at CalledByJava.<init>(CalledByJava.java:113)
at CallJython.main(CallJython.java:4)
Please help me out.
Thanks,
Raj

[Barry Gaunt]

I get the following

    java -classpath "C:\TEMP;C:\TEMP\jpywork;c:\jython-2.1\jython.jar" -ms32m -mx32m CallJython "1+2"
Calling Jython function :1+2
3.0
Process java exited with code 0

But to get it I had to change your .py file to:

import java
import sys
from math import *
class CalledByJava(java.lang.Object):
    def __init__(self, equation):
        """@sig public CalledByJava(String equation)"""
        self.equation = equation
    def solve(self):
        """@sig public float solve()"""
        result = evaluate(self.equation)
        return float(result)

if __name__ == '__main__':
    jt = CalledByJava(sys.argv[1])
    print jt.solve()

Notice the triple double-quotes (""") on the doc strings!
I also had to change eval to evaluate to get it into this post.
[ January 15, 2004: Message edited by: Barry Gaunt ]

[Barry Gaunt]

On second thoughts, I cannot see why using triple double-quotes should make any difference. In "Python Essentials" only single double-quotes were used. In the Jython site's example triple double-quotes were used. I don't know, the problem you are experiencing must be due to something else...
Can you get the example given in the FAQ to work?
[ January 15, 2004: Message edited by: Barry Gaunt ]

[Raj Sikka]

Barry,
Thanks for your help.
I am able to run the example with the changes you have done.
Raj