Functional Interfaces

A Functional Interface is an interface with exactly one abstract method.

@FunctionalInterface Annotation

Ensures the interface has only one abstract method.

@FunctionalInterface
interface Calculator {
    int calculate(int a, int b);
}

Predefined Functional Interfaces

Java 8 provides many built-in functional interfaces in java.util.function.

1. Predicate<T>

Takes an argument, returns boolean.

Predicate<String> isValid = s -> s.length() > 5;
System.out.println(isValid.test("Hello World")); // true

2. Function<T, R>

Takes argument T, returns result R.

Function<String, Integer> getLength = s -> s.length();
System.out.println(getLength.apply("Java")); // 4

3. Consumer<T>

Takes argument T, returns void (consumes it).

Consumer<String> printer = s -> System.out.println(s);
printer.accept("Hello");

4. Supplier<T>

Takes no argument, returns result T (supplies it).

Supplier<Double> random = () -> Math.random();
System.out.println(random.get());

5. UnaryOperator<T> & BinaryOperator<T>

Specialized versions of Function where input/output types are same.

UnaryOperator<Integer> square = x -> x * x;
BinaryOperator<Integer> add = (a, b) -> a + b;

Quiz