Lambdas & functional interfaces: interview questions & scenarios

Model answer

A functional interface is an interface with exactly one abstract method. That single abstract method is the shape a lambda or method reference implements. Default methods and static methods do not count against the one-method rule, and public methods inherited from Object, like toString, do not create extra abstract methods for this purpose.

@FunctionalInterface is not required for lambdas to work. It is a compiler check and a signal to readers. If someone later adds a second abstract method, the annotated interface stops compiling instead of quietly becoming unusable as a lambda target.

@FunctionalInterface
interface PriceRule {
    long apply(long cents);

    default PriceRule andThen(PriceRule next) {
        return cents -> next.apply(apply(cents));
    }
}

Model answer

A lambda gets its type from the target context: a variable declaration, method parameter, cast or return type. s -> s.length() can fit Function<String, Integer> because length() returns an int that can be boxed to Integer. It can also fit ToIntFunction<String> without boxing. It cannot fit Predicate<String> because a predicate must return boolean.

Function<String, Integer> boxedLength = s -> s.length();
ToIntFunction<String> primitiveLength = s -> s.length();
// Predicate<String> bad = s -> s.length(); // int is not boolean

The lambda text is not enough by itself. The target type pushes parameter and return expectations into the body.