Import GatewayServer from the py4j package so that the unqualified class can be used in the application

import py4j.GatewayServer;

You can call a Python method from Java by implementing a Java interface on the python side.

The steps are:

1. Create an interface in Java, e.g., py4j.examples.Operator
2. In Python, create a class and inside the class, create a Java class with an "implements" field.
3. In Python, instantiate a gateway with start_callback_server=True, e.g., gateway = JavaGateway(start_callback_server=True)
4. In Python, instantiate the class implementing a Java interface and send it to the Java side.
5. In Java, call the interface.

Example adapted from the Py4J documentation:

Java code:

// File 1
package py4j.examples;

public interface Operator {
        public int doOperation(int i, int j);
        public int doOperation(int i, int j, int k);
}

// File 2
package py4j.examples;

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

import py4j.GatewayServer;

public class OperatorExample {

    // To prevent integer overflow
    private final static int MAX = 1000;

    public List<Integer> randomBinaryOperator(Operator op) {
        Random random = new Random();
        List<Integer> numbers = new ArrayList<Integer>();
        numbers.add(random.nextInt(MAX));
        numbers.add(random.nextInt(MAX));
        numbers.add(op.doOperation(numbers.get(0), numbers.get(1)));
        return numbers;
    }
}

Python code:

from py4j.java_gateway import JavaGateway

class Addition(object):
    def doOperation(self, i, j, k = None):
        if k == None:
            return i + j
        else:
            return i + j + k

    class Java:
        implements = ['py4j.examples.Operator']

if __name__ == '__main__':
    gateway = JavaGateway(start_callback_server=True)
    operator = Addition()
    operator_example = gateway.jvm.py4j.examples.OperatorExample()

    # "Sends" python object to the Java side.
    numbers = operator_example.randomBinaryOperator(operator) 

Minimal working example:

//AdditionApplication.java
import py4j.GatewayServer;

public class AdditionApplication {

  public static void main(String[] args) {
    AdditionApplication app = new AdditionApplication();
    // app is now the gateway.entry_point
    GatewayServer server = new GatewayServer(app);
    server.start();
  }
}

Compile (make sure that the -cp path to the py4j is valid, otherwise adjust it such that it points to the right place):

javac -cp /usr/local/share/py4j/py4j0.9.jar AdditionApplication.java

Run it:

java -cp .:/usr/local/share/py4j/py4j0.9.jar AdditionApplication

Now, if you run your python script, in the terminal where the java AdditionApplication is running you should see something like:

>>> Hello World!