MINOR: Read configuration fields from ProducerConfig in example (#4601)

Reading the configuration field names from ProducerConfig class and taking the key and value serializer names from class name directly instead of hardcoding.

examples/src/main/java/kafka/examples/Producer.java

@@ -20,6 +20,9 @@ import org.apache.kafka.clients.producer.Callback;
 import org.apache.kafka.clients.producer.KafkaProducer;
 import org.apache.kafka.clients.producer.ProducerRecord;
 import org.apache.kafka.clients.producer.RecordMetadata;
+import org.apache.kafka.clients.producer.ProducerConfig;
+import org.apache.kafka.common.serialization.IntegerSerializer;
+import org.apache.kafka.common.serialization.StringSerializer;
 
 import java.util.Properties;
 import java.util.concurrent.ExecutionException;
@@ -31,10 +34,10 @@ public class Producer extends Thread {
 
     public Producer(String topic, Boolean isAsync) {
         Properties props = new Properties();
-        props.put("bootstrap.servers", KafkaProperties.KAFKA_SERVER_URL + ":" + KafkaProperties.KAFKA_SERVER_PORT);
-        props.put("client.id", "DemoProducer");
-        props.put("key.serializer", "org.apache.kafka.common.serialization.IntegerSerializer");
-        props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");
+        props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, KafkaProperties.KAFKA_SERVER_URL + ":" + KafkaProperties.KAFKA_SERVER_PORT);
+        props.put(ProducerConfig.CLIENT_ID_CONFIG, "DemoProducer");
+        props.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, IntegerSerializer.class.getName());
+        props.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, StringSerializer.class.getName());
         producer = new KafkaProducer<>(props);
         this.topic = topic;
         this.isAsync = isAsync;