Interview question

What is a functional interface in Java? Java में functional interface क्या है?

Answer

A functional interface is an interface with EXACTLY ONE abstract method, making it eligible to be implemented using a lambda expression. The @FunctionalInterface annotation enforces this at compile time.

@FunctionalInterface
interface Calculator {
    int calculate(int a, int b);  // exactly one abstract method
    
    // default and static methods are allowed, don't count against the 'one method' rule
    default void printInfo() {
        System.out.println('Calculator interface');
    }
}

Calculator add = (a, b) -> a + b;
Calculator multiply = (a, b) -> a * b;

System.out.println(add.calculate(3, 4));       // 7
System.out.println(multiply.calculate(3, 4));  // 12

// @FunctionalInterface catches mistakes at compile time
// @FunctionalInterface
// interface Broken {
//     void method1();
//     void method2();  // COMPILE ERROR - more than one abstract method
// }

// Java's built-in functional interfaces (java.util.function package)
Function<Integer, Integer> square = x -> x * x;        // takes T, returns R
Predicate<Integer> isEven = x -> x % 2 == 0;            // takes T, returns boolean
Consumer<String> printer = s -> System.out.println(s); // takes T, returns nothing
Supplier<String> greeting = () -> 'Hello';               // takes nothing, returns T
BiFunction<Integer, Integer, Integer> add2 = (a, b) -> a + b;  // two inputs, one output

System.out.println(square.apply(5));      // 25
System.out.println(isEven.test(4));       // true
printer.accept('Hello World');            // Hello World
System.out.println(greeting.get());       // Hello

Functional interface exactly एक abstract method वाला interface है, जो lambda expression से implement होने योग्य बनाता है। @FunctionalInterface annotation इसे compile time पर enforce करता है।

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

    default void printInfo() {
        System.out.println('Calculator interface');
    }
}

Calculator add = (a, b) -> a + b;
Calculator multiply = (a, b) -> a * b;

System.out.println(add.calculate(3, 4));       // 7
System.out.println(multiply.calculate(3, 4));  // 12

// Java के built-in functional interfaces
Function<Integer, Integer> square = x -> x * x;
Predicate<Integer> isEven = x -> x % 2 == 0;
Consumer<String> printer = s -> System.out.println(s);
Supplier<String> greeting = () -> 'Hello';
BiFunction<Integer, Integer, Integer> add2 = (a, b) -> a + b;

System.out.println(square.apply(5));      // 25
System.out.println(isEven.test(4));       // true
printer.accept('Hello World');
System.out.println(greeting.get());       // Hello

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