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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nijhof2axon

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Axon framework kullanıcılarına katkıda bulunmak amacıyla paylaştığım proje ile ilgili yorum ve sorular gelmeye başladı.

http://code.google.com/p/nijhof2axon/

>>You share the Data between the Domain and the Query Model?
When a command is dispatched, a CommandHandler uses the aggregate root to handle the command. Aggregate root fires DomainEvents.
=> Query layer related code listens to domain events and updates the query model. (Example: ClientTableUpdater)

Domain entities are first class objects but Query Model classes are just DTOs with getters and setters. We have Client as entity and ClientDetailsEntry as DTO.

>>The (generated?) table domainevententry looks like you can not replay events? missing the event data?

To observe how events are replayed by the framework, you can follow these steps:

Click on sample client
change name
stop server
start server
put a breakpoint to EventSourcingRepository.doLoad() method
put a breakpoint to Client.handleClientNameChangedEvent() method
start server in debug mode
open the client that you changed
change name
=> you stop at the doLoad() breakpoint
reach to this line:
aggregate.initializeState(...
Here the events will be replayed to initialize aggregate state.
=> hit the breakpoint of Client.handleClientNameChangedEvent()

So, we have to implement the handlers that handle state changes properly.

>>if i can force the query database to have a invalid state...
You can do that by not listening to domain events and not updating query tables. So updating the query tables properly is our duty.

>>how to replay events to integrate new features
I added a test class to nijhof2axon. Please have a look at SampleEventVisitorTest

>>how i can monitor the system (maybe the command and event queue)
We can look at org.axonframework.auditing.AuditingInterceptor

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