Workshop for CS-AP Teachers
Chapter 3
Advanced Object-Oriented
Concepts
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Learning Goals
• Understand at a conceptual and practical
level
– Objects
• Object variables and methods
– Classes
• Class variables and methods
– Inheritance
– Abstract Classes
– Interfaces
– Polymorphism
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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
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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
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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
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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
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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
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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
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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
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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
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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()
…
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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()
Sally:Student
firstName
lastName
pictureFile
grades
Student: Class
• The Class objects
holds information
from the class
definition
numStudents
addGrade(grade)
getGradePointAverage()
…
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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());
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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?
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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
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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
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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
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Inheritance in our Models
• One class can inherit
from another
– Gets all the fields and
methods
Person
Parent
Student
Child
• The class you inherit from
is called
– Parent, superclass, base
class
• The class doing the
inheriting is called
– Child, subclass, derived
class
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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
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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
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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
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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
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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?
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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
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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!
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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
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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
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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)
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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
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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
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Specialization
• 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
Person
firstName
lastName
pictureFile
Student
id
nickname
grades
numGrades
numStudents
getGradePointAverage()
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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
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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
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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
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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()
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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
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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
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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
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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
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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;
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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()
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Abstract Classes
• Abstract classes are classes that can’t be
instantiated. Abstract classes can only be
Food
subclassed.
price
calories
Hamburger
Coke
• Create an abstract class by using the
keyword abstract in the class declaration.
– public abstract class Food
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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
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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
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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 }
}
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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
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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,…
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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)
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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
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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
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List and Set Interfaces and Classes
<<interface>>
Collection
<<interface>>
List
ArrayList
Vector
<<interface>>
Set
LinkedList
HashSet
<<interface>>
SortedSet
TreeSet
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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();
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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();
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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
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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
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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
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Class Diagram for ShapePanel
ShapePanel
ShapeInterface
1
Shape
draw()
Oval
draw()
*
1
1
1
ShapeCanvas
paint()
Rectangle
draw()
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1
ButtonPanel
Interface Exercise
– Add a method to the ShapeInterface
public void clearShapes();
– Modify the ShapeCanvas class to implement
the method and clear the vector of shapes
and then call repaint();
– Modify the ButtonPanel class to add the clear
button and have it call clearShapes() when
pressed.
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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()
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Add Abstract Class Subclass Exercise
– Create a new class Line which is a subclass
of Shape.
– Add a constant for Line to the Shape class.
– Add a Line Button to the Button Panel and
when the line button is clicked on notify the
handler to set the shape to line.
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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
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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
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