1. AbstractClassesInterfacesPolymorphism1 Abstract Classes, Interfaces, Polymorphism Barb Ericson Georgia Tech ericson@cc.gatech.edu April 2010

2. Advanced-OO2 Learning Goals Understand at a conceptual and practical level –Inheritance –Abstract Classes –Interfaces –Polymorphism

3. Advanced-OO3 Object-Oriented Principles Objects with data (fields) and operations (methods) –Usually classes too Inheritance –Hierarchy of types –Generalization / Specialization Polymorphism –Executing the right method based on the type of the object at run-time

4. Advanced-OO4 Abstract Classes Abstract classes are classes that can’t be instantiated. Abstract classes can only be subclassed. Create an abstract class by using the keyword abstract in the class declaration. –public abstract class Food HamburgerCoke Food price calories

5. Advanced-OO5 Why use an Abstract Class? Represents an abstract idea (like a Shape or a List) Holds methods common to several related classes Holds attributes common to several related classes Enforce naming convention by abstract methods that must be overridden by children Allows for general algorithms based on abstract methods with customization by children

6. Advanced-OO6 Interfaces Interfaces are a description of behavior. –They are a special kind of abstract class that only has public abstract methods and constants. public interface ShapeInterface { public void setShape(int shape); } –You don’t have to declare the methods as abstract or public They automatically are

7. Advanced-OO7 Classes Implement Interfaces Classes that implement interfaces must provide the implementations for the methods specified in the interface. –Or be declared abstract as well public class ShapeCanvas implements ShapeInterface { public void setShape(int shape) { code to handle set shape } }

8. Advanced-OO8 Why use an Interface? Separates what from who –I don’t care who you are I just need a way to talk to you –Choose from several implementers A class can implement many interfaces but inherit from only one class –like multiple inheritance but easier to use –thinner than inheritance

9. Advanced-OO9 Interfaces Versus Inheritance When a class inherits from a parent class it inherits all the object attributes and methods. –With inheritance it inherits the structure and behavior of the parent class. –With an interface it inherits only the method names and parameter lists. A class can inherit from only one parent class –public class Person extends Object A class can implement more than one interface. –public class ShapeCanvas implements Interface1,Interface2,…

10. Advanced-OO10 Comparable Interface How would you compare any two objects? –And decide if one is less than, equal too, or greater than the other It would depend on the Class of the objects being compared –For String objects compare the letters in the string Implement the Comparable interface –public int compareTo(Object object)

11. Advanced-OO11 Comparable Exercise How would you compare two Person objects? –Implement the Comparable interface public class Person implements Comparable –Add a compareTo method public int compareTo(Person comparePerson) Compare the last names first –If they are equal compare the first names The String class implements Comparable so you can use the results of comparing the last name and first name

12. Advanced-OO12 Collections - java.util Used to hold objects –Uses wrapper classes to hold primitive values int numItems = 3; Integer numItemsInt = new Integer(numItems); Three basic types –List - ordered list of objects Can have duplicate objects –Set - group of objects without an order No duplicate objects allowed –Map - map of keys to objects

13. Advanced-OO13 List and Set Interfaces and Classes > Collection > List > Set > SortedSet ArrayList Vector LinkedList TreeSet HashSet

14. Advanced-OO14 Collection Methods Add an object to a collection boolean add(Object object); // optional Remove an object from a collection boolean remove(Object object); //optional See if the collection has the object in it boolean contains(Object object); Add all objects in another collection boolean addAll(Collection collection); // optional Get the intersection of two collections boolean retainAll(Collection collection); // optional Empty a collection Void clear();

15. Advanced-OO15 Use Interface Name as Type Declare the type of the collection variable to be one of the main interface types –List –Set SortedSet –Map SortedMap This allows you to change the implementation without changing much code Map addressMap = new HashMap(); Map addressMap = new Hashtable();

16. Advanced-OO16 Polymorphism Literally: many forms In Object-Oriented development it means that what happens when a message is sent to an object depends on the type (class) of the object at runtime

17. Advanced-OO17 How Does Polymorphism Work? If a class is declared to be final –then the compiler can figure out the location of a method that matches the message If a class can be subclassed –then a variable declared to be of the parent type can point to an object of the parent class or any subclass at run-time –the compiler can’t determine the method to invoke –the method to invoke is figured out at run-time

18. Advanced-OO18 Shape Panel Exercise Execute the main method of ShapePanel Click the Rectangle button and then click and drag to position the rectangle Click the Oval button and click and drag to position the oval

19. Advanced-OO19 Class Diagram for ShapePanel Shape draw() Oval draw() Rectangle draw() ShapeCanvas paint() ShapePanel ButtonPanel 1 1 * ShapeInterface 1 1 1

20. Advanced-OO20 Polymorphism - Many Forms Polymorphism is overloading that is resolved at execution time, and is also called dynamic or run-time binding. Say you have an array of Shapes that actually holds objects that are subclasses of shape. –When you ask a shape to draw itself what gets drawn depends on the run-time type. Shape draw() Oval draw() Rectangle draw()

21. Advanced-OO21 Add Abstract Class Subclass Exercise –Create a new class Line which is a subclass of Shape. Use Oval.java or Rectangle.java as starting points –Add “Line” to the array of shapeNames in ButtonPanel –Compile and run ShapePanel to try it out.

22. Advanced-OO22 Advantages to Polymorphism Used to create general algorithms that work on objects of different types –Collections that hold Objects List, Set, Stack, Queue, Map Makes it easy to add new types –Just create the new class and implement the required operations –Don’t change existing code

23. Advanced-OO23 Summary Class fields are on an object of the class Class –Not on objects of the class Class methods can only work on class fields –Not object fields Objects inherit fields and methods from a parent class –But need to use public methods to access private inherited fields Polymorphism allows you to write general methods based on a common parent or interface