Functional interfaces

Definition

A functional interface has exactly one abstract method, so a lambda can stand in for it. Java provides common ones: Function (input to output), Predicate (yes or no), Supplier (gives a value) and Consumer (uses a value).

Example code

The four built-in shapes, and your own

import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

@FunctionalInterface
interface Discount {                            // your own functional interface
    int apply(int price);
}

class Demo {
    public static void main(String[] args) {
        Function<String, Integer> length = s -> s.length();
        Predicate<Integer> isEven = n -> n % 2 == 0;
        Supplier<String> greeting = () -> "Hello";
        Consumer<String> printer = s -> System.out.println("Got: " + s);
        Discount tenPercent = price -> price * 90 / 100;

        System.out.println(length.apply("lambda"));
        System.out.println(isEven.test(7));
        printer.accept(greeting.get());
        System.out.println(tenPercent.apply(200));
    }
}

// Output:
// 6
// false
// Got: Hello
// 180