使用 Py4J 将双精度数组从 Java 传递到 Python

Question

I want to send arrays of double s (or float s) from Java to Python using Py4J. But it can't seem to be working. Here is my MWE on the Java side.

The primary program

public class TheExample {
    
    private double sq[];

    public double twoTimes(double element) {
        return 2*element ;
    }
    
    public double[] squares(double x, double y) {
        sq = new double[2];
        sq[0] = x*x ;
        sq[1] = y*y ;
        return sq ;
    }

}

The entry point program

import py4j.GatewayServer;

public class TheExampleEntryPoint {

    private TheExample thex;

    public TheExampleEntryPoint() {
      thex = new TheExample();
    }

    public TheExample getThex() {
        return thex;
    }

    public static void main(String[] args) {
        GatewayServer gatewayServer = new GatewayServer(new TheExampleEntryPoint());
        gatewayServer.start();
        System.out.println("Gateway Server Started");
    }

}

After starting the Gateway Server, I accessed the object from Python:

>>> from py4j.java_gateway import JavaGateway
>>> gateway = JavaGateway()
>>> thex = gateway.entry_point.getThex()
>>> thex.twoTimes(9.0)
18.0
>>> thex.squares(2.0, 3.0)
JavaObject id=o1
>>> ### ??? The surprise

How to properly send an Array of double s or float s from Java to Python?

My use case is simple: I need only to receive the values from Java. Not necessary to be able to modify the array (the list on the Python side). Hence if receiving it as a tuple is possible and easier, that's even better.

Thanks!

Answer 1

My current solution is to utilize the List in Java, as follows:

The primary program

import java.util.List;
import java.util.ArrayList;

public class TheExample {
    
    public double twoTimes(double element) {
        return 2*element ;
    }
    
    public List<Double> squares(double x, double y) {
        List<Double> sq = new ArrayList<Double>();
        sq.add(x*x) ;
        sq.add(y*y) ;
        return sq ;
    }

}

And it works as expected in the Python side:

>>> from py4j.java_gateway import JavaGateway
>>> gateway = JavaGateway()
>>> thex = gateway.entry_point.getThex()
>>> thex.twoTimes(9.0)
18.0
>>> thex.squares(2.0, 3.0)
[4.0, 9.0]