Interface in a Package

Interface in a Package

1. What Is an Interface? (beginner)

Interface = pure promise list (methods WITHOUT bodies! public abstract by default). Class implements (fulfills ALL!). Like job contract: sign = do all duties! Enables multiple-inheritance!

2. Code — Drawable Contract (exam classic!)

package shapes; // interface lives in package too!
public interface Drawable {
    void draw(); // public abstract (auto!) — no body!
    default void info() { System.out.println("shape"); } // Java 8+ body allowed!
}
class Circle implements Drawable { // fulfills!
    public void draw() { System.out.println("O"); } // MUST public! (can't weaken!)
}
Drawable d = new Circle(); // interface ref (polymorphism!)
d.draw(); // O (object wins!)

3. Rules (exam traps!)

• Methods: public abstract auto (Java 7). default/static bodies allowed (Java 8+).
• Fields: public static final constants ONLY (int X = 10; = constant!).
• Class implements ALL (else abstract!). implements A, B MANY (multiple ok!).
• interface extends interface MANY (interface C extends A, B!).

4. Interface vs Abstract (exam table!)

	Interface	Abstract class
Bodies	none (default ok)	mix!
Fields	constants only	any!
Inherits	many!	one!
new?	No	No

5. Exam Points

• Interface + implement program — 7 marks, very frequent.
• Rules + vs abstract — 5 marks, frequent.