Functional Interfaces

A functional interface has one abstract method; @FunctionalInterface asks the compiler to enforce that shape. Object methods and defaults do not count against it.

Standard vocabulary

Predicate<String> present = text -> text != null;
Predicate<String> useful = present.and(text -> !text.isBlank());
Function<String, Integer> length = String::length;
Supplier<Instant> clock = Instant::now;
Consumer<String> output = System.out::println;

Use primitive specializations such as IntFunction to avoid boxing in hot paths. UnaryOperator and BinaryOperator preserve a type. Prefer standard interfaces for interoperability; create a domain-specific one when its name, checked-exception contract, or extra semantics materially clarify the API.