Kotlin lambdas are fully compatible with Java functional interfaces.
But sometimes you need to give Kotlin a little nudge, using a SAM constructor.
SAM constructors (Single Abstract Method) allow you to convert a lambda expression to an instance of a functional interface. The syntax is pretty straightforward.
FunctionalInterfaceName { lambda_function }
The message in the title appears when you use a SAM constructor, when you don't have to. Kotlin is smart enough to create the appropriate anonymous class without us being specific in most cases.
I notice this happening sometimes when I have IntelliJ convert my Java class automatically to Kotlin.
A simple example:
// Java
public Builder addMapping(FieldMetadata field, Supplier<?> valueSupplier) {...}
public Builder addMapping(FieldMetadata field, Supplier<?> valueSupplier) {...}
// redundant Kotlin
val fieldMapping = FieldMapping.builder()
.addMapping(OrderItemField.ITEM_NR), Supplier { orderRepository.getOrderItem })
.build()
val fieldMapping = FieldMapping.builder()
.addMapping(OrderItemField.ITEM_NR), Supplier { orderRepository.getOrderItem })
.build()
// correct Kotlin
val fieldMapping = FieldMapping.builder()
.addMapping(OrderItemField.ITEM_NR) { orderRepository.getOrderItem }
.build()
val fieldMapping = FieldMapping.builder()
.addMapping(OrderItemField.ITEM_NR) { orderRepository.getOrderItem }
.build()
References
- Medium - Idiomatic Kotlin: Lambdas and SAM constructors
- https://medium.com/tompee/idiomatic-kotlin-lambdas-and-sam-constructors-fe2075965bfb
