Java — Interfaces

Basic interface

interface Drawable {
    void draw();
}

class Circle implements Drawable {
    @Override
    public void draw() {
        System.out.println("Drawing circle");
    }
}

public class Main {
    public static void main(String[] args) {
        Drawable d = new Circle();
        d.draw();
    }
}

Multiple interfaces

interface Printable {
    void print();
}

interface Loggable {
    void log();
}

class Document implements Printable, Loggable {
    @Override
    public void print() {
        System.out.println("Printing document");
    }

    @Override
    public void log() {
        System.out.println("Logging document");
    }
}

Interface with default methods

interface Logger {
    void log(String message);

    default void error(String message) {
        log("[ERROR] " + message);
    }

    static void info(String message) {
        System.out.println("[INFO] " + message);
    }
}

class ConsoleLogger implements Logger {
    @Override
    public void log(String message) {
        System.out.println("[LOG] " + message);
    }
}

Functional interface

@FunctionalInterface
interface MathOperation {
    int apply(int a, int b);
}

public class Main {
    public static void main(String[] args) {
        MathOperation add = (a, b) -> a + b;
        MathOperation mul = (a, b) -> a * b;

        System.out.println(add.apply(3, 4)); // 7
        System.out.println(mul.apply(3, 4)); // 12
    }
}

Abstract class vs interface

Feature	Abstract Class	Interface
Multiple inheritance	No	Yes
Constructors	Yes	No
State (fields)	Yes	Only constants
Methods	Abstract + concrete	Abstract + default

Mini Practice

Write Java code that:

1. Defines an interface with abstract and default methods
2. Implements multiple interfaces in one class
3. Creates a functional interface with a lambda

Up Next

In the next lesson, you'll learn about Enums — enumerations in Java.