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A functional interface in the Java language refers to an interface with a single abstract method. @FunctionalInterface is an annotation which requires a particular interface declaration to conform to this specification. The target type of a lambda expression or method reference must be a functional interface. Functional interfaces are part of the Java 8 feature set.

A functional interface in the java language refers to an interface with a single abstract method. @FunctionalInterface is an annotation which requires the interface to conform to this specification. The target type of a lambda expression or method reference must be a functional interface. Functional interfaces are part of the java-8 feature set.

The following is a simple functional interface:

@FunctionalInterface
interface StringFunction<T> {
    String applyToString(T obj);
}

The single abstract method declaration allows it to be the target of a lambda expression:

StringFunction<Integer> toHexStringFn =
    (Integer n) -> Integer.toHexString(n);

If the interface has more than one abstract method, it is not longer a functional interface and can no longer be the target of a lambda.

Functional interfaces should be annotated with the @FunctionalInterface annotation, which requires the interface to conform to the functional interface specification. An interface annotated with @FunctionalInterface will cause a compilation error if the interface has e.g. more than one abstract method. (This is similar to the behavior of the @Override annotation, which will cause a compilation error if a method override is incorrect.)