Interfaces in Java

An interface is a contract. It says: “Any class that signs this contract promises to provide these methods.” The interface defines what must exist. Each class decides how it works.

interface Greetable {
    void greet();
}

class EnglishPerson implements Greetable {
    @Override
    public void greet() {
        System.out.println("Hello!");
    }
}

Using it:

Greetable g = new EnglishPerson();
g.greet(); // → "Hello!"