title: Apache kafka sending java object failed
body: <p>I have this object</p>
  
  <pre><code>       HashMap message = new HashMap();
         message.put("x", "xxxxx");
         message.put("y", "yyyyy");
         message.put("z", 100);
  
         ProducerRecord producerRecord = new ProducerRecord(topic, message);
         producer.send(producerRecord); 
  </code></pre>
  
  <p>I am getting </p>
  
  <blockquote>
    <p>Exception in thread "main" org.apache.kafka.common.errors.SerializationException: Can't convert value of class java.util.HashMap to class org.apache.kafka.common.serialization.StringSerializer specified in value.serializer</p>
  </blockquote>