Is lesson me hum seekhenge:

• Functional Interface kya hota hai
• Lambda ke saath relation
• @FunctionalInterface annotation
• Built-in Functional Interfaces
• Real examples

Functional Interface ka matlab:

aise interface jisme sirf ek abstract method hota hai

interface MyInterface {
    void show();
}

✔ sirf ek abstract method → valid functional interface

Functional interface ko use karte hain:

lambda expression ke saath

Example:

MyInterface obj = () -> {
    System.out.println("Hello");
};

obj.show();

@FunctionalInterface
interface MyInterface {
    void show();
}

✔ compile-time check karta hai
✔ ensure karta hai sirf 1 abstract method ho

interface Test {
    void m1();
    void m2(); // error
}

interface Test {

    void show(); // abstract

    default void display(){
        System.out.println("Default method");
    }

    static void print(){
        System.out.println("Static method");
    }
}

✔ allowed hai
✔ count nahi hote abstract methods me

Java me already ready interfaces hote hain:

Predicate<Integer> isEven = n -> n % 2 == 0;

System.out.println(isEven.test(4));

Function<Integer, Integer> square = x -> x * x;

System.out.println(square.apply(5));

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

print.accept("Hello");

Supplier<Integer> random = () -> 100;

System.out.println(random.get());

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

Use:

Calculator c = (a, b) -> a + b;

System.out.println(c.add(10, 20));

Button click listener
event handling

✔ sirf ek abstract method hona chahiye
✔ lambda ke saath use hota hai
✔ default & static allowed

✔ code short hota hai
✔ readable hota hai
✔ functional programming support

1. Functional interface kya hota hai?
2. @FunctionalInterface annotation ka use kya hai?
3. kya multiple methods allowed hain?
4. lambda ka relation kya hai?

Is lesson me humne seekha:

✔ Functional Interface concept
✔ Lambda relation
✔ Built-in interfaces
✔ Custom examples

Functional Interface Java me lambda expressions ka backbone hota hai.