Kotlin: Double Colon vs. Lambda in similarish Class

Question

I understand my first second example uses a lambda function with a single parameter passed in. I'm trying to understand why it would be different from my second boilerplate example where a double colon is used instead of a lambda. (still a kotlin newb trying to wrap my head around double colons coming from a python background)

class Service(services: PluginServiceHub) {
    init {
        services.registerFlowInitiator(Landlord::class.java) { Landlord(it) }
    }
}

VS

class Service(services: PluginServiceHub) {
    init {
        services.registerFlowInitiator(IssuanceRequester::class.java, ::Issuer)
    }
}

What does the ::Issuer represent exactly?

score 9 · Accepted Answer · answered Jul 27 '17 at 18:29

9

Assuming there is a class Issuer, ::Issuer will be a function reference to its constructor. A constructor taking the appropriate number of arguments (one in this case) will be resolved and used, which is equivalent to a lambda { Issuer(it) }.

If there's no such a class, a function named Issuer and taking one argument will be used, if it exists.

See: Are there constructor references in Kotlin?

answered Jul 27 '17 at 18:29

hotkey

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this is a very simple and clarifying explanation. thank you! – mleafer Jul 28 '17 at 17:23

Kotlin: Double Colon vs. Lambda in similarish Class

1 Answers1