Kotlin Language Features Related to Null Handling

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Any software engineer with a Java background would find the null handling features in the Kotlin language interesting. Let's summarize this topic with some examples. Nullable types: In Kotlin, types are non-nullable by default. If you want a variable to be able to hold a null value, you need to explicitly declare its type as nullable using the Type? syntax. For example, String? denotes a nullable string, while String represents a non-nullable string. Safe calls (?.): Kotlin introduces the safe call operator (?.) for handling nullable types. It allows you to safely invoke a method or access a property on a nullable object. If the object is null, the expression returns null instead of throwing a NullPointerException. Example: data class Person(val name: String, val age: Int, val address: String?) fun main() {     // Create a person with a nullable address     val person1 = Person("John Doe", 25, "123 Main Street")     val person2 = Person("Jane Doe", 30,...
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Revisiting the Axon Framework

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Axon is a CQRS Framework for scalable and high-performance applications. Lets look at the evolution of the API from Axon 1 to Axon 3:


With Axon 3, Aggregate  interface is introduced and we pass the method to the execute() method:

Axon 1:

@CommandHandler
public void handle(final ChangeClientNameCommand command, UnitOfWork unitOfWork) {

    Client client = clientRepository.load(new StringAggregateIdentifier(command.getClientIdentifier()));

    client.changeNameAs(command.getNewName());
}


Axon 3:

@CommandHandler
public void handle(final ChangeClientNameCommand command) {

    Aggregate client = clientRepository.load(command.getClientIdentifier().toString());

    client.execute(aggregateRoot -> aggregateRoot.changeNameAs(command.getNewName()));
}





In the past Client entity was derived from a class called "AbstractAnnotatedAggregateRoot". Now in Axon 3, it is just a POJO and the Repository does not return a Client entity but an "aggregate". We pass the function as a parameter. At the time of Axon 1.0, Java language did not have support for lambda expressions.







Axon 1:









public class Client extends AbstractAnnotatedAggregateRoot {



    private String name;



    private Address address;



    private String phoneNumber;









Axon 3:








@AggregateRootpublic class Client {

    @AggregateIdentifier    private ClientId companyId;

    private String name;
    private Address address;
    private String phoneNumber;





Domain entities are not dependent to a base class anymore. We see this pattern in many frameworks like Unit Test frameworks. There is even a project called Lombok that uses annotations for simpler code.







To be continued..




















































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