...

KafkaFuture.Function and KafkaFuture.BiCosumer are currently public abstract classes with only one abstract method.

This means you cannot implement use them with a java 8 lambda, as that only works for functional interfaces.

Replacing those classes with interfaces makes them lambda compatible.

Public Interfaces

In the public interface, only org.apache.kafka.common.KafkaFuture has helper classes which can better be expressed as functional interfaces.

...

As the Biconsumer class was not used in the public interface, I decided to stick there with a backwards incompatible change.

See the pull request for more details.

Compatibility, Deprecation, and Migration Plan

...