Anonymous implementation of an interface method in Kotlin

user_12125755 0
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In java, you can use implementations of interfaces like this:

import org.springframework.data.mongodb.core.query.Query;

@FunctionalInterface
public interface Filter {
    Query apply(Query query);
}

public class FlagFilterFactory {
  public Filter create(SearchQuery searchQuery) {
    return q -> {
      q.addCriteria(...)
      return q;
    };
  }

But I did not find how it can be written using Kotlin.

@FunctionalInterface
public interface Filter {
    fun apply(query: Query): Query
}

How can one write an anonymous implementation of this interface? Or do I definitely need to create a class in Kotlin that will implement it?

functional-interface kotlin lambda java
1 Answers

You can write a fun interface in Kotlin (no pun intended)

fun interface Filter {
    fun apply(query: Query): Query
}

Usage:

fun create(searchQuery: SearchQuery) =
    Filter { q ->
        q.addCriteria(...)
    }

Alternatively, don't declare a new type at all, and just use the function type (Query) -> Query. (You can make a type alias for this)

typealias Filter = (Query) -> Query

fun create(searchQuery: SearchQuery): Filter = { q ->
    q.addCriteria(...)
}
user_51335850
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