Java Interfaces

An interface in Java is a blueprint of a class that contains abstract methods (methods without a body). Interfaces are used to achieve abstraction and multiple inheritance in Java. It defines a set of methods that a class must implement but does not provide the implementation.

Key Points About Interfaces:

An interface can contain:

Abstract methods (methods without a body).
Default methods (methods with a body, introduced in Java 8).
Static methods (introduced in Java 8).
Private methods (introduced in Java 9)

A class can implement multiple interfaces (unlike extending classes, which is limited to one).
Interfaces cannot have instance variables; they can have constants (public, static, and final by default).
All methods in an interface are implicitly public and abstract (except default, static, and private methods).

Syntax:

interface InterfaceName {
    // Abstract method
    void method1();

    // Default method (introduced in Java 8)
    default void method2() {
        System.out.println("Default Method");
    }

    // Static method (introduced in Java 8)
    static void method3() {
        System.out.println("Static Method");
    }
}

Example: Basic Interface

interface Animal {

void sound(); // Abstract method

default void sleep() { // Default method

System.out.println("Sleeping...");

}

class Dog implements Animal {

public void sound() {

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

}

public class Main {

public static void main(String[] args) {

Dog dog = new Dog();

dog.sound(); // Calls the overridden method

dog.sleep(); // Calls the default method

}

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Example: Multiple Interfaces

interface Printable {

void print();

}

interface Showable {

void show();

}

class Demo implements Printable, Showable {

public void print() {

System.out.println("Printing...");

}

public void show() {

System.out.println("Showing...");

}

public class Main {

public static void main(String[] args) {

Demo demo = new Demo();

demo.print();

demo.show();

}

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Example: Default Method

interface Vehicle {

default void start() {

System.out.println("Vehicle is starting");

}

class Car implements Vehicle {

public void start() {

System.out.println("Car is starting");

}

public class Main {

public static void main(String[] args) {

Vehicle car = new Car();

car.start(); // Calls the overridden method in Car

}

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Example: Static Method

interface MathUtils {

static int add(int a, int b) {

return a + b;

}

public class Main {

public static void main(String[] args) {

System.out.println(MathUtils.add(5, 10)); // Directly calls the static method

}

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Real-World Example: Interface for Multiple Objects

interface Shape {

double area();

double perimeter();

}

class Circle implements Shape {

double radius;

Circle(double radius) {

this.radius = radius;

}

public double area() {

return Math.PI * radius * radius;

}

public double perimeter() {

return 2 * Math.PI * radius;

}

class Rectangle implements Shape {

double length, width;

Rectangle(double length, double width) {

this.length = length;

this.width = width;

}

public double area() {

return length * width;

}

public double perimeter() {

return 2 * (length + width);

}

public class Main {

public static void main(String[] args) {

Shape circle = new Circle(5);

Shape rectangle = new Rectangle(10, 20);

System.out.println("Circle Area: " + circle.area());

System.out.println("Circle Perimeter: " + circle.perimeter());

System.out.println("Rectangle Area: " + rectangle.area());

System.out.println("Rectangle Perimeter: " + rectangle.perimeter());

}

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