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In these exercises, you’ll practice some of the concepts taught in this module.

First, either create a new Java project, adding the reactor-bom and reactor-core dependencies to your build file (Maven or Gradle) or use the stub you can find at:

I’ll give you the instructions (and sometimes hints) so you can put all the code together in the main method of a class and observe the output.

Here you can find the solution for the coding exercises:

Exercise 1

This exercise simulates fetching user data from a remote API. The API can return either a list of user names or an error.

  1. Given the following code where the method simulateApiCall simulates an API call and based on a random value, either returns a list of user names or throws an exception:
     import reactor.core.publisher.Flux;
     import java.util.Arrays;
     import java.util.List;
     import java.util.Random;
     public class Exercise01 {
         public static void main(String[] args) {
             Flux<List<String>> userNamesFlux = Flux.defer(() -> {
                 // TODO Create either a Flux with the result or with an error
                 return null;
             userNamesFlux.subscribe(userNames -> System.out.println("User data: " + userNames),
                     error -> System.out.println("Error fetching user data: " + error.getMessage()));
         public static List<String> simulateApiCall() {
             List<String> userNames = Arrays.asList("Alice", "Bob", "Carol", "David");
             Random random = new Random();
             if (random.nextBoolean()) {
                 throw new RuntimeException("Remote API error");
             return userNames;
  2. Implement the body of the lambda expression passed to Flux.defer to call the method simulateApiCall:
    • Create a Flux that uses the Flux.error method to emit an error if the simulateApiCall method throws an exception.
    • Otherwise, create a Flux with the list returned by the method.
  3. Run the Exercise01 class and analyze the output.

Exercise 2

In the following exercise, you’ll implement the reactive equivalent of the following imperative try-catch-finally block.

  1. Create a class named Exercise02 with a main method.
  2. Given the following code:
     List<Integer> inputList = Arrays.asList(1, 2, -3, 4);
     try {
         for (Integer num : inputList) {
             if (num < 0) {
                 throw new IllegalArgumentException("Negative numbers are not allowed.");
             System.out.println(num * 2);
     } catch (IllegalArgumentException e) {
         System.out.println("Error: " + e.getMessage());
     } finally {
         System.out.println("Finished processing the list.");
  3. Create a Flux with the values of inputList (if you prefer, you can create a Flux from this list).
  4. Implement the reactive equivalent of the above try-catch-finally block using the map, doOnError, doFinally, and subscribe operators.
  5. Run the Exercise02 class and analyze the output.


  • Use the map operator to double each integer and throw an IllegalArgumentException if a negative number is found.
  • Handle the error using the doOnError operator, printing the appropriate message.
  • Use the doFinally operator to print the appropriate message regardless of whether an error occurred or not.
  • Subscribe to the Flux to print each doubled value, also passing an empty error consumer to avoid printing the stack trace of the exception.

Exercise 3

In this exercise, you are given a list of URLs, each containing a JSON string. Your task is to create a program that fetches JSON data from the URLs and parses them into strings. Some of the URLs may return invalid JSON responses, causing a exception to be thrown during the parsing process. In case of an invalid JSON string, you should fetch data from a fallback URL, parse it, and return the result.

  1. Create a class named Exercise03 with a main method.
  2. Given the following code were the methods fetchData and parseJson simulate the fetching and parsing of JSON data, respectively:
     import reactor.core.publisher.Flux;
     public class Exercise03 {
         public static void main(String[] args) {
             String fallbackUrl = "fallbackUrl";
             Flux.just("url1", "url2", "url3", "url4")
                     // TODO Use flatMap/map and onErrorResume to implement the functionality
         public static String fetchData(String url) {
             // Simulates fetching JSON data from the URL
             return "{\"data\": \"" + url + "\"}";
         public static String parseJson(String jsonString) {
             // Simulates the parsing of jsonString and throwing an exception if it is invalid
             String str = "";
                 throw new RuntimeException("Invalid JSON");
                 str = jsonString.toUpperCase();
             return str;
  3. Fetch and parse the JSON data handling any exception using the onErrorResume operator. In case of an exception, fetch data from the fallback URL, parse it, and return the result.
  4. Run the Exercise03 class and analyze the output.

Note: This exercise can be implemented in more than one way. Probably one of the easiest is to get, inside a flatMap operator, the JSON string as a Mono, and then, use map to parse it and onErrorResume to use the fallback URL. Take a look at the solution to review this approach if you’re having a hard time with this exercise.

Exercise 4

In this exercise, you are given a list of integers. Your task is to implement the error handling of a program that processes a number using doOnError and onErrorResume.

  1. Create a class named Exercise04 with a main method.
  2. Given the following code:
     import reactor.core.publisher.Flux;
     import reactor.core.publisher.Mono;
     public class Exercise04 {
         public static void main(String[] args) {
             Flux.just(1, 2, 3, 4, 5)
                     .flatMap(number -> {
                         // TODO Implement reactive sequence using doOnError and onErrorResume
                         return null;
         public static Mono<Integer> processNumber(int number) {
             int doubled = number * 2;
             if (doubled % 4 == 0)
                 return Mono.error(new IllegalArgumentException("Result is divisible by 4"));
                 return Mono.just(doubled);
  3. Inside the flatMap operator:
    • Call the method processNumber to get a Mono<Integer>.
    • Use the doOnError method to print “Error processing [number]: “ followed by the error message, but only for IllegalArgumentException.
    • Use the onErrorResume operator to handle any error by returning a Mono that emits -1 as the fallback value.
  4. Run the Exercise04 class and analyze the output.

Exercise 5

In this exercise, you’ll use Exceptions.propagate(Throwable) to propagate a checked exception as a RuntimeException.

  1. Create a class named Exercise04 with a main method.
  2. Given the following code:
     import reactor.core.publisher.Flux;
     public class Exercise05 {
         public static void main(String[] args) {
             Flux<String> stringFlux = Flux.just("1", "2", "three", "4", "5");
    -> {
                         // TODO Parse each element and propagate any NumberFormatException
                         return null;
                             error -> System.out.println("Error: " + error.getMessage())
  3. Inside the map operator, parse the strings into integers.
  4. If a string is not a valid number, throw a NumberFormatException and propagate it as a RuntimeException using Exceptions.propagate(Throwable) method.
  5. Run the Exercise05 class and analyze the output.