CODEWITHSUNDEEP
kotlin
Jim
Jim

Reputation: 4425

Use the default implementation of an interface if nothing passed as function parameter

In Kotlin I have an interface with default implementation: 

  interface MyInterface {
        fun foo() = "foo"
        fun bar() = 120        
    }

So I am trying to create a function that would expect as a parameter a MyInterface or use the default implementation if one is not passed.
The following does not compile:
fun(param1: String, param2: Int, myInterface: MyInteface = MyInterface())

How can I fix this?

Upvotes: 0

Views: 139

Answers (3)

gidds
gidds

Reputation: 18627

As other answers have said, you need an object which implements your interface.

Perhaps the simplest way of creating one is using the object : <ClassOrInterface> syntax:

fun myFun(param1: String, param2: Int, myInterface: MyInterface = object : MyInterface{}) {
    // …
}

That may create a new object for each call which uses the default*.  If that matters, you might want to create an object separately:

val defaultImpl = object : MyInterface

fun myFun(param1: String, param2: Int, myInterface: MyInterface = defaultImpl) {
    // …
}

(* Kotlin 1.3.50 always seems to create a new instance each time the default is used.  But that behaviour isn't specified in the docs, so it could potentially be optimised in future releases.)

Upvotes: 1

Kwame Opare Asiedu
Kwame Opare Asiedu

Reputation: 2345

The default then needs to be an object which implements the interface...

class MyInterfaceImplementation: MyInterface

fun (param1: String, param2: Int, myInterface: MyInteface = MyInterfaceImplementation()) {
    /* Function implementation */
}

Upvotes: 1

JB Nizet
JB Nizet

Reputation: 692231

Create a class that implements the interface, and use 

class MyDefaultImplementation : MyInterface

fun(param1: String, param2: Int, myInterface: MyInterface = MyDefaultImplementation())

Or even better, create an object (singleton) that implements this interface, and use that object as default value.

object MyDefaultImplementationObject : MyInterface
fun(param1: String, param2: Int, myInterface: MyInterface = MyDefaultImplementationObject)

Upvotes: 2

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