Functional Interfaces

A Functional Interface is an interface with exactly one abstract method. It forms the foundation for lambda expressions in Java.

Declaration

@FunctionalInterface
interface MyFunctionalInterface {
    void execute();
}
  • The @FunctionalInterface annotation is optional but recommended.
  • Having more than one abstract method will result in a compilation error if the annotation is present.

Example with Lambda

public class Main {
    public static void main(String[] args) {
        MyFunctionalInterface f = () -> System.out.println("Executed!");
        f.execute();
    }
}

Built-in Functional Interfaces (in java.util.function)

  • Predicate<T> โ€“ returns boolean
  • Function<T, R> โ€“ returns a value
  • Consumer<T> โ€“ performs an action
  • Supplier<T> โ€“ provides a value
Predicate<String> isLong = s -> s.length() > 5;
System.out.println(isLong.test("Hello")); // false

Benefits

  • Enables cleaner, more modular code
  • Essential for lambda expressions
  • Supports functional programming

๐Ÿ”— Related Notes