Workshop for CS-AP Teachers

Workshop for CS-AP Teachers Chapter 3 Advanced Object-Oriented Concepts Georgia Institute of Technology

Learning Goals • Understand at a conceptual and practical level • Objects • Object variables and methods • Classes • Class variables and methods • Inheritance • Abstract Classes • Interfaces • Polymorphism Georgia Institute of Technology

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 Georgia Institute of Technology

Advantages to Objects • Data and related operations are combined • No passing data around • Data is protected • Objects are responsible • If code needs to be fixed you can figure out what class needs to be changed • Methods can change as long as they still do the job • Easier to maintain and extend • Objects don’t change as often as procedures • The program is a simulation of the domain • Classes can be reused Georgia Institute of Technology

Class Definition Exercise • Write the definition for a class Student. • We need to know about the student’s name (first and last), store the name of a file with his or her picture in it, and keep the grades for this student. We want to be able to add grades and compute the grade point average. • Remember to define methods to get and set all fields • Write the main method to create a Student Georgia Institute of Technology

Steps in a Class Definition • Underline the nouns and verbs • Nouns may be attributes • Sometimes other classes • Verbs may be methods • Do a simple UML class • Class name, attributes (fields), methods • Specify the types of the attributes • Specify the return type and parameter types • Code the class Georgia Institute of Technology

Class Definition • A class definition in Java defines • Object Fields (Variables) • Fields that all objects of the class will have • Class Fields (Variables) • Fields that will only be on the class definition class • Constructors • Used to initialize object fields • Object Methods • Methods that have the current object passed implicitly (this) • Class Methods • Methods that operate on class variables Georgia Institute of Technology

Adding a Student Id • What if we want to add an id as an object field? • Each object should have a unique id • One way to assign a unique id is to have a counter that counts how many students we have • Increment the current count each time a new student is created • Use the current count as the id Georgia Institute of Technology

Adding Id Exercise • Modify the Student class to add a unique id for each student • Add the field id • Add the field numStudents that starts out as 0 • Each time a new student is created • Increment numStudents • Set the id to the numStudents • Modify the toString method to add the id • Create several students in the main method Georgia Institute of Technology

What went wrong? • id and numStudents are object fields • A separate copy is in each object • So all objects have a count of 0 • But we want there to only be only one copy of the number of students • That all Student objects have access to Sally :Student JaKita :Student Thomas :Student numStudents Georgia Institute of Technology

Class Fields (Variables) • Each object has a link to an object that describes the class • An object of the class “java.lang.Class” • So if we create a class field • All objects will have access to the one field • Create class fields • Using the keyword static Sally:Student Id firstName lastName pictureFile grades Student: Class numStudents addGrade(grade) getGradePointAverage() … Georgia Institute of Technology

An Object “knows” what Class it is • You can check that an object is of a class instanceof ClassName • You can get the Class object from an object getClass() • The Class objects holds information from the class definition Sally:Student firstName lastName pictureFile grades Student: Class numStudents addGrade(grade) getGradePointAverage() … Georgia Institute of Technology

Class Exercise • Loop up the class java.lang.Class • How do you get the parent class of a class? • How do you get the methods of a class? • How do you get the fields of a class? • How do you tell if it is an interface? • Try the following in DrJava • Class stringClass = “Test”.getClass(); • System.out.println(stringClass); • System.out.println(stringClass.newInstance()); Georgia Institute of Technology

Class Variable Exercise • Make numStudents a class variable by adding the keyword static • Now run the main method and check that the ids are unique and incrementing • Create another student in the main and run it again • What id does the first student start with? Georgia Institute of Technology

Object Methods versus Class Methods • Object methods are implicitly passed the current object • And work on the data in that object • Class methods only have access to class fields • They are not passed an object of the class • They can not work on object data • They are useful when you don’t have a current object • They are useful for working with class fields Georgia Institute of Technology

