Java producer client in Kafka - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kafka producer code snippet?

Consider the following Java Kafka producer code snippet. What will be printed to the console?

Kafka

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");

KafkaProducer<String, String> producer = new KafkaProducer<>(props);
ProducerRecord<String, String> record = new ProducerRecord<>("test-topic", "key1", "value1");

try {
    RecordMetadata metadata = producer.send(record).get();
    System.out.println("Sent to partition: " + metadata.partition());
} catch (Exception e) {
    System.out.println("Send failed");
}
producer.close();

A Sent to partition: 0

B Send failed

C Sent to partition: 1

D Compilation error

💡 Hint

By default, Kafka assigns messages to partition 0 if no custom partitioner is set and only one partition exists.

The code sends a message to the topic "test-topic". If the topic has only one partition (partition 0), the message will be sent there. The code prints the partition number, so the output is "Sent to partition: 0".

🧠 Conceptual

intermediate

Which property is required to specify the Kafka broker address in a Java producer client?

When configuring a Kafka producer in Java, which property key must be set to specify the Kafka broker address?

A bootstrap.servers

B broker.address

C kafka.broker

D server.address

💡 Hint

It usually contains the word 'bootstrap'.

The correct property to specify Kafka broker addresses is bootstrap.servers. It tells the producer where to connect initially.

🔧 Debug

advanced

What error does this Kafka producer code raise?

Examine the following Java Kafka producer code snippet. What error will it raise when run?

Kafka

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
// Missing value.serializer property

KafkaProducer<String, String> producer = new KafkaProducer<>(props);
ProducerRecord<String, String> record = new ProducerRecord<>("test-topic", "key1", "value1");
producer.send(record);
producer.close();

A NullPointerException

B ClassNotFoundException for StringSerializer

C No error, message sent successfully

D org.apache.kafka.common.KafkaException: Missing value serializer

💡 Hint

Check if all required serializer properties are set.

The producer requires both key.serializer and value.serializer properties. Missing value.serializer causes a KafkaException indicating the serializer is missing.

📝 Syntax

advanced

Which option fixes the syntax error in this Kafka producer code?

Identify the option that fixes the syntax error in the following Java Kafka producer snippet:

KafkaProducer producer = new KafkaProducer(props);
ProducerRecord record = new ProducerRecord<>("topic", "key", "value")
producer.send(record);
producer.close();

A Remove diamond operator and add semicolon after ProducerRecord line

B Add diamond operator in constructor and semicolon after ProducerRecord line

C Add semicolon after producer.send(record) line only

D No changes needed, code is correct

💡 Hint

Check missing semicolons and proper generic usage.

The ProducerRecord line is missing a semicolon. Also, the KafkaProducer constructor should use diamond operator <> for type safety. Option B fixes both issues.

🚀 Application

expert

How many messages are sent by this Kafka producer code?

Given the following Java Kafka producer code, how many messages will be sent to the topic?

Kafka

Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");

try (KafkaProducer<String, String> producer = new KafkaProducer<>(props)) {
    for (int i = 0; i < 5; i++) {
        ProducerRecord<String, String> record = new ProducerRecord<>("my-topic", "key" + i, "value" + i);
        producer.send(record);
        if (i == 2) {
            break;
        }
    }
}

A 0

B 5

C 3

D 2

💡 Hint

Look at the loop and the break statement carefully.

The loop runs from 0 to 4 but breaks when i == 2, so it sends messages for i = 0, 1, 2 only, totaling 3 messages.

Practice

1. What is the primary role of a Java producer client in Kafka?

easy

A. To send messages to Kafka topics

B. To consume messages from Kafka topics

C. To manage Kafka brokers

D. To monitor Kafka cluster health

Solution

1. Step 1: Understand Kafka client roles

   Kafka has producers that send messages and consumers that receive messages.

2. Step 2: Identify the producer's function

   A Java producer client specifically sends messages to Kafka topics.

