Functional Interface

title: Functional Interface in Java 8 description: Functional interfaces aur @FunctionalInterface annotation

Exactly one abstract method wala interface. Lambda expression target type hota hai.

@FunctionalInterface Annotation

java exampleWoHoTech

@FunctionalInterface  // Compiler enforce karta hai — 1 se zyada abstract method nahi
interface Greeting {
    String greet(String name);

    // Default methods allowed
    default void sayBye(String name) {
        System.out.println("Bye, " + name);
    }

    // Static methods allowed
    static Greeting formal() {
        return name -> "Good day, " + name;
    }
}

Greeting hindi = name -> "Namaste, " + name + "!";
Greeting english = name -> "Hello, " + name + "!";

System.out.println(hindi.greet("Ram"));    // Namaste, Ram!
System.out.println(english.greet("John")); // Hello, John!

Built-in Functional Interfaces

Predicate<T>

java exampleWoHoTech

Predicate<String> isEmpty = String::isEmpty;
Predicate<String> isLong = s -> s.length() > 5;

// Compose predicates
Predicate<String> isNonEmptyAndLong = isEmpty.negate().and(isLong);

List<String> words = Arrays.asList("hi", "hello", "java", "programming");
words.stream().filter(isNonEmptyAndLong).forEach(System.out::println);
// hello, programming

Function<T, R> — Chaining

java exampleWoHoTech

Function<String, String> trim = String::trim;
Function<String, String> upper = String::toUpperCase;
Function<String, Integer> length = String::length;

// andThen: trim → upper → length
Function<String, Integer> combined = trim.andThen(upper).andThen(length);
System.out.println(combined.apply("  hello  ")); // 5

// compose: reverse order
Function<Integer, Integer> doubleIt = x -> x * 2;
Function<Integer, Integer> addThree = x -> x + 3;
doubleIt.compose(addThree).apply(5); // addThree(5)=8, doubleIt(8)=16

Consumer<T> — Chaining

java exampleWoHoTech

Consumer<String> print = System.out::println;
Consumer<String> upper = s -> System.out.println(s.toUpperCase());

Consumer<String> both = print.andThen(upper);
both.accept("hello");
// hello
// HELLO

title: Functional Interface in Java 8 description: Functional interfaces aur @FunctionalInterface annotation

Exactly one abstract method wala interface. Lambda expression target type hota hai.

@FunctionalInterface Annotation

java exampleWoHoTech

@FunctionalInterface  // Compiler enforce karta hai — 1 se zyada abstract method nahi
interface Greeting {
    String greet(String name);

    // Default methods allowed
    default void sayBye(String name) {
        System.out.println("Bye, " + name);
    }

    // Static methods allowed
    static Greeting formal() {
        return name -> "Good day, " + name;
    }
}

Greeting hindi = name -> "Namaste, " + name + "!";
Greeting english = name -> "Hello, " + name + "!";

System.out.println(hindi.greet("Ram"));    // Namaste, Ram!
System.out.println(english.greet("John")); // Hello, John!

Built-in Functional Interfaces

Predicate<T>

java exampleWoHoTech

Predicate<String> isEmpty = String::isEmpty;
Predicate<String> isLong = s -> s.length() > 5;

// Compose predicates
Predicate<String> isNonEmptyAndLong = isEmpty.negate().and(isLong);

List<String> words = Arrays.asList("hi", "hello", "java", "programming");
words.stream().filter(isNonEmptyAndLong).forEach(System.out::println);
// hello, programming

Function<T, R> — Chaining

java exampleWoHoTech

Function<String, String> trim = String::trim;
Function<String, String> upper = String::toUpperCase;
Function<String, Integer> length = String::length;

// andThen: trim → upper → length
Function<String, Integer> combined = trim.andThen(upper).andThen(length);
System.out.println(combined.apply("  hello  ")); // 5

// compose: reverse order
Function<Integer, Integer> doubleIt = x -> x * 2;
Function<Integer, Integer> addThree = x -> x + 3;
doubleIt.compose(addThree).apply(5); // addThree(5)=8, doubleIt(8)=16

Consumer<T> — Chaining

java exampleWoHoTech

Consumer<String> print = System.out::println;
Consumer<String> upper = s -> System.out.println(s.toUpperCase());

Consumer<String> both = print.andThen(upper);
both.accept("hello");
// hello
// HELLO

Functional Interface

@FunctionalInterface Annotation

Built-in Functional Interfaces

Predicate<T>

Function<T, R> — Chaining

Consumer<T> — Chaining

Continue learning this concept

Functional Interface

@FunctionalInterface Annotation

Built-in Functional Interfaces

Predicate<T>

Function<T, R> — Chaining

Consumer<T> — Chaining

Continue learning this concept