Difference Between Class and Interface

A class is a complete blueprint: it says what data an object holds and exactly how each method works, and you can build objects from it with new. An interface is a contract: it lists what an implementing class must be able to do, but you can't build anything from the contract alone. A class can sign that contract with implements. Beginners sometimes see an interface as "a class without code", but they play different roles, and this lesson lines them up.

Side by Side

Both in One Example

public class Library {
    interface Borrowable {
        int MAX_DAYS = 14;
        boolean isAvailable();
        void borrow(String member);
    }
 
    static class Book implements Borrowable {
        private final String title;
        private String borrower;
 
        Book(String title) { this.title = title; }
 
        public boolean isAvailable() { return borrower == null; }
 
        public void borrow(String member) {
            borrower = member;
            System.out.println(member + " borrowed " + title
                + " for " + MAX_DAYS + " days");
        }
    }
 
    public static void main(String[] args) {
        Borrowable item = new Book("Java Basics");
        System.out.println("Available: " + item.isAvailable());
        item.borrow("Srihitha Anamaneni");
        System.out.println("Available: " + item.isAvailable());
    }
}

Available: true
Srihitha Anamaneni borrowed Java Basics for 14 days
Available: false

How it works

1. Borrowable (the interface) declares a constant and two methods without bodies. It holds no data about any book.
2. Book (the class) holds the state, title and borrower, a constructor, and the code for each method.
3. new Book(...) creates the object; the variable uses the interface type Borrowable, so it could later hold a Magazine or a Dvd too.

Try it yourself: add a Dvd class that implements Borrowable with a borrowing period of 7 days instead of MAX_DAYS.

Common Mistakes

Trying new Borrowable(). Interfaces can't be instantiated; create an implementing class.

Expecting interface fields to be per object. MAX_DAYS is one shared constant.

Using extends for a class and an interface. A class implements an interface; only interfaces extend interfaces.

Interview Corner

Practice

1. Easy: List three things a class can contain that an interface can't.
2. Harder: Create an interface Rentable with long dailyRate() and two classes, Car and Bike, that implement it. Print the cost of renting each for 3 days from a Rentable[] array.

Key Takeaways