3. Final Answer:

   To send messages to Kafka topics -> Option A

4. Quick Check:

   Producer = send messages [OK]

Hint: Producer sends, consumer receives messages [OK]

Common Mistakes:

• Confusing producer with consumer
• Thinking producer manages brokers
• Assuming producer monitors cluster

2. Which of the following is the correct way to set the serializer for the key in a Java Kafka producer?

easy

A. props.add("key.serializer", "StringSerializer");

B. props.set("key.serializer", "StringSerializer");

C. props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");

D. props.put("key.serializer", String.class);

Solution

1. Step 1: Recall how to set properties in Kafka producer

   Kafka producer uses Properties object with put(key, value) method.

2. Step 2: Identify correct serializer setting syntax

   The key.serializer must be set to the full class name as a string.

3. Final Answer:

   props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer"); -> Option C

4. Quick Check:

   Use props.put with full serializer class name [OK]

Hint: Use props.put with full serializer class name string [OK]

Common Mistakes:

• Using props.set or props.add instead of put
• Not using full class name string
• Passing class type instead of string

3. What will be the output of the following Java Kafka producer code snippet?

import java.util.Properties;
import org.apache.kafka.clients.producer.*;
Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");

Producer<String, String> producer = new KafkaProducer<>(props);
ProducerRecord<String, String> record = new ProducerRecord<>("my-topic", "key1", "value1");
producer.send(record);
producer.close();
System.out.println("Message sent");

medium

A. Message sent

B. Compilation error due to missing imports

C. Runtime error due to missing Kafka server

D. No output

Solution

1. Step 1: Analyze the code flow

   The code creates a producer, sends a message, closes the producer, then prints "Message sent".

2. Step 2: Consider output behavior

   Assuming Kafka server is running and imports are correct, the print statement executes after send and close.

3. Final Answer:

   Message sent -> Option A

4. Quick Check:

   Print after send and close = "Message sent" [OK]

Hint: Print statement runs after send, shows "Message sent" [OK]

Common Mistakes:

• Assuming send() blocks until message delivered
• Ignoring that print runs regardless of Kafka server
• Confusing compile errors with runtime issues

4. Identify the error in this Java Kafka producer code snippet:

import java.util.Properties;
import org.apache.kafka.clients.producer.*;
Properties props = new Properties();
props.put("bootstrap.servers", "localhost:9092");
props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");

KafkaProducer<String, String> producer = new KafkaProducer<>(props);
ProducerRecord<String, String> record = new ProducerRecord<>("my-topic");
producer.send(record);
producer.close();

medium

A. Producer not closed properly

B. Incorrect serializer class name

C. Missing bootstrap.servers property

D. ProducerRecord missing key argument

Solution

1. Step 1: Check ProducerRecord constructor usage

   The constructor used has only topic, but the generic type expects key and value.

2. Step 2: Identify mismatch in arguments

   ProducerRecord<String, String> expects key and value, but no key/value is provided, causing a compile error.

3. Final Answer:

   ProducerRecord missing key argument -> Option D

4. Quick Check:

   ProducerRecord requires key and value [OK]

Hint: ProducerRecord needs key and value if generics specify both [OK]

Common Mistakes:

• Using wrong ProducerRecord constructor
• Ignoring generic type mismatch
• Assuming value-only constructor works with key type

5. You want to send multiple messages to different Kafka topics using a single Java producer instance efficiently. Which approach is best?

hard

A. Create a new KafkaProducer for each message

B. Reuse one KafkaProducer and send ProducerRecords with different topics

C. Use multiple threads each with its own KafkaProducer

D. Send all messages to one topic and then move them to others

Solution

1. Step 1: Understand KafkaProducer resource usage

   Creating a producer is expensive; reusing one instance is efficient.

2. Step 2: Sending to multiple topics

   ProducerRecords can specify different topics; one producer can send to many topics.

3. Step 3: Evaluate other options

   Creating many producers wastes resources; sending all to one topic is incorrect.

4. Final Answer:

   Reuse one KafkaProducer and send ProducerRecords with different topics -> Option B

5. Quick Check:

   One producer, multiple topics via records [OK]

Hint: Reuse one producer; specify topic per ProducerRecord [OK]

Common Mistakes:

• Creating many producers wastes resources
• Sending all messages to one topic incorrectly
• Assuming each thread needs its own producer