iodigital-com/coroutines-google-day

Java reactive to Kotlin reactive

1. unzip resource file data.sql.zip
2. use the VM flag -XX:+AllowRedefinitionToAddDeleteMethods
3. set property to false the second time the application runs: com.io.reload-db-data=false

HTTP script

The project contains the file scripts/Weather-info-requests.http that you can use to make api calls directly from your IDE

Migration to Kotlin

As a first step, we would like to migrate the project to kotlin. The project is already configured for kotlin but the files are all java.

Kotlin syntax help

As some of you are not familiar with kotlin, here are a few tips to help you with the syntax:

• The primitive types and their boxed versions do not exist: int and Integer become -> Int in Kotlin
• The types all exist in 2 variants: nullable and non-nullable so the java String becomes -> String and String?
• every nullable variable has to be accessed safely using ?. instead of .: for example: a.b?.c?.d -> a is not nullable, but everything after is. It is also possible to perform null checks manually and the value will automatically be cast to non-null version. See docs
• you can use ?: to conditionally access the right part when the left part is null. You can use (but shouldn't) !! to assert something as not nullable (a!!.b will throw NPE at Runtime if a is null)
• Functions and types are written differently: Mono<ServerResponse> foo(Bar bar) {} will become -> fun foo(bar: Bar): Mono<ServerResponse> {}
• the variables can be assigned with val (constant reference) or var (mutable reference), you should only use var if you can't use val
• the lambdas are written differently: (a, b) -> {} will be written { a, b -> } in kotlin, if you don't provide a name for the parameter, the default it becomes available
• lambda parameters: in kotlin, if the only parameter is a lambda, then you omit the parenthesis in the function call: collection.filter({ item -> item }) will be written collection.filter { it }
• You can chain instructions, without variables with the widely used functions apply, also, run and let. For example: listOf(1,3).map { it += 1 }.let { setOf(it) }.also { log.debug(it) } will return a Set and also log its content
• The default collections and data structures available are not mutable, if you want their mutable version, you need to request it: listOf(1,3) is not mutable, mutableListOf(1,3) is mutable.
• What you would write as a java record should be a kotlin data class: data class A(val a: String?, val b: Int) (be aware of the val in the parameter)
• The kotlin class and its file don't need to have the same name, similarly you can have as many classes as you want per file.
• instead of using static values, you can define object which are singletons, and usually used as companion object for a class to have its static methods.

You can find the REPL if you want to try some kotlin code:

IDE auto migration

You can use the IDE functionality to get a direct code translation java to kotlin. While the code migration works, you need to re-arrange it to make it more human-readable.