Interfaces

An interface is a contract defining WHAT a class must do, not HOW.

interface

interface Drawable { void draw(); }
class Circle implements Drawable { public void draw() { } }

implements vs extends

implements is for interfaces, extends is for classes. A class can implement MULTIPLE interfaces.

interface Flyable { void fly(); }
interface Swimmable { void swim(); }
class Duck implements Flyable, Swimmable { }

default (Java 8)

interface Vehicle { void start(); default void honk() { } }

static (Java 8), private (Java 9)

interface Util { static int add(int a, int b) { return a+b; } }

✅ Итоги урока

• Interface = contract (what, not how).
• implements for interfaces, extends for classes.
• Multiple interface implementation is allowed.
• default methods (Java 8): methods with body.
• static (Java 8), private (Java 9) methods.
• Interface fields are always public static final.