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

Public Interfaces

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

Proposed Changes

The current proposal implemented in PR#4033 makes from those abstract classes interfaces.

This is a binary incompatible change.  Which might be acceptable as KafkaFuture is annotated as InterfaceStability.Evolving, where compatibility may be broken at minor release.

This change also helps to move KafkaFuture closer to the interface of a java 8 CompletableFuture.  As KafkaFuture's javadoc states:

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introduces a FunctionInterface which defines the new functional interface.  The old Function class is kept and marked as @Deprecated.

This class is there to guarantee binary compatibility, also for the thenApply function (the only public function which was using this Function object) the old signature is kept next to the new one.

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

Compatibility, Deprecation, and Migration Plan

When the change is implemented like proposed, everybody using this api will need to implement instead of extend their implementations of KafkaFuture.Function and KafkaFuture.Biconsumer will get a deprecation warning.

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Rejected Alternatives

• Keep the current interface as is, as a nicer syntax to address this api might not be worth breaking backwards compatibility
• Wait for a kafka release which will not support java 7 anymore.  A backwards incompatible change can then be done to replace KafkaFuture with CompletableFutureSpend more effort and update the definition of KafkaFuture in such a way that it gradually switches to CompletableFuture.  By following an approach which looks like this:
• Create new interface versions of KafkaFuture.Function and KafkaFuture.BiConsumer, create new methods which handle those functional interface and preferably line their names up with those of CompletableFuture.  So for example KafkaFuture.addWaiter(BiConsumer) becomes KafkaFuture.whenComplete(BiConsumerInterface).  Mark the old methods as @Deprecated.
• When kafka does not support java 7 anymore, let KafkaFuture extend CompletableFuture (or even CompletionStage)
Later on the deprecated methods can be removed, and KafkaFuture becomes a shim on Completablefuture or CompletionStage.And replace or extend KafkaFuture with the java 8 CompletionStage. This KIP can then become part of KIP-118: Drop Support for Java 7 in Kafka 0.11