Interfaces and abstract classes in Java

Class given at ReDI School Berlin in April 2020.

Transcript

1. Interfaces & Abstract Classes Program: Digital Career Program Semester: Spring 2020 Course: Java Intermediate Date: April 6, 2020
2. Learning Objectives ✅ Define what an interface is ✅ Create and use interfaces in Java ✅ Explain what interfaces are useful for ✅ Define what an abstract class is ✅ Create and use abstract classes in Java ✅ Explain what abstract classes are useful for ✅ Explain the differences between abstract classes and interfaces
3. Abstract Classes
4. Abstract Classes What do we know about inheritance?
5. An abstract class in Java is a class that cannot be instantiated. An abstract class can only be inherited or subclassed. What’s an abstract class?
6. How to declare an abstract class? public abstract class GeometricShape { abstract double area(); void printArea() { System.out.println("The area of this shape is " + this.area()); } }
7. How to declare an abstract class? public abstract class GeometricShape { abstract double area(); void printArea() { System.out.println("The area of this shape is " + this.area()); } }
8. How to declare an abstract class? public abstract class GeometricShape { abstract double area(); void printArea() { System.out.println("The area of this shape is " + this.area()); } }
9. How to use an abstract class? public class Rectangle extends GeometricShape { private double width; private double height; @Override double area() { return width * height; } }
10. How to use an abstract class? public class Circle extends GeometricShape { private double radius; @Override double area() { return Math.PI * radius * radius; } }
11. How to use an abstract class? public class Main { public static void main(String[] args) { Rectangle rectangle = new Rectangle(300, 200); Circle circle = new Circle(42); System.out.print("Rectangle: "); rectangle.printArea(); System.out.print("Circle: "); circle.printArea(); } } > Task :Main.main() Rectangle: The area is 60000.0 Circle: The area is 5541.769440932396
12. Why abstract classes?
13. Let’s build Super Mario
14. Let’s build Super Mario
15. Let’s build Super Mario Luigi Mario Toad ★Can run ★Can jump ★Can run ★Can jump ★Can run ★Can jump
16. Let’s build Super Mario public class Character { private String name; public Character(String name) { this.name = name; } public void run() { System.out.println("I'm " + name + "! I'm running !!..."); } public void jump() { System.out.println("I'm " + name + "! I'm jumping !!..."); } }
17. Let’s build Super Mario public class Game { Character character; public Game(Character character) { this.character = character; } public void makeCharacterRun() { character.run(); } public void makeCharacterJump() { character.jump(); } }
18. Let’s build Super Mario > Task :Main.main() I'm Mario! I'm running ""... I'm Mario! I'm jumping ""... public class Main { public static void main(String[] args) { Character mario = new Character("Mario"); Game game = new Game(mario); game.makeCharacterRun(); game.makeCharacterJump(); } }
19. Let’s build Super Mario Luigi Mario Toad ★Can run ★Can jump very high ★Can run ★Can jump ★Can run very fast ★Can jump
20. Let’s build Super Mario public abstract class Character { abstract void jump(); abstract void run(); }
21. public class Mario extends Character { @Override void jump() { System.out.println("I'm a Mario! And I'm running normally !!..."); } @Override void run() { System.out.println("I'm a Mario! And I’m jumping normally !!..."); } } Let’s build Super Mario
22. public class Main { public static void main(String[] args) { Character mario = new Mario(); new Character("Mario") Game game = new Game(mario); game.makeCharacterRun(); game.makeCharacterJump(); } } Let’s build Super Mario > Task :Main.main() I'm a Mario! And I'm running normally ""... I'm a Mario! And I'm jumping normally ""...
23. public class Luigi extends Character { @Override void jump() { System.out.println("I'm a Luiiiigi! And I'm jumping very high !!..."); } @Override void run() { System.out.println("I'm Luiiiigi! And I'm running normally!!..."); } } Let’s build Super Mario
24. Let’s build Super Mario public class Main { public static void main(String[] args) { Character luigi = new Luigi(); new Character(“Luigi") Game game = new Game(luigi); game.makeCharacterRun(); game.makeCharacterJump(); } } > Task :Main.main() I'm Luiiiigi! And I'm running normally ""... I'm a Luiiiigi! And I'm jumping very high…
25. public class Toad extends Character { @Override void jump() { System.out.println("I'm Toad Toad! And I'm jumping normally!!..."); } @Override void run() { System.out.println("I'm Toad Toad! And I'm running very fast !!..."); } } Let’s build Super Mario
26. Let’s build Super Mario public class Main { public static void main(String[] args) { Character toad = new Toad(); new Character(“Toad") Game game = new Game(toad); game.makeCharacterRun(); game.makeCharacterJump(); } } > Task :Main.main() I'm Toad Toad! And I’m running very fast ""... I'm Toad Toad! And I'm jumping normally ""...
27. Why abstract classes?
28. Why abstract classes? Similarly to super classes (normal inheritance), abstract classes are used to share common capabilities among subclasses. The difference here is that the specifics of the capabilities are known to subclasses only.
