How to instantiate an anonymous class that implements an interface in Kotlin

Question

In Java, instantiate an interface object is as easy as new Interface()... and override all the required functions as below, on AnimationListener

private void doingSomething(Context context) {
    Animation animation = AnimationUtils.loadAnimation(context, android.R.anim.fade_in);
    animation.setAnimationListener(new Animation.AnimationListener() {
        // All the other override functions
    });
}

However, in Kotlin when we type

private fun doingSomething(context: Context) {
    val animation = AnimationUtils.loadAnimation(context, android.R.anim.fade_in)
    animation.setAnimationListener(Animation.AnimationListener(){
        // All the other override functions
    })
}

It error complaints unresolved References AnimationListener.

Possible duplicate of [How to create an instance of anonymous interface in Kotlin?](http://stackoverflow.com/questions/37672023/how-to-create-an-instance-of-anonymous-interface-in-kotlin) — miensol, Jun 14 '16 at 08:16

JB Nizet · Accepted Answer · 2017-10-20T16:01:20.157

47

As explained in the documentation:

animation.setAnimationListener(object : Animation.AnimationListener {
    // All the other override functions
})

edited Oct 20 '17 at 16:01

answered Jun 14 '16 at 07:44

JB Nizet

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Thanks! Great. When I search in https://kotlinlang.org/docs/reference/interfaces.html, it didn't even touch anything on that. The documentation is quite obscured. Search on the internet also can't find it. The only place to find this is to manually write a Java code, and convert it... Hopefully my stackoverflow question would help others searching for this quickly to find it. Thanks! – Elye Jun 14 '16 at 07:48
I suggested to improve the documentation in the kotlin slack. I agree that the information isn't that easy to find in the doc. – JB Nizet Jun 14 '16 at 07:57
That sounds great! – Elye Jun 14 '16 at 08:01
2

The documentation has been improved: https://kotlinlang.org/docs/reference/nested-classes.html#anonymous-inner-classes and https://kotlinlang.org/docs/reference/classes.html#creating-instances-of-classes – JB Nizet Jun 14 '16 at 10:28
Cool for improving the documentation. Look at the number of 'tick' at your answer within two days. Clearly this is a hot question and your answer helps to answer them. – Elye Jun 15 '16 at 11:53
I dont know why you rollback my edit but can you tell IntelliJ to fix this: https://ibb.co/mHZuqm – Ultimo_m Oct 20 '17 at 12:07
1

@Ultimo_m sorry for that. The syntax with parentheses is correct to create an anonymous object extending a class and calling its default constructor. When implementing an interface, the parentheses should indeed not be there. – JB Nizet Oct 20 '17 at 16:03

score 6 · Answer 2 · answered Nov 30 '16 at 15:30

Apparently the latest way (using Kotlin 1.0.5) of doing it is now without the parenthesis, given there's no empty constructor for the interface.

animation.setAnimationListener(object : Animation.AnimationListener {
    // All the other override functions
})

How to instantiate an anonymous class that implements an interface in Kotlin

2 Answers2