Class Method Exercise • When should you use a class method and when an object method? • To read a file and create student objects from the information in the file • To get the grade point average for a student • To get the number of student objects created • To print the information in a student object Georgia Institute of Technology

Inheritance • Did you get features or abilities from your parents? • People inherit physical characteristics • Some people inherit abilities: music • Inheritance in OO means receiving data and methods from your parent • In Java you can only have one parent Georgia Institute of Technology

Inheritance in our Models • One class can inherit from another • Gets all the fields and methods • The class you inherit from is called • Parent, superclass, base class • The class doing the inheriting is called • Child, subclass, derived class Person Parent Student Child Georgia Institute of Technology

Inheritance Exercise • Open the Picture class (Picture.java) • See if you can find the show method • Notice that the Picture class extends (inherits) from SimplePicture • See if you can find the show method in SimplePicture • Since Picture inherits from SimplePicture you can ask a Picture object to show itself Georgia Institute of Technology

How Inheritance Works • When an object gets a message it checks to see if it has a corresponding method • If it does it executes that method • If it doesn’t it checks the parent class • And keeps looking at parents until the method is found or it reaches Object • The Java compiler makes sure that a method is available • based on the declared type of the object Georgia Institute of Technology

Teaching Inheritance • Point out inheritance in common objects • What is a book? • What is a dictionary? • Talk about the things that are the same • Talk about the things that are different Georgia Institute of Technology

The Object Class • If you don’t specify a parent for a class using “extends” it will inherit from Object • public class Name { • All objects in Java inherit from Object • Directly or indirectly • Object is in the package java.lang • All objects inherit • toString() and equals() but you will usually override these Georgia Institute of Technology

Inheritance Exercise • Look up Object in the api • http://java.sun.com/j2se/1.4.2/docs/api/ • In package java.lang • What other public methods are in Object? • Look up String in the api • In package java.lang • What class does it inherit from? • Look up JButton in the api • In package javax.swing • What are all the classes it inherits from? Georgia Institute of Technology

Private Visibility and Inheritance • When a class inherits from another class it gets all the fields and methods • But, it can’t directly access fields or methods that are private • Use public methods to ask for changes to private fields • Call public methods that call private methods • This is to allow an object to protect its data and keep private methods private Georgia Institute of Technology

Protected and Inheritance • Some books use protected visibility with inheritance for fields • To give children objects direct access to fields • This is a bad idea • Protected gives access to subclasses • Protected also gives access to all classes in the same package • You can’t guarantee the data isn’t messed up by objects of other classes if you use protected fields! Georgia Institute of Technology

When to Use Inheritance • Use inheritance when one class is really a sub-type of another • You must be able to substitute an object of the child class for an object of the parent class and still have it make sense • Don’t use it just to have access to some fields and methods of the parent class • Use an association (has a) link instead Georgia Institute of Technology

Correct Use of Inheritance • Students and Teachers are people • You could use a student or teacher when you need a person • A course period is not a course • You wouldn’t include a course period in a list of available courses • Instead a course period would have a course associated with it • Have a field that is a Course object Georgia Institute of Technology

Substitution • Is a dictionary a book? • If you need a book will a dictionary work? • You can use a child object when a variable refers to a parent object • SimplePicture p = new Picture(fileName); • You can’t substitute a parent for a child • Picture p = new SimplePicture(fileName) Georgia Institute of Technology

Generalization • Generalization means that classes have things in common • If they are several classes that all have the same or similar fields and methods • pull out the common items and put them in a parent class Georgia Institute of Technology

Generalization - Exercise • Pull out common fields and methods in Student and Teacher and put them in a new class Person • Modify constructors as needed • Have student and teacher inherit from Person • Run the main methods in Student and Teacher after the changes • To make sure they still work Georgia Institute of Technology

Specialization Person firstName lastName pictureFile • A class differs from the parent class in some way • It add fields • It adds methods • It does something different when given the same message as the parent Student id nickname grades numGrades numStudents getGradePointAverage() Georgia Institute of Technology

