Create a directory for the Java files in this project: 
mkdir -p src/main/java/io/streamnative/developer
Let’s create the Java producer application by pasting the following code into a file src/main/java/io/streamnative/developer/ProducerExample.java. 
package io.streamnative.developer;

import org.apache.kafka.clients.producer.KafkaProducer;
import org.apache.kafka.clients.producer.Producer;
import org.apache.kafka.clients.producer.ProducerRecord;
import org.apache.kafka.common.serialization.StringSerializer;

import java.util.Properties;
import java.util.Random;

import static org.apache.kafka.clients.CommonClientConfigs.BOOTSTRAP_SERVERS_CONFIG;
import static org.apache.kafka.clients.CommonClientConfigs.SECURITY_PROTOCOL_CONFIG;
import static org.apache.kafka.clients.producer.ProducerConfig.*;
import static org.apache.kafka.common.config.SaslConfigs.*;

public class ProducerExample {

    public static void main(final String[] args) {
        final Properties props = new Properties() {{
            // User-specific properties that you must set
            put(BOOTSTRAP_SERVERS_CONFIG, "<BOOTSTRAP SERVERS>");
            put(SASL_JAAS_CONFIG,         "org.apache.kafka.common.security.plain.PlainLoginModule required username='unused' password='token:<API KEY>';");

            // Fixed properties
            put(KEY_SERIALIZER_CLASS_CONFIG,   StringSerializer.class.getCanonicalName());
            put(VALUE_SERIALIZER_CLASS_CONFIG, StringSerializer.class.getCanonicalName());
            put(ACKS_CONFIG,                   "all");
            put(SECURITY_PROTOCOL_CONFIG,      "SASL_SSL");
            put(SASL_MECHANISM,                "PLAIN");
        }};

        final String topic = "purchases";

        String[] users = {"eabara", "jsmith", "sgarcia", "jbernard", "htanaka", "awalther"};
        String[] items = {"book", "alarm clock", "t-shirts", "gift card", "batteries"};
        try (final Producer<String, String> producer = new KafkaProducer<>(props)) {
            final Random rnd = new Random();
            final int numMessages = 10;
            for (int i = 0; i < numMessages; i++) {
                String user = users[rnd.nextInt(users.length)];
                String item = items[rnd.nextInt(items.length)];

                producer.send(
                        new ProducerRecord<>(topic, user, item),
                        (event, ex) -> {
                            if (ex != null)
                                ex.printStackTrace();
                            else
                                System.out.printf("Produced event to topic %s: key = %-10s value = %s%n", topic, user, item);
                        });
            }
            System.out.printf("%s events were produced to topic %s%n", numMessages, topic);
        }

    }
}
Fill in the appropriate <BOOTSTRAP SERVERS> endpoint and <API KEY> in the BOOTSTRAP_SERVERS_CONFIG and SASL_JAAS_CONFIG properties where the client configuration props object is created. You can test the syntax before proceding by running the following command: 
gradle build
And you should see the following output: 
BUILD SUCCESSFUL in 1s