What is a functional interface, and how do Java lambdas use it?

@FunctionalInterface is optional but useful because the compiler rejects accidental changes that add another abstract method. Lambdas make behavior easy to pass to collection, stream, retry, or validation APIs without creating a named class for every small strategy. A method reference can improve readability when it clearly names the same operation. If the lambda contains several branches and side effects, a named method or class may communicate the behavior better.