Can I add Kotlin extension function to Java class?-CodePudding

I'm trying to create extension function like this:

fun UHFTAGInfo.toReadUhfTagInfo(): ReadUhfTagInfo {
    return ReadUhfTagInfo(this.epc, this.count, this.rssi.toIntOrNull())
}

It is supposed to convert UHFTAGInfo (from java library) to ReadUhfTagInfo (my class in Kotlin).

I'm trying to use it like this:

UHFTAGInfo i = getUHFTAGInfo();
ReadUhfTagInfo ri = i.toReadUhfTagInfo();

At this moment my toReadUhfTagInfo function is at top level, but finally I want to put it in my ReadUhfTagInfo class, like this:

class ReadUhfTagInfo(var epc: String, var cnt: Int, var rssi: Int?)
{
    fun UHFTAGInfo.toReadUhfTagInfo(): ReadUhfTagInfo {
        return ReadUhfTagInfo(this.epc, this.count, this.rssi.toIntOrNull())
    }
}

CodePudding user response：

You can call Kotlin extension functions from Java, sure, but you can't call them with extension function syntax, you must call them like static methods. If you, for example, define

// file: Foo.kt

fun Bar.baz() { ... }

then in Java, you would call this as

FooKt.baz(bar);

CodePudding user response：

I don't think you want to have toReadUhfTagInfo as a member function on the ReadUhfTagInfo class. That would imply that, in order to convert a UHFTAGInfo to a ReadUhfTagInfo, you already need a ReadUhfTagInfo object (which will presumably go unused except to serve as the receiver object.

Extension functions defined inside a class are member extensions and essentially have two receivers.

You can declare extensions for one class inside another class. Inside such an extension, there are multiple implicit receivers - objects whose members can be accessed without a qualifier. An instance of a class in which the extension is declared is called a dispatch receiver, and an instance of the receiver type of the extension method is called an extension receiver.

If you want the extension method to act like a static method in Java (i.e. not require an instance of the enclosing class to execute), then you do the same thing we do with all static methods in Kotlin: We put it in a companion object.

class ReadUhfTagInfo(var epc: String, var cnt: Int, var rssi: Int?)
{
  companion object {
    fun UHFTAGInfo.toReadUhfTagInfo(): ReadUhfTagInfo {
      return ReadUhfTagInfo(this.epc, this.count, this.rssi.toIntOrNull())
    }
  }
}

As pointed out in the comments, this will still require the name to be imported into the current scope (as all extension methods do), but it won't require a receiver of type ReadUhfTagInfo to call anymore.