Interface

1. What is an Interface?

An interface is a completely abstract type in Java that defines a set of methods a class must implement, without providing (in most cases) any implementation itself. It represents a "contract" that implementing classes must follow.

2. Why is it used?

Interfaces let unrelated classes share a common set of expected behaviours, and they allow Java to support something close to multiple inheritance, since a class can implement multiple interfaces at once.

3. Real-Life Example

Think of a job advertisement listing required skills, like "must know driving and cooking." Anyone who takes the job (implements the interface) must actually have those skills — the advertisement itself doesn't do the driving or cooking.

4. Syntax

java
interface InterfaceName {
    void methodName(); // implicitly public and abstract
}

class ClassName implements InterfaceName {
    public void methodName() {
        // implementation
    }
}

5. Example Program

java
interface Drivable {
    void drive();
}

class Car implements Drivable {
    public void drive() {
        System.out.println("Car is being driven");
    }
}

public class InterfaceDemo {
    public static void main(String[] args) {
        Drivable d = new Car();
        d.drive();
    }
}

Output:

Car is being driven

6. Key Points to Remember

• A class uses implements to use an interface, and must provide implementations for all its methods.
• A class can implement multiple interfaces at once, unlike extending multiple classes.
• Since Java 8, interfaces can also have default and static methods with actual implementations.