PY4J callback server error

I’m trying to run the example for the callback server in the PY4J website
here

But I’m getting the following exception:
“py4j.protocol.Py4JNetworkError: An error occurred while trying to start the callback server”

This is the code:

Java:

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;
    }

    public List<Integer> randomTernaryOperator(Operator op) {
            Random random = new Random();
            List<Integer> numbers = new ArrayList<Integer>();
            numbers.add(random.nextInt(MAX));
            numbers.add(random.nextInt(MAX));
            numbers.add(random.nextInt(MAX));
            numbers.add(op.doOperation(numbers.get(0), numbers.get(1), numbers.get(2)));
            return numbers;
    }

    public static void main(String[] args) {
            GatewayServer server = new GatewayServer(new OperatorExample());
            server.start();
    }

}

Interface:

package py4j.examples;

public interface Operator {

    public int doOperation(int i, int j);

    public int doOperation(int i, int j, int k);

}

Python:

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()
    numbers = gateway.entry_point.randomBinaryOperator(operator)
    print(numbers)
    numbers = gateway.entry_point.randomTernaryOperator(operator)
    print(numbers)
    gateway.shutdown()

As I mentioned, I’m getting this exception

py4j.protocol.Py4JNetworkError: An error occurred while trying to start the callback server.

This is the stack trace:

Traceback (most recent call last):
  File "/home/amir/Python code/callback_example.py", line 14, in <module>
    gateway = JavaGateway(start_callback_server=True)
  File "/usr/local/lib/python2.7/dist-packages/py4j-0.8.2.1-py2.7.egg/py4j/java_gateway.py", line 851, in __init__
    self._start_callback_server(python_proxy_port)
  File "/usr/local/lib/python2.7/dist-packages/py4j-0.8.2.1-py2.7.egg/py4j/java_gateway.py", line 867, in _start_callback_server
    self._callback_server.start()
  File "/usr/local/lib/python2.7/dist-packages/py4j-0.8.2.1-py2.7.egg/py4j/java_gateway.py", line 1091, in start
    raise Py4JNetworkError(msg)
py4j.protocol.Py4JNetworkError: An error occurred while trying to start the callback server
[Finished in 0.5s with exit code 1]