Interfaces

An interface is like a contract. It says "any class that implements me MUST have these methods." It's similar to an abstract class, but even more strict: an interface can't have regular fields or constructors (with some exceptions we'll skip for now)

interface Printable {
    void printInfo();
}

That's it. No body, no fields, just method signatures. Any class that says implements Printable MUST provide a printInfo() method

class Book implements Printable {
    String title;
    String author;

    Book(String title, String author) {
        this.title = title;
        this.author = author;
    }

    @Override
    public void printInfo() {
        System.out.println("Book: " + title + " by " + author);
    }
}

Notice the method in the class must be public. Interface methods are always public by default, so your implementation must be too

The big difference between interfaces and abstract classes: a class can implement multiple interfaces, but can only extend ONE class

interface Driver {
    void drive();
}

interface Shooter {
    void shoot();
}

class Associate implements Driver, Shooter {
    @Override
    public void drive() {
        System.out.println("Driving the getaway car");
    }

    @Override
    public void shoot() {
        System.out.println("Covering the crew");
    }
}

This is Java's answer to multiple inheritance: one parent class, but as many interfaces as you want

Think of it like the abilities in Vice City. Tommy Vercetti is a Criminal (his parent class). But he also implements Swimmer, Driver, Shooter: those are "contracts," abilities he has. Different characters implement different combinations: Lance implements Driver, Shooter but maybe not Swimmer. Cortez implements Commander, Negotiator

Here's a complete example

interface Printable {
    void printInfo();
}

class Book implements Printable {
    String title, author;

    Book(String title, String author) {
        this.title = title;
        this.author = author;
    }

    @Override
    public void printInfo() {
        System.out.println("Book: " + title + " by " + author);
    }
}

class Movie implements Printable {
    String title, director;

    Movie(String title, String director) {
        this.title = title;
        this.director = director;
    }

    @Override
    public void printInfo() {
        System.out.println("Movie: " + title + " directed by " + director);
    }
}

public class Main {
    public static void main(String[] args) {
        Book b = new Book("The Mobster's Handbook", "Sonny Forelli");
        Movie m = new Movie("Scarface", "Brian De Palma");
        b.printInfo();
        m.printInfo();
    }
}

Output

Book: The Mobster's Handbook by Sonny Forelli
Movie: Scarface directed by Brian De Palma

And just like with polymorphism, you can use the interface type for variables

public class Main {
    public static void main(String[] args) {
        Printable[] items = { new Book("The Mobster's Handbook", "Sonny Forelli"), new Movie("Scarface", "Brian De Palma") };
        for (Printable p : items) {
            p.printInfo();
        }
    }
}

When to use interface vs abstract class?

• Interface: when you want to define a capability that unrelated classes can share. A book and a movie aren't related, but both can be Printable
• Abstract class: when you have a family of related classes that share common code. A Driver and a Shooter are both Criminal and share fields like name

You can even combine them: abstract class Criminal implements Printable

Mission: Tommy's Catalog

Tommy keeps a mixed collection in his Vice City mansion: books and movies. He wants a catalog that lists every item, no matter the type. Build it using a shared Printable interface so every item can describe itself.

1. Create an interface Printable with a method printInfo()
2. Create a Book class with title and author fields that implements Printable; printInfo() prints "Book: TITLE by AUTHOR"
3. Create a Movie class with title and director fields that implements Printable; printInfo() prints "Movie: TITLE directed by DIRECTOR"
4. In main, store the book's title and author in title1 and author1, and the movie's title and director in title2 and director2 (start with "Vice City Chronicles"/"Tommy Vercetti" and "Top Gun"/"Tony Scott"). Create a Book from title1 and author1 and a Movie from title2 and director2, then call printInfo() on both

Output

Book: Vice City Chronicles by Tommy Vercetti
Movie: Top Gun directed by Tony Scott