You declare an interface with the interface keyword instead of class. Every method you write inside is public and abstract by default, so you don't need to type those modifiers yourself.
interface Drawable {
void draw();
double area();
}
Notice there's no method body. The interface only says what must exist, not how it works. You can also declare constants inside an interface. Any field you define is automatically public static final, meaning it's a compile-time constant shared across all implementing classes.