Recipe.md

Controller Recipes

In this recipe we will convert the classes in the controller package to Kotlin.

The RecipesController contains the REST methods of our Recipes API, and the RecipesControllerAdvice catches exceptions and returns the proper error code and response.

Convert RecipesController & RecipesControllerAdvice

1. Convert the RecipeController Java files to Kotlin
2. As the log property was created based on Lombok annotation @Slf4, we will have to replace it. Add a companion object to the class, and declare val log: Logger = LoggerFactory.getLogger(RecipeController::class.java) to it
3. Declare the private recipeService property as a non-nullable argument to the primary constructor. This way you can call the members on it without a safe call (?.) or a bang-bang (!!)
4. Convert RecipeControllerAdvice to Kotlin
5. Use the listOf() instead of Java's List.of()
6. When ready, run all tests:

   (cd ../.. && ./mvnw clean verify)

Convert ErrorMessageMapper

The ErrorMessageMapper is used by the RecipesControllerAdvice to transforms the error messages from the exceptions thrown by the application to the body of an error response. For example, if the client POST request contains multiple incorrect fields a MethodArgumentNotValidException is thrown by the application, because of the constraints put on certain request fields (e.g. @NotNull, @NotEmtpy etc.). The ErrorMessageMapper will map all the error messages (constraint violations) to a single String (with a pre-fix), and creates an RecipeErrorResponse.
Which gets serialised to JSON and returned to the client.

1. Convert the ErrorMessageMapper Java files to Kotlin. Because this class contains only static methods, IntelliJ converts it into a singleton object

2. Refactor both the toErrorMessage() method the following way:

   1. Use Kotlin's Collection extension functions instead of Java Streams
   2. Assign default values to the optional parameters
   3. Use sortedBy() to sort the messages
   4. Replace the mutable StringBuilder by more lean and safe joinToString() implementation.

3. When ready, run all tests:

   (cd ../.. && ./mvnw clean verify)

8. If all tests have passed, continue to the next recipe.

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