Add FunctionalInterface Annotation to most of the interfaces primarily used by Lambdas #19234
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…y used for lambdas A lot of the Kafka Stream documentation makes liberal use of Java lambdas, which is fine. The FunctionalInterface annotation isn't strictly required for Java to do this, as Java will automatically coerce, however this breaks the lambda syntax in Clojure 1.12, as it needs the FunctionalInterface in order to coerce a Clojure function into a Java lambda. As far as I can tell, there's no reason *not* to have the FunctionalInterface annotation in all the places that regularly use the lambda syntax, and this annotation would make it more clear.
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A lot of the Kafka Stream documentation makes liberal use of Java lambdas, which is fine. The FunctionalInterface annotation isn't strictly required for Java to do this, as Java will automatically coerce, however this breaks the lambda syntax in Clojure 1.12, as it needs the FunctionalInterface in order to coerce a Clojure function into a Java lambda.
As far as I can tell, there's no reason not to have the FunctionalInterface annotation in all the places that regularly use the lambda syntax, and this annotation would make it more clear, as well as make it easier to user in other JVM languages.