Overriding Methods • One way a child class can specialize is to handle the same message in a different way than the parent • If you inherit parent methods how can you do this? • The child can redefine a method with the same name and parameters as the parent method • The new method is called instead of the parent method • This is called overriding Georgia Institute of Technology

How Does Overriding Work? • When a message is sent to an object • The object checks with its class to see if the corresponding method exists in the class • If it doesn’t exist in the class then it checks in the parent class • It keeps looking up the inheritance tree till it finds a corresponding method Georgia Institute of Technology

Using Super • What if the method in the child class wants to call the method in the parent class? • I want to do the same operation as my parent but also add something to it • But my method overrides the parent method • Use the keyword super to invoke an overridden parent method • super.method() • It starts looking for a corresponding method in the parent class Georgia Institute of Technology

Overriding equals and hashCode • All classes inherit from Object the method • public boolean equals(Object o) • the default result is true if they are the same object else false • You will often want to override this method • And check if some field or fields are equal • If you override equals you must also override hashCode • public int hashCode() Georgia Institute of Technology

What is a hash code? • An integer used to index into • hashing based collection classes such as Hashtable, HashMap, HashSet • What should you return as a hash code? • Best if each object has a unique hash code • Okay if most objects have a unique hash code • But some have duplicates • Bad if most objects result in duplicate hash codes Georgia Institute of Technology

What is a Hashing Collection? • Map of key to value • You put a value (object) into a collection for a key (object) • You can get back the value for that key • Similar to a safety deposit box • The hashCode() method is used on the key to find the memory location holding the value • The address of it Georgia Institute of Technology

Overriding Exercise • Edit Student.java to override equals() • public booelan equals(Object object) • Return true if the passed object isn’t null and is of the class Student • And the id of the current object and passed student object are equal • Override hashCode() • You can return the id since each object will have a unique id • Test equals in the main method Georgia Institute of Technology

Advantages to Inheritance • Handles commonality • Common attributes and operations are factored out and put as high as possible in a hierarchy • Not copied to several locations • Handles differences • Children can inherit the parts that are common from the parent • They can add attributes and operations to handle how they differ from the parent • They can override operations (methods) to do something different from the parent Georgia Institute of Technology

Constants • The final keyword means that the item will not change. • By using the final keyword along with public and static you can create class fields that are constant. • public static final int MALE = 0; • These can be accessed by using the Class name.attribute name. • this.gender = Person.MALE; Georgia Institute of Technology

Constant Exercise • Add constants for unknown, male, and female gender to Person.java • public static final int UNKNOWN = 0; • public static final int FEMALE = 1; • public static final int MALE = 2; • Add a gender field to Person.java • private int gender = UNKNOWN; // default • Add get and set gender methods • Add gender to the result of toString() Georgia Institute of Technology

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 Georgia Institute of Technology

Why use an Abstract Class? • Represents an abstract idea (like Shape) • 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 Georgia Institute of Technology

Interfaces • Interfaces are a description of behavior. • They are a special kind of abstract class that has only abstract methods and constants. public interface ShapeInterface { public void setShape(int shape); public void setShapeColor(Color shapeColor); } • You don’t have to declare the methods as abstract • They automatically are Georgia Institute of Technology

Classes Implement Interfaces • Classes that implement interfaces must provide the implementations for the methods specified in the interface. public class ShapeCanvas implements ShapeInterface { public void setShape(int shape) { code to handle set shape } public void setShapeColor(Color shapeColor) { code to handle set shape color } } Georgia Institute of Technology

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 Georgia Institute of Technology

Interfaces Versus Inheritance • When a class inherits from a parent class it inherits all the 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,… Georgia Institute of Technology

Compare 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) Georgia Institute of Technology

Comparable Exercise • How would you compare two Person objects? • Implement the Comparable interface public int compareTo(Object object) • 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 Georgia Institute of Technology

Collections - java.util • Used to hold objects • Use 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 Georgia Institute of Technology

Workshop for CS-AP Teachers