DynamoDb now supports running a local test version of the Database. This is how you connect to it with the AWS Java SDK.
int port = 8800;
ClientConfiguration config = new ClientConfiguration().withProtocol(Protocol.HTTP);
AWSCredentials creds = new BasicAWSCredentials("test","madeup");
AmazonDynamoDBClient client = AmazonDynamoDBClient(creds,config);
client.setEndpoint("localhost:"+port);
Kotlin use the same syntax as Java for Generics but generics behave a bit different. In Kotlin all generics type parameters must be specified (it does not default to “*”, even though Kotlin does have a “*” wildcard). Generics are invariant in Kotlin (like Java), making this assignment not allowed
val stringSet : HashSet<String>()
stringSet.add("a")
stringSet.add("b")
var anySet : Set<Any> = stringSet //Not allowed
This is for a good reason, type-safety. If the above assignment was allowed, getting values from anySet would work fine because String is Any. However when values are added into anySet we run into trouble.
anySet.add(Date())
var totalLength = 0;
for( str in stringSet ) {
totalLength += str.length;
//This will fail when date is reached
}
However there are times where you know you will be just pulling values out of an object, for those cases Kotlin uses the out modifier.
fun printAll(set : Set<out Any>) {
for( item in anySet ) {
//item is Any
println(item);
}
}
printAll(stringSet)
Kotlin distinguishes between generic types passed into functions and generic types returned from functions (in/out modifiers). When you specify the generic type as out, all functions which reference that generic as an input parameter will be unavailable.
val anyOutSet : Set<out Any> = stringSet anyOutSet.add(Date()); //will not compile
Now back to the “*” wildcard.
Java
Set<*> => Set<Object>
Kotlin
Set<*> => Set<out Any?>
Java Set<Object> is similar to Set<out Any?> in Kotlin however it is not the same. As we know out generics limit the use to only functions which do not take the generic type as a parameter. Due to Kotlins enhanced type-safety there are times we have to think through the uses case a bit more than in Java.
I have been using Kotlin now for about 2 weeks to build our new DynamoDB Driver and have come up with some notes on using Kotlin with the standard Java Reflections system.
That is it, java reflections appear to work fine