What are Functional Interfaces in Java (Predicate, Function, Consumer, Supplier)?

Explanation & Concept

A Functional Interface has exactly one abstract method and is eligible to be target for lambda expressions. Annotated with `@FunctionalInterface`.

• `Predicate<T>`: `T -> boolean` (filtering) • `Function<T, R>`: `T -> R` (transformation) • `Consumer<T>`: `T -> void` (side-effects) • `Supplier<T>`: `() -> T` (lazy instantiation)

Code Example

java

import java.util.function.*;

public class FunctionalDemo {
    public static void main(String[] args) {
        Predicate<Integer> isPositive = n -> n > 0;
        Function<String, Integer> stringLength = String::length;
        Consumer<String> logger = System.out::println;
        Supplier<Double> randomGen = Math::random;

        if (isPositive.test(10)) {
            logger.accept("Length: " + stringLength.apply("Antigravity"));
        }
    }
}

How It Works & Key Takeaways

Functional interfaces form the foundational contracts for the Java Stream API and asynchronous `CompletableFuture` chains.

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Functional InterfacesAdvanced

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java

What are Functional Interfaces in Java (Predicate, Function, Consumer, Supplier)?

Explanation & Concept

A Functional Interface has exactly one abstract method and is eligible to be target for lambda expressions. Annotated with `@FunctionalInterface`.

• `Predicate<T>`: `T -> boolean` (filtering) • `Function<T, R>`: `T -> R` (transformation) • `Consumer<T>`: `T -> void` (side-effects) • `Supplier<T>`: `() -> T` (lazy instantiation)

Code Example

java

import java.util.function.*;

public class FunctionalDemo {
    public static void main(String[] args) {
        Predicate<Integer> isPositive = n -> n > 0;
        Function<String, Integer> stringLength = String::length;
        Consumer<String> logger = System.out::println;
        Supplier<Double> randomGen = Math::random;

        if (isPositive.test(10)) {
            logger.accept("Length: " + stringLength.apply("Antigravity"));
        }
    }
}

How It Works & Key Takeaways

Functional interfaces form the foundational contracts for the Java Stream API and asynchronous `CompletableFuture` chains.

Mock Test · 1 Questions · Optional

Ready for Optional & Null Safety?

Take a quick test to evaluate yourself, or move straight to the next module.

Advanced1 questions

Skip