Calling a java method, in Python, using Jython.

Question

I have a Java class called matrixMult() with two methods, one populates a Matrix with 1's and 0's, and the other receives 3 arrays, multiplies 2 of them, and returns the result in an array.

import java.util.Timer;

public class matrixMult {

//Method to populate Matrices with 1's and 0's//
public static int [][] populatingMatrices(int A [] [])
{
    for (int i = 0; i < A.length; i++) 
    {   
        for (int j = 0; j < A[0].length; j++) 
        {   
            if (i == j) 
            {
                A[i][j] = 1;
            }
            else 
            {
                A[i][j] = 0;
            }
        }
    //Returns Matrix populated with 1's and 0's//
    }return A;
} 

//Method to multiply Identity Matrices//
public static int [][] mmm (int a [][], int b [][], int c [][])
{
    int nr;
    int nc;

    nr = nc = a.length;

    for (int i = 0; i < nr; i++) 
    {
        for (int j = 0; j < nr ; j++ ) 
        {
            for (int k = 0;k < nr ;k++) 
            {
                c[i][j] = c[i][j] + a[i][k] * b[k][j];      
            }   
        }
    }
    return c;
}

}

Then, I wrote a python script, where I'm importing "matrixMult". I want to create 3 multidimensional arrays, populate 2 of them using the method populatingMatrices(), multiply them using the mmm() method and time how much was the operation.

import timeit
import matrixMult

#matrixMult Object#
a = matrixMult()

listA = [[]]
listB = [[]]
listC = [[]]

#Populating matrices#
a.populatingMatrices(listA)
a.populatingMatrices(listB)
a.populatingMatrices(listC)

#Timing the Matrix Multiplication#
start_time = timeit.default_timer()
listC = a.mmm(listA,listB,listC)
elapsed = timeit.default_timer() - start_time

print elapsed

Once I've got my timings, the plan is to plot it using Gnuplot, and compare the cost of three different operations (Linear, Quadratic and cubic).

When I try to run the python scrip through the command line I get this error:

File "mmmPY.py", line 20, in <module>
listC = a.mmm(listA,listB,listC)
at matrixMult.mmm(matrixMult.java:40)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:
    at  sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorIm
    at java.lang.reflect.Method.invoke(Method.java:606)

java.lang.ArrayIndexOutOfBoundsException: java.lang.ArrayIndexOutOfBoundsException: 0

My guess is that the arrays are empty, and they're not getting populated with the populatingMatrices() method.

Does anybody find a way to fix it? It's safe to say I'm a noob in Python and not the best java programmer, but I really can't get my head around this.

Thanks again for your help!

Answer 1

I think its all about your lists that send from

#Populating matrices#
a.populatingMatrices(listA)
a.populatingMatrices(listB)
a.populatingMatrices(listC)

are not initialized with a length so when java controls it, it sees 0 as length then throws java.lang.ArrayIndexOutOfBoundsException: 0