Java Interface

An interface in Java is a blueprint of a class. It defines a set of abstract methods (behaviors) that any implementing class must follow.

Interfaces help achieve abstraction and multiple inheritance in Java.

Key Points About Interfaces

Feature	Description
Contains abstract methods	Yes (before Java 8: all methods were abstract)
Supports multiple inheritance	✔ Yes
Objects allowed?	❌ Cannot create objects
Variables	Always `public static final` (constant)
Methods	Can be `abstract`, `default`, `static`, and `private` (after Java 8)

🧩 Basic Interface Example

interface Animal {
void sound(); // abstract method
}class Dog implements Animal {
public void sound() {
System.out.println("Dog barks");
}
}public class Main {
public static void main(String[] args) {
Animal a = new Dog();
a.sound();
}
}

interface Animal {

void sound(); // abstract method

}class Dog implements Animal {

public void sound() {

System.out.println("Dog barks");

}

}public class Main {

public static void main(String[] args) {

Animal a = new Dog();

a.sound();

}

✔ Output:

Multiple Interfaces Example

A class can implement more than one interface:

interface Animal {
void sound();
}interface Pet {
void play();
}class Dog implements Animal, Pet {
public void sound() {
System.out.println("Dog barks");
}
public void play() {
System.out.println("Dog loves to play!");
}
}public class Main {
public static void main(String[] args) {
Dog d = new Dog();
d.sound();
d.play();
}
}

interface Animal {

void sound();

}interface Pet {

void play();

}class Dog implements Animal, Pet {

public void sound() {

System.out.println("Dog barks");

}

public void play() {

System.out.println("Dog loves to play!");

}

}public class Main {

public static void main(String[] args) {

Dog d = new Dog();

d.sound();

d.play();

}

✔ Output:

Interface with Default Method (Java 8+)

Default methods let interfaces have method bodies.

interface Vehicle {
void start();default void horn() {
System.out.println("Beep beep!");
}
}class Car implements Vehicle {
public void start() {
System.out.println("Car started");
}
}public class Main {
public static void main(String[] args) {
Car c = new Car();
c.start();
c.horn();
}
}

interface Vehicle {

void start();default void horn() {

System.out.println("Beep beep!");

}

}class Car implements Vehicle {

public void start() {

System.out.println("Car started");

}

}public class Main {

public static void main(String[] args) {

Car c = new Car();

c.start();

c.horn();

}

✔ Output:

Static Methods in Interface

✔ Output:

Real-Life Example

Interface works like a contract—if a company signs it, they must follow rules.

interface Payment {
void pay();
}class CreditCard implements Payment {
public void pay() {
System.out.println("Payment done using Credit Card");
}
}class UPI implements Payment {
public void pay() {
System.out.println("Payment done using UPI");
}
}public class Main {
public static void main(String[] args) {
Payment p1 = new CreditCard();
Payment p2 = new UPI();p1.pay();
p2.pay();
}
}

interface Payment {

void pay();

}class CreditCard implements Payment {

public void pay() {

System.out.println("Payment done using Credit Card");

}

}class UPI implements Payment {

public void pay() {

System.out.println("Payment done using UPI");

}

}public class Main {

public static void main(String[] args) {

Payment p1 = new CreditCard();

Payment p2 = new UPI();p1.pay();

p2.pay();

}

✔ Output:

🏁 When to Use Interface?

Use interface when:

✔ Multiple classes should follow the same rules
✔ You need multiple inheritance
✔ You want total abstraction

🎉 Summary

Feature	Yes / No
Supports multiple inheritance	✔
Object creation allowed	❌
Contains only abstract methods (before Java 8)	✔
Can contain default and static methods (after Java 8)	✔

Java Interface

Java Interface

Key Points About Interfaces

🧩 Basic Interface Example

✔ Output:

Multiple Interfaces Example

✔ Output:

Interface with Default Method (Java 8+)

✔ Output:

Static Methods in Interface

✔ Output:

Real-Life Example

✔ Output:

🏁 When to Use Interface?

🎉 Summary

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