如何在Kotlin中定义可为空的委托成员？

Question

I need to decorate an instance in Java and would like the delegation to be in Kotlin (easier).

The problem is that I get a compilation error on the definition.

How can I define inner to be able to receive null?

open class ConnectionDecorator(var inner: Connection?) : Connection by inner // Getting an error on the right inner

Example usage from Java:

new ConnectionDecorator(null).close();

* This is a simplified example, trying to use Kotlin's delegation in Java, where passed content can be null.

Answer 1

You can provide a Null Connection Object if the inner is null, for example:

//                             v--- the `var` is unnecessary
open class ConnectionDecorator(var inner: Connection?) : Connection by wrap(inner)


fun wrap(connection: Connection?): Connection = when (connection) {
    null -> TODO("create a Null Object") 
    else -> connection
}

In fact, there is no need such a ConnectionDecorator , it doesn't make sense, because when you using delegation you also need override some methods to provide additional operations, eg: log . you can use wrap method directly, for example:

val connection:Connection? = null;

wrap(connection).close()

You should make the inner to non-nullable and create ConnectionDecorator instance by wrap , for example:

//                                        v--- make it to non-nullable
open class ConnectionDecorator(var inner: Connection) : Connection by inner {
    fun close(){
       inner.close();
       log.debug("connection is closed");
    }
}

val source:Connection? = null;

//                                          v--- wrap the source
val target:Connection = ConnectionDecorator(wrap(source))

target.close()

Answer 2

Try This

ConnectionDecorator(null).close();

open class ConnectionDecorator(var inner: Connection?) : Connection by inner!!

Hope its work