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Abstract Classes, Interfaces, Polymorphism

Abstract Classes, Interfaces, Polymorphism. Barb Ericson Georgia Tech ericson@cc.gatech.edu April 2010. Learning Goals. Understand at a conceptual and practical level Inheritance Abstract Classes Interfaces Polymorphism. Object-Oriented Principles.

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Abstract Classes, Interfaces, Polymorphism

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

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

  3. 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 Advanced-OO

  4. 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 Food price calories Hamburger Coke Advanced-OO

  5. 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 Advanced-OO

  6. 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 Advanced-OO

  7. 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 } } Advanced-OO

  8. 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 Advanced-OO

  9. 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,… Advanced-OO

  10. 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) Advanced-OO

  11. Comparable Exercise • How would you compare two Person objects? • Implement the Comparable interface public class Person implements Comparable<Person> • 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 Advanced-OO

  12. 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 Advanced-OO

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

  14. 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(); Advanced-OO

  15. 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(); Advanced-OO

  16. 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 Advanced-OO

  17. 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 Advanced-OO

  18. 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 Advanced-OO

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

  20. Polymorphism - Many Forms • Polymorphism is overloading that is resolved at execution time, and isalso 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 Rectangle draw() draw() Advanced-OO

  21. 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. Advanced-OO

  22. 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 Advanced-OO

  23. 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 Advanced-OO

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