29. Abstract Classes: Recap ✅ Declared using the abstract keyword ✅ Can have methods and attributes ✅ Can have abstract methods (with no implementation) ✅ Can not be instantiated ✅ Can only be inherited ✅ Used to define common capabilities among subclasses but leaves the specifics of those capabilities to the subclasses
30. Interfaces
31. What’s an interface? An interface is a mechanism by which Java objects can adopt certain traits or behavior.
32. How to declare an interface? public interface Drawable { void draw(); }
33. How to declare an interface? public interface Drawable { void draw(); void print() { !// Prints this drawable !!... } } ❌
34. How to use an interface? public class Rectangle extends Shape implements Drawable { private double width; private double height; @Override double area() { return width * height; } @Override public void draw() { !// Drawing this rectangle on screen! } }
35. How to use an interface? public class Paint { public void draw(Drawable drawable) { drawable.draw(); } }
36. How to use an interface? public class Main { public static void main(String[] args) { Paint paint = new Paint(); Rectangle rectangle = new Rectangle(); paint.draw(rectangle); } }
37. Why interfaces?
38. Mario and friends to the rescue again :)
39. Mario PowerUps
40. Fire Balls Throwing public class Mario extends Character { @Override void jump() { System.out.println("I'm a Mario! And I'm running normally !!..."); } @Override void run() { System.out.println("I'm a Mario! And I’m jumping normally !!..."); } }
41. Fire Balls Throwing public class Mario extends Character { @Override void jump() { System.out.println("I'm a Mario! And I'm running normally !!..."); } @Override void run() { System.out.println("I'm a Mario! And I'm jumping normally !!..."); } void throwFireBall() { System.out.println("I'm a Mario! And I'm throwing fire balls "); } }
42. Fire Balls Throwing →
43. Fire Balls Throwing public interface FireBallThrowable { void throwFireBall(); }
44. Fire Balls Throwing public class Mario extends Character implements FireBallThrowable { @Override void jump() { System.out.println("I'm a Mario! And I'm running normally !!..."); } @Override void run() { System.out.println("I'm a Mario! And I'm jumping normally !!..."); } @Override public void throwFireBall() { System.out.println("I'm a Mario! And I'm throwing fire balls "); } }
45. Fire Balls Throwing public class Luigi extends Character implements FireBallThrowable { @Override void jump() { System.out.println("I'm a Luiiiigi! And I'm running normally !!..."); } @Override void run() { System.out.println("I'm Luiiiigi! And I'm jumping very high !!..."); } @Override public void throwFireBall() { System.out.println("I'm Luiiiigi! And I'm throwing fire balls "); } }
46. Giant Mario
47. Giant Mario public interface GiantTransformable { void stomp(); }
48. Giant Mario public class Mario extends Character implements FireBallThrowable, GiantTransformable { @Override public void throwFireBall() { System.out.println("I'm a Mario! And I'm throwing fire balls "); } @Override public void stomp() { System.out.print("I'm a Mario! And I'm destroying anything in my path!"); } }
49. Giant Mario public class Mario extends Character implements FireBallThrowable, GiantTransformable { @Override public void throwFireBall() { System.out.println("I'm a Mario! And I'm throwing fire balls "); } @Override public void stomp() { System.out.print("I'm a Mario! And I'm destroying anything in my path!"); } }
50. Bonus Level ⏳
51. Bonus Level public class Game { public void startBonusLevel(GiantTransformable character) { !// Play the bonus level } }
52. Bonus Level public class Main { public static void main(String[] args) { Mario mario = new Mario(); Game game = new Game(); game.startBonusLevel(mario); } }
53. Why interfaces?
54. Why interfaces? Interfaces are used to abstract a specific behavior, by defining a contract that objects who wish to adopt the behavior, should conform to.
55. Interfaces: Recap ✅ Declared using the interface keyword ✅ Can not have methods with implementations ✅ Classes conform to an interface by using the implements keyword ✅ A class can implement multiple interfaces ✅ Different and unrelated classes can implement the same interface ✅ Interfaces are used to capture behaviors shared by different objects
56. Interfaces vs. Abstract Classes
57. ✅ Declared using the abstract keyword ✅ Can have methods with implementations ✅ Can have public, private, protected attributes ✅ Adopted by using the extends keywordd ✅ A class can extend only one abstract class ✅ All the classes inheriting an abstract class are in the same “family” ✅ Used to define common capabilities among subclasses but leaves the specifics of those capabilities to the subclasses ✅ Declared using the interface keyword ✅ Can not have methods with implementations ✅ Can not have public, private, protected attributes ✅ Adopted by using the implements keywordd ✅ A class can implement as many interfaces as necessary ✅ Different and unrelated classes can implement the same interface ✅ Used to capture and abstract behaviors shared by different objects Abstract classes Interfaces
58. Practice Time