🔷 Interface Changes

Before Java 8

• An interface contains only abstract methods

• A method without a body is called an abstract method

• A class implements an interface using the implements keyword

• A class must implement all abstract methods, otherwise it will cause a compile-time error

Example (Before Java 8)

java

interface Vehicle {
    void startVehicle();
}

class Car implements Vehicle {
    public void startVehicle() {
        System.out.println("Car started...");
    }
}

class Bus implements Vehicle {
    public void startVehicle() {
        System.out.println("Bus started...");
    }
}

class Bike implements Vehicle {
    public void startVehicle() {
        System.out.println("Bike started...");
    }
}

❗ Problem

If we add a new method inside the Vehicle interface:

java

void stopVehicle();

All implementing classes (Car, Bus, Bike) must implement this method.
Otherwise, they will fail at compile time.

Solution in Java 8 – Default & Static Methods

Java 8 allows concrete methods inside interfaces using:

• default keyword

• static keyword

This feature was introduced for backward compatibility.

🔹 Default Method

• Has a method body

• Can be overridden in implementing classes

• Provides a default implementation

🔹 Static Method

• Belongs to the interface

• Cannot be overridden

• Must be called using the interface name

Example – Interface with Default & Static Methods

java

interface Vehicle {

    void start();  // abstract method

    default void m1() {
        System.out.println("Default Method m1()");
    }

    default void m2() {
        System.out.println("Default Method m2()");
    }

    static void clean() {
        System.out.println("Cleaning completed...");
    }
}

//  Implementation Class

public class Car implements Vehicle {

    public void start() {
        System.out.println("Car started...");
    }

    public static void main(String[] args) {
        Car c = new Car();

        c.start();        // calling abstract method
        c.m1();           // calling default method
        Vehicle.clean();  // calling static method
    }
}

📝 Important Points

✔ Interface can have multiple default and static methods
✔ Default methods can be overridden
✔ Static methods cannot be overridden
✔ Static methods must be called using interface name
✔ Introduced for backward compatibility